Zinc Coating and corrosion prevention

By Nick Karakasch, a retired principle of Total Corrosion Consultants Melbourne Australia, his experience spans over 55 years, specializing in services to the Protective Coating, Galvanizing, and Structural Fire Protection Industries.

Kenya Engineer

September 25, 2024

ABSTRACT

Breakthroughs take different forms. It is almost 90 years since a breakthrough in the science of corrosion prevention gave us the extraordinary benefits of long-life steel protection that we now take for granted. This article discusses a range of issues, together with a general overview related to the protection of steel with metallic zinc coatings. The content is not new to those with industry experience nevertheless the matter is worth a revisit for those who have over recent times entered the Protective Coating Industry. This review is based on personal and practical experience and is not all inclusive. The intent is to provide a general insight into the historical background, performance, and a basic understanding of the principals associated with the use of metallic zinc coatings.

INTRODUCTION

An early reference to zinc-based coatings was back to May 1837(ref 1) when Frenchman Stanislaus Sorel filed a patent for Hot Dip Galvanizing with the use of molten zinc. A further addition was added in June of that year specifically referring to Galvanic Paint. In 1839, Professor Standin, Inspector General French Military Academy wrote:

‘A patent was recently taken out by S. Sorel ‘for galvanic protection of iron, by either coating it in a bath of molten zinc or by covering it with a so-called galvanic paint.”

“Zinc reduced to very small powder is mixed with oils and other substances used in ordinary paints and varnish, all substances which are used to make different colours can also be put into galvanic paint”.

By today’s standards early zinc coatings were disappointing, they had limited success in providing long term protection because binders such as linseed oil and resins derived from trees were generally used. The other problem was the high zinc oxide content which coated and electrically insulated the metallic zinc particles. With time and improved technology, a breakthrough occurred in 1937 with the advent of the first Inorganic Zinc Silicate Coating( IOZ) Other developments introduction were Organic Zinc Rich Primers Coatings(OZRP).The term Inorganic Zinc Silicate was not used till the early 1960s previously it was known as “Silicated Composition Containing Finely Divided Zinc.”

Zinc coatings are classified into two broad categories, Inorganic Zinc Silicate (IOZ), incorporating binders that are inorganic based, meaning they are inert to UV and atmospheric deterioration. They come in two forms water or solvent based materials available as single or two pack products. Organic Zinc Rich Primers(OZRP) are either single or two pack materials using a variety of binders all based of organic matter, meaning they are not inert and will degrade in atmospheric exposure. Present day binders incorporate materials such as , Epoxy, Urethane Alkyds, Polyurethane, Polyester/Polystyrene, and Chlorinated rubber etc.

INORGANIC ZINC SILICATE (historical development background)

The world’s first Inorganic Zinc Silicate (IOZ) was “Galvanite” later renamed ‘Dimetcote 2 ’( ref 2) invented by Victor Nightingall (Melbourne, Australia 1937) the founding director of the Dimet Coating company. The Australian patent was issued in 1939 (ref 2), other countries covered in later years were USA 1949 (ref 3), Canada, UK, NZ, Belgium, Singapore, and Ireland. Dimetcote was licenced to the US Ameron Corporation in 1949, who were world leaders in Vinyl Paint Technology. This arrangement resulted through close collaboration(1954) into the first IOZ that did not require heat treatment for curing purposes. The 1954 version meant for the first-time IOZ offered flexibility, large steel objects such ships, offshore oil platforms, bridges, tanks etc, together with site projects could be accommodated, and existing structures refurbished to provide long term protection. In those days long term meant four years.

Nightingall’s invention was the ‘holy grail‘ of the old age corrosion problem, the protection of iron and steel in a sea water environment. It revolutionized the protective coating industry throughout the world. For the first time structural steel components too large for a galvanizing bath, could be given long term protection. Nightingall is not well-known, few people realise this important principle of steel protection had its origins in Melbourne Australia. His contribution to the Corrosion Industry was like what Microsoft bestowed to the computing industry.

The first IOZ coating (1937) was water based and heat cured. Over the years numerous upgrades and variations throughout Australia and the USA were developed. The first in Australia 1945, Zincilate 100 another heat cured version with the addition of sodium sulphide to stop gassing and included red lead. In 1954 Dimetcote 3 the first Chemically cured coating appeared( USA/Australia). This was followed by two variations of Self-Curing Solvent Borne Ethyl Silicate, Acid hydrolysed by Carboline Corporation USA 1959 ( Ref 4),and Alkali hydrolysed ( Australia/USA1965). Also 1965 a Water Borne (potassium silicate) appeared, Colloidal and Lithium silicate followed (late 1960s) coming onto the market.

The latest variation in general use is High ratio (water based) patented 1980 in the US by NASA. High ratio relates to the liquid component “silica-alkali ratio”. The zinc component remained largely unchanged, any liquid component with a ratio over 1.4 is considered high ratio. IOZ are unique their films closely resemble Silica Sand in chemical composition and are sometime referred to as ”liquid Glass”.

Dimet had numerous high ratio materials well before the 1980 patent, the first in 1965 “ Dimetcote 5” (1.47 ratio). These at the time were viewed as only slight raw material variations to their original patent. In their view there was no need to upgrade their patent. One formular, was based on potassium silicate, the next a combination of Potassium / lithium, and the third based on straight lithium. These variations were developed to emulate curing properties similar to that of Solvent Borne Inorganic Zinc Silicates. Tidal zone testing at the RAAF Air Base Victoria testing facility showed the lithium material to outperform the other two by a factor of 3. However, it was expensive at the time and only ever used in the early days by the shipping industry where the cost could be justified.

PERFORMANCE

On performance, the original heat cured IOZ ( Galvanite/ Renamed Dimetcote 2) is still ongoing exceeding 80 years on the 359km Morgan Whyalla Pipeline. The chemically cured 60 years, water-based materials 55 years on many steel bridge and offshore structures are also ongoing. The performance figures outlined have average Metallic Zinc content of 500-600 grams/ M2. The protection of offshore platforms presents one of the most difficult corrosion problems unlike a ship, fixed Platforms are extremely difficult to maintain as the they remain at sea for life of the well. It is likely that there is no other system that has been able to provide the same level of performance under these very harsh marine conditions. Hundreds off Offshore Platforms worldwide were successfully coated some of which I believe are still in operation.

Solvent Borne IOZ are excluded from offshore environments unless top coated. However, the offshore industry over recent times now allows their use on the basis the platforms are near their use by dates and don’t require the same level of durability as water HR Zinc provides.

Metalizing processes such as Hot Zinc Flame Spray or Hot Dip Galvanizing have outstanding “Onshore Marine” performance. However, performance Offshore and in Shipping is limited, usually confined to fasteners, stair treads, and gratings due to their geometrical configuration making them impactable to paint. The success of IOZ cannot be denied, putting this into prospective known performance in the Shipping Industry and Offshore Marine Environment, an average of 500/600 grams/M2 of metallic zinc held in a matrix has exceeded 40 years of service. Whereas, galvanizing with 700 grams under the same harsh conditions provides 3-5 years maximum.

What this demonstrates is, pure metallic zinc system’s go into solution considerably faster in harsh marine environments than zinc held in an inert silica matrix. Another outstanding area is structural steel within containment shells of nuclear power plants IOZ is immune to UV attack and unaffected by radiation or radioactivity. It is important to understand IOZ is not a metalizing process it’s a Coating, a layer of paint incorporating metallic zinc particles as a pigment held in a matrix. The performance and consumption of pure Zinc in Metallic materials (Galvanizing, Hot Zinc Flame Spray, Zinc Plating, Sherardizing ), is linear purely related to Zinc Volume. There is no matrix barrier similar to IOZ coatings to regulate consumption, therefore performance for these materials can be predicted to a known corrosion environment.

ZINC COATING PRINCIPLES

The function of a zinc coating is primarily to provide corrosion protection to the underlining steel surface, for this to occur an electrolyte needs to be present (water, oxygen, and soluble salts). In electrochemical terms steel surfaces are divided into anodic and cathodic areas. There must be a release of electrons at the anodic surface where oxidation of the metal occurs. Cathodic areas accept the electrons, this reaction occurs at the same time and at equivalent rates. However, deterioration only occurs at the anodic areas, these sites change from time to time and can give the appearance of uniform corrosion.

Galvanic action or Cathodic protection is when an electric current is generated between differing metals in contact with one another in an active electrolyte solution or corrosive environment. Relatively speaking electrons will flow from metals that have high electrical potential to metals with low electrical potential. Compared to steel, zinc has a higher electrical potential therefore it is sacrificed going into solution. If the current can be blocked, broken, or reversed corrosion will not occur. The anodic reaction is sometimes referred to as deterioration, oxidation, or sacrificial action of the zinc.

The term Galvanic was named after the Italian Physician/Physicist Luigi Galvani (1737-1798). Professor (Count) Alessandro Volta another Italian who invented the first electric battery (1800) coined the term Galvanism meaning a current produced by chemical action when two different metals in an electrolyte are in contact with one another. This was later reinforced by the research conducted at Cambridge University(1932-1943) by Messer’s Evans & Mayne that clearly showed conductivity and cathodic protection occurs with materials that have high metallic zinc content.

The current flow or amperage which determines performance is less when metallic zinc is held in a matrix binder, compared to other forms of protection where pure zinc is exposed in its own right. The matrix interrupts and regulates the current flow. The consumption of zinc under these circumstances slows considerably, meaning less metallic zinc is required for protection to a given environment. Apart from the zinc component the importance of the matrix cannot be overstated, it is the main element that has contributed to IOZ success.

The simplest way to describe IOZ coatings is a zinc anode in the form of fine particles of metallic zinc held in a matrix of silica. When the two components combine chemical changes take place during the curing process creating the matrix which in turn holds the zinc in place. After the initial curing process, the coating is somewhat porous, porosity decreases on atmospheric exposure when initial galvanic process takes place protecting underling steel. The corrosion by-products produced are essentially inert insoluble compounds i.e., zinc carbonate, hydroxide, and zinc oxide, all retained within the film filling the voids which then blocks and regulates the current demand on the zinc.

Up to 3-40% of the metallic zinc at this stage is consumed, the remaining zinc needs to be dislodged or exposed for the galvanic action to continue the protection process. At the same time if either Zinc, Calcium or Magnesium ions are present in the electrolyte, a Microcrystalline deposit is formed at the steel surface which has a stifling effect on the cathodic area. Thereby minimizing the current demand on the zinc and consequently extending the protective period, by holding unreacted zinc which only becomes operative if the coating is damaged or the zinc is exposed at some later stage.( Refer Fig 1&2)

The other action that occurs is the exposure to carbon dioxide this results in the formation of Insoluble Zinc Carbonate. This basic carbonate held in the film is initially a semi permeable barrier which protects the zinc from corroding too rapidly while at the same time providing sufficient permeability to allow the necessary electrical current to flow keeping the steel from corroding.

Formation of zinc carbonate is not unique to IOZ, it also applies to Galvanizing and Hot Zinc Flame spray. With Galvanizing it’s a 2–3-micron layer on the surface, with Hot Flame spray it is retained on the surface and in the film. When these layers are depleted generally through atmospheric erosion it activates the galvanic action.

In some exposers the consumption of metallic zinc is so rapid IOZ coatings are impactable, exposure to strong acidic, or alkali environments are not recommended, neither is under buried conditions nor fire proofing compounds unless appropriately top coated. Zinc being an amphoteric metal means it will react with both acidic and alkali environments.

The ideal pH range is between 5-10 for sacrificial action to be kept at a minimum. If used outside this range top coating is always necessary to protect the zinc primer. Top coating may also be required to improve the aesthetic appeal of the installation. In the early years enamel (alkyd) paints were often used, these products should never be applied directly to any zinc surface as it produces a soapy substance at the interface leading to loss of adhesion. Scientifically it is called Saponification.

If enamel paint is to be used, then an application of non-reactive barrier intermediate coat such as an epoxy/primer is necessary, chlorinated rubber coatings in the past had been widely used for this purpose however their use has been superseded.

Over coating IOZ provides another important feature, a Synergistic Effect meaning, working together or a combined action to increase performance.

IOZ have a unique ability to contain corrosion at the point of damage (Fig 3),there is no under film corrosion creep. In simple terms where the corrosion is visible its contained, this progresses slowly as the surrounding zinc is gradually consumed and spreads. This benefit has implications to repair costs as it can be accurately costed.

Porosity has a significant influence on both selection and application where topcoats are required. The quality of application must be controlled to minimize the risk of pin holing, or dry over spray. These aspects also need consideration when estimating material and labour requirements. In many cases to overcome these problems when top coating a mist coat/full coat technique is used. The acceptability to the end client or inspector will vary, and the appearance can be subject for questioning or rejection, in some cases the final appearance can be less than expected. Following are some procedures that can minimize this dilemma.

Apply IOZ to achieve the smoothest possible finish.
Remove any dry or over spray.
In the case of SB-IOZ use slow evaporating thinners.
Use the mist coat-full coat technique.
Correct visible pinhole areas with an additional spray pass of the topcoat material.

SUREFACE PREPARATION, ADHESION and DRY FILM THICKNESS (DFT).

The degree of surface preparation is the most important factor controlling the performance. For IOZ complete removal of mill scale, rust and all foreign matter is mandatory. Abrasive blast cleaning is always necessary, although with the original IOZ Acid Descaling was also acceptable. The blasting process should impart a rough angular surface profile. The origins of this technique can be traced back to the USA patented by B.C. Tilghman in 1869, the concept came to him during the US civil war when he witnessed the effects of windblown sand on glass windows, Tilghman formed his own company now called the Wheelabrater Group a world leader and major supplier of abrasive blasting equipment.

Minimum standard for surface preparation is SA2.5 (near white metal), although SA3 (white metal) is the preferred standard, particularly for severe environments. The importance the of surface profile is that if IOZ was applied to a smooth surface there would be poor adhesion, no film strength, and be brittle. The silicate component in the material reacts with the steel in a similar way it does with metallic zinc. A microcryline layer (fig1&2) is formed at the interface which greatly assists with adhesion. When fully cured the film becomes extremely hard and is highly abrasion resistant.

Prior to abrasive blast cleaning all surface contaminates need to be removed either by appropriate decreasing or high-pressure water washing, otherwise long-term adhesion is compromised which in turn effects the overall coating performance. If contaminants remain, particularly if IOZ is top coated, osmoses will occur resulting in osmotic blisters within the topcoat materials.

Surface roughness is important for attaining good adhesion, the surface profile is generally subject to an agreement between the contracting parties, the general rule, this should not exceed one third of the specified total dry film thickness (DFT). The same principal applies to all coating systems. Allowance must be made to eliminate the surface profile effect which can range between 15-20 microns depending on the size of the abrasive medium used. Abrasive medium size is important for two reasons 1) if the profile height is too high there may be insufficient coating cover in relation to coating thickness, 2). Contractor’s coating usage will increase due to the increased surface area which can be as high as 10%. The following guidelines should be used when specifying profile height.

Coating DFT Average Profile Height

0 – 75um 25um

150um 50um

300um 75um

400um 100um

Adhesion can be divided into Mechanical or Chemical. Mechanical involves the anchoring of the film to the substrate, this is determined and influenced by the degree of surface cleanliness and roughness obtained. (Surface profile). This aspect is critical as it provides an anchor Patten for the IOZ to adhere to and increases the surface area for adhesion. Chemical Adhesion generally depends on the chemical affinity with the substrate to form a relatively strong bond.

Adhesion strength has a direct bearing on long term coating performance and durability. Paint cohesive properties play a role in performance, which is derived from the strength of the molecular forces within the paint film. Paint coatings require good wetting properties with good flow characteristics they needed to be able to fill crevasses, micro voids etc to displace any trapped air at the interface. Molecules within the coating need to flow freely sharing electrons attached by negative and positive regions.Scientifcally the process is called Absorption. Pin holes and dry spray can occur after application via solvent entrapment and release. Application in hot and low humidity conditions can increase the tendency for these defects.

When using IOZ in a multi-coat systems there are several points of potential failure of which the most important is failure of the coating to adhere to the metal surface, or a clean separation of one coat from the other. Cohesive failures within a paint layer are of less importance and are preferable to adhesion failures to the metal substrate. Delamination between coats can in broad terms be related to over thickness, solvent entrapment, overcoating time limits ,incompatibility between coats or where they have been applied before the IOZ cure is complete.

Cohesion film strength is not to be confused with adhesion to the substrate. If for example topcoat systems are applied before IOZ is cured well above the required thickness and structural movement takes place, the system overall will tend to crack in most cases delamination from the substrate occurs. The weakest point in the system in the early stage’s is somewhere in the IOZ film. The reason is that topcoat materials such as epoxy have a much higher initial cohesive strength.

Another major concern has been the radius of steel edges, whilst clearly outlined in the abrasive blast standard and necessary it is rarely performed by the steel fabricator. The contracting parties need to be made aware of this requirement during the design and tendering stage and reinforced in a pre-start meeting attended by fabricator, painting contractor, coating supplier, inspector, and owner’s representative. Unfortunately, when failures occur usually starting on edges. Invariably the painting contractor and or the paint suppler are blamed. In my view the steel fabricator is primarily responsible to ensure steel edges are properly radiused. If this facet is absent, it could be argued successfully that this is a cojoined responsibility by all those associated with the project if this aspect of the specification is not performed during fabrication of the steel components.

ASSESSMENT

Assessing IOZ can be confusing, it’s worth noting conventional enamel paint systems can be assessed by accelerated testing which have previously been associated with practical performance. These system which may last for three to four years can be made to fail in a few hundred hours in a Salt Spray Cabinet test. The best IOZ coatings usually require approximately 20000 hours plus, or over a two-year period to fail for the best of these coatings. Proven case history performance in the relevant environment is the most practical way for selection.

There are four aspects that are measurable, Friction Grip Bolting, Dry Film Thickness, Metallic Zinc content, and Abrasion Resistance.

IOZ is unique, being Inorganic they will not burn and have outstanding friction grip resistance to bolted steel connections, Painted Surfaces and Galvanizing are generally prohibited. IOZ has a coefficient of friction 0.599 well above the minimum for design purposes of 0.45 this aspect cannot be overstated considering the importance of bolted connections to the steel construction industry.

For technical reasons it is not possible to incorporate the same high zinc loading in solvent borne materials as with water-borne coatings, nevertheless they provide considerable better application properties under adverse weather conditions due to their greater tolerance to high humidity and lower temperatures during application particularly under site conditions.

DRY FILM THICKNESS (DFT)

The DFT for single coat applications varies, Solvent-based materials usually between 75-100microns, water-based materials up to 200microns depending on environmental exposure. IOZ does not shrink when cured like normal organic based paints reason being, the matrix created on application increases the volume of material by as much as 10%. The method used to determine theoretical coverage is called the Void Content Method, whereas all other paint coatings use the standard Volatile Method.

Void Content Method: is based on the volume of actual solid material, zinc dust etc and includes the void spaces created on application.

Volatile Method: takes into consideration only the solid material available on application.

The void method will give a higher figure which is the true reflection on what is achievable in relation to coverage. It is important to identify the method being used so that an appropriate comparison can be made. This enables the coating applicator to determine the coating quantities required for the project. Typical examples of coverage based on 75 microns DFT.:

IOZ(W/B) Volatile Method = 9 / M2

Void Method = 9.9/ M2

IOZ (S/B) Volatile Method = 7.3 /M2

Void Method = 8/M2

Organic Zinc Rich (Epoxy based 45% volume solids) Volatile Method only = 5.91/M2 at 75 microns.

SB-IOZ materials are very susceptible to crazing and mud cracking at thicknesses above 75microns,the reason being they are initially unstable. On application the pH factor is acidic which converts back to neutral when full curing takes place. Small amounts of organic resins are used which act as reinforcing agents to control thickness and possible subsequent mud cracking on application Water-based coatings can also suffer this problem if film thickness exceeds 200 microns.

SB-IOZ are susceptible to dry spray particularly under hot and windy conditions. The way to overcome this is to add a small quantity of a slow volatile solvent at no more than 10%. which has the tenancy to slow solvent evaporation out of the film after application. High volatile hydrocarbon solvents such as Xylol and Oxytol are commonly used in their formulation, this means the solvent release during application is fast under certain conditions hence the dry spray problem. However, the user should take the advice of the coating manufacturer if faced with windy, high temperatures or low humidity conditions.

DFT values below 60 microns and greater than 150 microns should be cause for rejection, first to ensure long term protection and secondly to minimise mud cracking. If cracking should occur this needs to be limited to no more than 0.05% of the total area as failure can results from splitting and delamination from the steel surface. Cracking should not cause any loss of adhesion or cohesion of the film and be limited to complex corners and 90-degree surfaces. Minor cracking in some circumstances may be acceptable. IOZ have a self-regulating repair mechanism, the capacity to self-heal for areas of 3-5mm in width. On completion it should be well cured, firmly adhered to the substrate and when physically stressed eroded rather than separate at the steel interface or within itself.

Over the years many contactors have experienced difficulties applying SB-IOZ. Whilst there are numerous reasons there appear to be two common factors. Firstly, the use of Airless Spray Equipment instead of Conventional Air Atomised spray units together with Inexperienced Applicators. With airless equipment it is either on or off, contactors tend to over build in corners and 90-degree surfaces. In their attempt to achieve the normal requires thickness of 75-100 microns, airless applications are often not successful. Airless equipment was originally designed for increased productivity to allow deposition of highly viscos coating materials. Whereas air atomised equipment allows a competent applicator to use the material air and fan controls on the spray gun to “feather” and better control the deposition of a more liquid coating to suit the surface configuration.

Applicators may argue they have been applying these products for many years and have rarely experienced problems. This may or may not be so. However, being highly susceptible to mud cracking and human error it is reasonable to expect if care is not exercised DFT will exceed the required thickness value and mud cracking is highly likely. Compliance issues for painting contractors are part of the contracting industry, a prequalified contactor is essential.

METALIC ZINC CONTENT

The critical question is how much metallic zinc goes into a IOZ coating. The answer depends on the intent and skill of the supplier. If a coating is formulated for maximum performance, in most cases it will not be competitive. If formulated for low cost its performance will usually fall short of requirements. It can be said with some confidence long term performance is much related to the binder created and the volume of metallic zinc in the coating film. It is volume of zinc and how it is fixed within the film is what determines the length of performance.

In many cases manufacturers literature is often unclear, quoting their product contains “ 90% zinc “. This statement is ambiguous and incomplete, there is no mention of zinc purity, percentage of zinc oxide, or more importantly the figure is not qualified as Weight or Volume. It is metallic Volume per square metre of surface area that determines overall performance, not weight in the can.

Zinc oxide does not assist galvanic protection, it only acts as an inert filler. Metallic zinc is the most expensive component, the selling cost to industry is largely influenced by this content. The Australian Standard AS/NZS 3750.15.1998 for IOZ nominates zinc pigment composition i.e., minimum metallic zinc and oxide content. Independent testing has shown some remarkable results, zinc oxide content is meant to be 5%, yet oxide levels for non-conforming materials ranged between 6.7 to 40%.The inference from the testing indicates a wide variety of formulations.

The test method used was an Induction Couple Plasma Assay followed by a Spectrum Trace, this is a very sophisticated test not common but entirely accurate compared to the Hydrogen Evolution Method used by the paint industry. Conformance to the Australian Standard DFT at 75 microns should result in 300 grams of metallic zinc per square meter for water-based and 210 grams for solvent-based materials .

Organic zinc rich primers at 75microns average between 75-150 gm/M2. The Australian standard for this class only nominates zinc pigment composition manufactures are free to formulate their own materials. International standards do not assist as the following descriptions used by various Standard Societies in describing the amount of zinc as:

Mass solid when mixed

Zinc % in wet paint

Zinc % dust in the dry film.

Total zinc in the dry film (zinc oxide and other impurities not mentioned)

Properties of a paint film usually reflect the pigment volume concentrations rather than the pigment weight loading. Zinc coatings should be formulated with the highest metallic zinc by volume that the binder will tolerate to provide satisfactory application properties. There are many applications where zinc plays no part in the protective process, yet long term protection can be achievable i.e., tank linings, buried pipelines, wharf piles, and chemical environments to name a few.

ABRASION RESISTANCE

Abrasion resistance is a strong indicator of the wearing ability, which is often clearly demonstrated by very little damage suffered during transportation and erection. The method commonly used for comparison is a Taber Abrasion Federal Test (No 141-6192), in this test a standard abrasive wheel under a standard load revolves against a coated test plate (75 microns), which itself is made to rotate. The number of cycles required to wear completely through is calculated and is quoted as “wear cycles”. Typical comparison values obtained showed the following results:

Heat cured IOZ 150,000 wear cycles.
Water- borne IOZ 25 -50000 wear cycles.
solvent – borne SB-IOZ 5 -25000 wear cycles.

Another measure is” wear index” the weight loss over 5000 revolutions:

1.Water-borne. 0.7-gram loss.

2.SB-IOZ . 0.30- gram loss.

SB-IOZ coatings are somewhat softer demonstrated by commercial examples showing a wider range of wear cycles, some manufactures frequently quote 3000 hours which has no practical value in relation to IOZ. Specifying authorities should protect their position by insisting only those coatings with a minimum of at least 20,000 wear cycles be approved. When steel requires transportation the effect of lower resistance is compounded by the increased risk of more damage. Site repairs have a significant impact on a project cost on completion and should be part of the evaluation process. This is rarely costed as it is not possible to assess the effects of site time savings, however experience clearly favours materials which exhibit greater abrasion resistance.

The debate regarding zinc quality and quantity is a long standing one which has highlighted the competitive nature of the competing coating suppliers. Unfortunately, all too often perceptions are created to gain market acceptance or advantage. Specifying authorities expect to rely on well informed technical advice, and this should be unbiased, technically correct, non-contradictory, and above all based on proven performance in a given environment.

SELECTING THE APPROPRIATE IOZ COATING

In choosing between the two groups, there are advantages and disadvantages for each type, the simplest basis is:

Use Water Borne coatings for shop application, where temperature resistance of 400C (dry) is required or up to 540C intermittently or where the conditions of exposure are to be considered harsh.

Use Solvent Borne coatings for site applications in the open or where adverse weather is to be encountered. Temperature resistance is 315C

The facts behind the recommendations are:

1) Water borne coatings have higher zinc levels, harden more quickly, and reach a greater ultimate hardness making them more suitable for rapid handling and long distant transport, with less risk of damage. They are non-flammable and do not give rise to toxic or unpleasant odours, making them suitable for applications in confined spaces. The continuing concern with environmental conditions, industrial toxicity, and energy consumption makes this an important consideration. The curing process basically relies on the evaporation of water and chemical reaction between components to convert to the silica matrix binder which is temperature driven.

2) Solvent Borne coatings are available as either single or two pack materials and cure more rapidly under adverse conditions, making them suitable for site application. They reach water insolubility more rapidly and cure more reliably in high humidity ,and cold weather conditions. They reach water insolubility more rapidly and cure more reliably.

The curing procedure was designed to convert alkali-silicate , usually one of the commercial forms of ethyl silicate solutions. Adjustment is made to the pH with the use of Phosphoric Acid so that hydrolysis of the silicate occurs when exposed to atmosphere, resulting in the evaporation of the solvent content leaving the zinc encased in a matrix. On application the material is susceptible to mud cracking and dry spray if the correct procedures are not followed. There are three stages of drying during were hardness increases over time.

Solvent evaporation.
Completion of the hydrolysis and a condensation reaction.
Weathering of the zinc.

(a) Solvent evaporation takes place during the first 10-30 min, after which the steel can be handled and turned without undue damage.

(b) Completion of the hydrolysis and a condensation reaction takes place during the few days depending on atmospheric conditions. Hydrolysis is a reaction with water. Condensation means a reaction that produces and gives off water. The pre-hydrolysed ethyl silicate contains a small amount of water in the formulation and absorbs more water vapour from the atmosphere on application which then reacts to produce Ethyl Alcohol and Polymeric Silica to form the matrix binder. This reaction is slow but is assisted by high humidity which is proportional to the weight of water vapour present in the air. The ideal condition for application is minimum 58% humidity which corresponds to 8 grams/water per cubic metre. In some circumstances it may take up to five days to reach the desired hardness if the atmospheric condition are not suitable and left to its own devises.

For example, if the humidity is below 58% curing may be assisted by applying a very light water mist spray to the surrounding atmospheric area. This needs to be strictly controlled as to much water interferes with the curing process by rinsing out the alcohol component before the silicate can react. Alternatively, if application is inhouse on completion hose the shop floor with water and cover the steel over to confine water vapour to the immediate area. If humidity happens to be higher than 95% in some circumstances the material can be applied over a slightly dump surface.

Weathering of the zinc can take 3-6 months depending on weather conditions to form basic zinc carbonates etc and continues to harden until all the porosity is filled.

Mud cracking and crazing is a common feature. As film thickness increases the effect becomes more pronounced. It is caused by the stresses and strains set up in the coating as drying and curing takes place. The faster the curing, the more likely the film is to craze. The blast profile has the effect of reducing the likelihood of crazing as the peaks of the profile have a reinforcing effect on the film, distributing the load points more effectively. Crazed areas are filled with zinc oxidation products when the galvanic progress occurs this in no way detracts from the effectiveness of the coating in preventing corrosion. Whilst the curing process between water and solvent borne are slightly different the cured film structure is chemically very similar.

To summarise IOZ from an environmental aspect, they display a much lower environmental impact for the level of performance they provide with less overall use of zinc and energy associated resources. There are no industrial emissions apart from SB IOZ where small levels hydrocarbon solvents go to atmosphere during the curing process. There is no toxic waste, with only minor pollution emissions in comparison to other processes in the Corrosion Industry. For further details, those who have an interest in Environmental Sustainability which is a global issue I would refer them to an article referenced herein(12) titled Corrosion Protection & the Environment an interesting read, available on request.

ORGANIC ZINC- RICH COATING

Whilst Victor Nightingall had the first World IOZ patent ,he also held four Australian patents for Organic Zinc -Rich Coatings, three granted in 1941 and one in 1942.The coatings were called” Zincaron” consisting primarily of Zinc dust and Tung oil resins, classified into four separate versions Pipeline,Marine,Baked, and Synthetic. It was recognised they could not provide the same level of service, nevertheless these were developed primarily for applications where heat cured IOZ was impracticable, also as a repair material, should welding or transportation and should site damage occur or where sound surface preparation was impactable.

Organic zinc- rich coatings regardless of whether they are two or single pack are generally considered primarily barrier coatings providing limited galvanic protection. The reason is that they are highly filled with organic resins which completely encapsulate zinc particles. Organic binders are poor electrical conductors, for galvanic protection to occur. Metallic zinc particles need to be in contact with one another and connected to the steel substrate. Apart from this restriction Organic zinc materials contain considerably less metallic zinc than Inorganic products in the order of 30-50%.

When Organic Zinc Coatings comes up for consideration, those that incorporate Micaceous Iron Oxide (MIO) are my preferred preference. Organic materials have an inherent weakness to UV attack and Water Permeability. This ingredient provides extra protection properties. MIO has been in use for well over a century, is a natural mineral (Haematite), when processed consists of shiny metallic-grey flaky lamellar particles. When incorporated into paint coatings, the inert flaky particles orientate themselves in multiple interleaving overlapping layers ,creating a tortuous path for water ingress and UV damage.

As the encapsulated zinc is primarily acting as a non-reactive barrier coating. Up to 30-40% has been replaced with MIO. This raises the question if the zinc component is encapsulated and providing limited galvanic protection what percentage of zinc is really necessary?

There are four main benefits: 1) An added effective barrier against a corrosive environment. 2) Increased water resistance (3 Abrasion resistance, the layers physically strengthen the coating to minimise incidences of possible cracking or crazing ,4) Increased UV resistance, blocks radiation and slows the effects of chalking and film erosion. (Fig 4&5)

Australian Standard AS2312-2017, table 6 outlines expected performance no single coat is recommended in all exterior environments, they are all part of multi coat systems. Nevertheless, there has been extensive external use mainly confined to low corrosive environments. For internal mild environments the situation is quite different, they have a distinct advantage over conventual three coat enamel systems. Providing there are no colour requirements, and the client is prepared to accept grey. Organic zinc is very cost effective under these circumstances. There are case history’s dating back to the1970s where the coating is still in relatively good condition to this day. One example sighted recently comes to mind the old Ford Motor Company facility in Geelong Victoria where an epoxy-based Dimet material called Met-pon-93 was used.

There is no doubt Organic Zinc Primers have a role in the corrosion industry, comparisons between generic coating types need to be understood. Generally, it will be found that each coating type has a set of end use and, environmental requirements, where a products special inherent properties make it the first choice. The quality criteria related to performance is generally recognised in order of best to last and described by the binder type as:1) Two pack amine cured epoxy. 2) Phenoxy. 3) Epoxy ester. 4) Urethane alkyd. 5) Polyester / polystyrene, 6) Chlorinated rubber.

If a project requires a multi-coat system, the use of Organic versus Inorganic Zinc becomes a matter of choice as over coating any zinc material converts it to a non-reactive barrier primer. My choice would be to recommend Organic zinc for its ease of application properties and chemical compatibility with a wide range intermediate and topcoats. However, there are exceptions, when exposure is harsh marine, under insulation, or beneath fire proofing compounds, IOZ is the choice, for the reason there are two principal mechanisms for protection, Galvanic and Barrier. The use of IOZ gives you both, if the barrier topcoats happen to fail the galvanic mechanism of the zinc take over.

After all, at this point it is the properties of the topcoats that matter with their ability to protect the underlying zinc primer. Topcoats needs to have good Water and UV resistance and be of high molecular weight materials, meaning they are tightly structured to stop or slow the ingress of water, oxygen, and soluble salts through to the substrate. A variety of topcoat materials have been used over the years to varying degrees, the best performers in decreasing performance going down the list of coatings has generally shown to be,1.) Polyurethane (Two pack), 2) Polysiloxane, 3) Amine epoxy, 4.) Vinyl, 5) Chlorinated rubber (unmodified), 6) Polyester, 7) Alkyd enamels, 8). Acrylic.

There are economic advantages with Organic Zinc when processing 100 kilos of raw material (exmple 90 kilos zinc plus other ingredients), Organic materials on average produce 22.78 litres compared to 16.45 for Inorganic (Ref table 1). The reason for the difference relates to Specific Gravity (SG) of the binders and additives used., Organic binders have a much lower SG than those used in Inorganic materials thereby providing larger volume of finished product.

Table 1

ZINC CONTENT COMPARISON ORGANIC VIS INORGANIC
Organic			Inorganic
	Weight ÷ Density = Volume		Weight ÷ Density = Volume
Binder (epoxy)	9 ÷ 1.1 =	8.18	(Silicate) 10 ÷ 2.6 = 3.85
Additives	1 ÷ 0.5 =	2.00
Zinc Particles	90 ÷ 7.14 =	12.60	90 ÷ 7.14 = 12.60
	100KL = 22.78 litres		100 KL = 16.45 litres
Zinc Content	90% wt. = 55% vol		90% wt. = 77% vol
Density used to convert to volume: Epoxy Binder 1.1, Additives 0.5, Inorganic Silicates Binder 2.6, Zinc Dust 7.14 (e.g., zinc 90KL ÷ 7.14 density = 12.60 volume)

Water based inorganic materials have the highest “volume loading” of metallic zinc in any of the zinc coatings.

Zinc Content: 1) 12.6 zinc volume as a % of 22.78litres = 55% / volume

2) 12.6 zinc volume as a % of 16.45/litres = 77% / volume

There is wide range of binders in use, whist some may have the same zinc” weight” loading, they can differ as much as 10-15% on a finished volume basis, due to the varying SG values within each binder type, zinc content can range between 45-80%, i.e., by weight not volume. Paint properties usually reflect the pigment volume concentration rather than the pigment weight loading. There is no test method nor volume requirements outlined in the appropriate standard related to the minimum quantity of metallic zinc, producers are free to formulate their own metallic zinc content.

To summarize Organic Zinc Rich Primers, their formulated characteristics favour application techniques, and speed rather than higher corrosion resistance. Main advantages 1) have a greater tolerance to poorer surface preparation, 2) easer to topcoat “no voids”, 3) good film flexibility, 4) some chemical resistance, 5) easy to repair or upgrade.

PRICING COMPARISON VIS COVERAGE

For some time, there appears to be a concerted effort by EU suppliers in promoting Organic Zinc Rich Coating. Specifying authorities and purchasers need to be aware that prices quoted by EU suppliers is per” kilo”, against local equivalents priced per litre. Products priced at $25/litre compared to $20/kilo may appear to be more expensive, however one needs to understand the difference between litres vis kilos.

The formula for the following comparison relates to specific gravity (SG) of the finished material i.e.: $20 /kilo multiplied by the nominated SG = litres.

For example, $20/kilo x SG (average 2.00) = $40.00/ litre. Compared to local $25.00/litre. Assuming the coverage rate is equal it becomes obvious that the litre price is the best option.

When evaluating any paint coating it is the cost related to coverage not the cost per litre that matters. Coverage will vary between proprietary brands and even within the same generic coating type. Most paint coatings contain some form of solid material and solvent. The solvent function is to keep the solid component in a liquid condition suitable for uniform application. After application the solvent evaporates leaving the solids on the surface. Putting cost aside it’s the Volume Solid component which ultimately determines the spreading rate at a given thickness which is then related back to cost. For example:

Product A at $35.00/litre may contain 65% volume solids.

Product B at $30.00/litre may only contain 50% volume solids.

Therefore, product A has 25% better coverage and is 15% cheaper.

Knowledge of volume solids is vital for estimating purposes it enables the purchaser to determine quantities necessary and more importantly it is the mechanism to control thickness and overall usage. The coating industry use the volume solid method for all paint materials with the exception of IOZ where the volatile/ void method is used.

GENERAL DISCUSSION & COMMENT.

Economics is the prime consideration in corrosion prevention . The most suitable coating system is the one which can be applied most economically, while protecting the steel structure for the expected service life. Evaluating authorities are faced with decisions between alternate materials and systems each having strengths, limitations, selective properties, and merits for specific applications. Selection should be based on established performance appropriate to each project rather than attempting to meet all requirements with one product type.

In some circumstances monetary constraints are implemented to minimize initial construction costs. When low-cost inferior systems are selected , the long-term implications relate to future maintenance costs, construction delays, possible revenue loss due to production stoppages, together with environmental considerations.

From a project perspective structural steel is always on the critical path of construction. One construction delay would render any initial prime cost inconsequential. Coating delays or premature failures become very costly to the facility owner. The eventual cost to the owner is made up of, Material, Labour, and Cost during service. IOZ has a minimal bearing on the” applied “cost. Labour costs for surface preparation and application is generally three times the cost of material.

There is a principle in law called “cojoined responsibility” which means tacit approval, a shared responsibility by all parties associated with a project if anything untoward occurs. It applies to all the aspects associated with the project., design, specifications, fabrication, protective coatings, site erection, inspection and all the other aspects associated with the project together with possible doubt, to conformance or perceived performance.

In areas of high labour cost the component of protective coating materials in the overall project is small .A 10% reduction in paint materials in most cases generally represents as little as 1-2% of the total project cost. it’s not worth considering, when quality materials are on offer or where the repetitive period of normal maintenance painting is considered. Early failure can cause increased maintenance costs and can be several times that of the original protection cost.

Finally, it is the maintenance free period that ensures a return on investment via increased productivity , it is more economical to provide long term protection in the first instance than to face future higher cost of maintenance, production delays, or early depreciation of plant and equipment. Good quality protection costs money, however endless maintenance costs more.

This article is only an overview analysis, for further in-depth or specific information it is recommended to consult recognized technical or published literature.

Nick. Karakasch

Total Corrosion Consultants.

Melbourne Australia.

About the author

Nick Karakasch is the retired principle of Total Corrosion Consultants Melbourne Australia, his experience spans over 55 years, specializing in services to the Protective Coating, Galvanizing, and Structural Fire Protection Industries. Nick spent over half of his working career with the Dimet Coating Company, the inventors of Inorganic Zinc Silicate Coatings by their founding director Victor Nightingall, Nick has also been the Executive Marketing Manager to the Galvanizers Association of Australia. Whilst living in South Africa in the mid-1970s he was employed as a Site Contracts Manager for R.J. Southey P/Ltd. Africa’s largest Corrosion Prevention Contractors.

Acknowledgement

This article was the result of industry experience and an association with the author over many years from numerous individuals during his and their employment at Dimet Coatings, together with the US Ameron Corporation. The author gratefully acknowledges the following individuals:

David Donald Chief Formulating/ Research Chemist (. Recipient Victor Nightingall Award 1998 Australian Corrosion Association)

Frank Worsnop General Manager Technical Services

Col O’Malley General Manager Marine. ( Member Institute Marine Engineers)

Barry Shepherd International Manager.

Graham Robilliard Manager Cathodic Protection.( Honorary Member Australian Corrosion Association)

Geoff White QLD State Manager, Technical Marketing Manager New. Zealand,

Australian Business Development Manager, Middle East Technical Consultant.

Ted Riding Technical Manager, Dimet / Jotun Coatings.

(Recipient of Victor Nightingall Award 2019 Australian Corrosion Association ).

Alex Szokolik IMP/ Dimet Coatings, A&A Szokolic Consultants,

(Recipient of Victor Nightingall Award 2005 Australian Corrosion Association )

Ameron Corporation USA, Chuck Munger (President), and Jay Herpel (International manager) now part PPG Coatings USA.

References

Sorel French patent (10 May/June1837) Hot Dip Galvanizing.
Australian patent 104231(improvements to Silicate Compositions 1939.
Dimet USA patent (1949) 2462763 Protectively Coated Ferrous Metal Surfaces & Methods of Producing Same.
USA patent 1959) Solvent Borne Inorganic Zinc Silicate Coating Carboline Corporation.
NASA patent 1980) High Ratio Inorganic Zinc Silicate.
USA patent 108408 Abrasive Blasting 1869 B.C. Tilghman.
Australian AS/NZS standards:( AS1627.4-2005), (AS/NZS 3750.15), (AS/NZS 2313.1.2014). (AS/NZS 3750.9.2009), (Taber Abrasion Federal Test Method No141-6192
Volta. Italy (Electric battery Invention 1800 with the use of two dissimilar metals).
Inorganic Zinc Silicate Coatings 70 years on N Karakasch.
Zinc Coating Review.( Galvanizing & Zinc based Paints). N Karakasch.
Painting Galvanized Structures. N Karakasch.
Corrosion Protection & The Environment. N Karakasch.
Corrosion Protection with Cash Flow Predictability N. Karakasch
A Szokolik 1992) Evaluation Inorganic Zinc Silicate for Oil/Gas Production/Marine Environments.
Szokolik 1995) Field Testing Inorganic Zinc Silicates.( Journal Protective Coatings & Linings).
Nightingall ‘Zincaron’ patents, P113891,P113946, P1666224(1941), P115134(1942)
Evans & Mayne Researchers Cambridge University (1932-43).
You Tube : Dimetcote: The Story of Inorganic Zinc Coatings.

Postscript:

Dimet Coating Company no longer exists, purchased by Private Equity Investors, during the mid to late1980s then finally after three years going into receivership. The demise of a highly profitable company,( 30% net return to sales) that had a world reputation, by inexperienced administration is something to be noted. The lesson here is “Simple solutions only exist in the minds of Cowboys, Fools, and Private Equity Investors”. The remnants of the organization were acquired by the Norwegian Paint Company Jotun in 1990 .Had Dimet been acquired in the first instance by a likewise coating company the outcome may have been somewhat different.

It is hard to understand why the name “Dimetcote ” was changed to ”Resist“ considering the Dimet name

is well known worldwide throughout the Corrosion Industry and beyond for the technological principles created in 1937 now used in over 70 countries. ”Some see an opportunity to be grasped, others see something distinctly to pass them by.”

Manufacturing in Australia has now ceased products are imported having to compete against competitive locally produced materials. Overall, there were 15 patents related to IOZ attributed to Dimet in various nations throughout the world between 1939 and 1956, and four Organic Zinc Rich Coatings in Australia between 1941 and 1942. The last patent in Australia 1956, was for the first non-heat cured IOZ material “ Dimetcote 3” developed in conjunction with the US Ameron Corporation. This was the coating that cemented Dimetcote’s reputation throughout the world being used successfully on its creation in the Shipping Industry and on Offshore Oil /Gas platforms.

Other unrelated patents by Nightingall ,

1) First Australian patent for Electric stove,

2) Wireless set communication ship to ship,

3) Electromagnetic removal steel from eyes.

The first large scale project using inorganic zinc silicate in the world was in South Australia – Morgan to Whyalla, pipeline in 1940. The 359km x 750mm diameter pipeline was built to supply water to the growing industrial centres of Port Pirie, Port Augusta, and Whyalla. A second pipeline was constructed in 1962, with a capacity of 2.5 times larger than the original. The original coating was applied by brush in situ whereas the duplicate line was spray applied in the workshop and transported to site. It is significant to note that both lines are still in outstanding condition.

The history of the project is fascinating; Nightingall had to plead his case for the use of Galvanite (Dimetcote 2), in front of a parliamentary select committee. His claims on performance were not entirely accepted; however, the decision was made to use Galvanite with the proviso that warranty money would be withheld as a form of guarantee on his claims. The government paid all withheld money, well in advance of the twenty-year expiry period when it became obvious that the coating survived many grass and bushfires that would have previously destroyed conventional organic coatings as originally proposed.

In memory Victor Nightingall (1881 – 1947) of Heidelberg, who in 1937 patented a unique protective coating now known as inorganic zinc silicate.

This coating protects millions of square metres of steelwork around the world from the ravages of corrosion.

Erected by members of the Australasian Corrosion Association November 1997 Warringal Cemetery, Heidelberg, Melbourne

Inventor of Inorganic Zinc Silicate Coatings, Melbourne Australia. Patent No 104231 – 20^th March 1939

Victor Nightingall invented the ‘Holy Grail’ of Protective Coatings, a material that can protect steel against the ravages of corrosion in sea water. It was a defining moment in the Protective Coating industry, which revolutionised the conception of corrosion protection throughout the world.

Formulation Classification of IZS.

INORGANIC ZINC COATINGS

1939

ACID HYDROLYSED

ALKALI

HYDROLYSED

1965

ALKALI SILICATES

WATER BASED

SELF CURED

1965

CHEMICAL CURED

1954

HEAT CURED

1939

WATER BASED

HIGH RATIO

1980

ETHYL SILICATES

SOLVENT BASED

COLLOIDAL

SILICA

1960’s

formulation sequence and USE DATES

	Product	Cure Type	Origin	Date
1	Galvanite (re-named Dimetcote 2)	Heat Cure	AUS	1939-1970’s
2	zincilate 100	heat cure	aus	1945-1966
3	dimetcote 3	chem / cure	usa/aus	1954-1965’s
4	carbozinc ii	ethyl silicate (acid hydrolysed)	usa	1959 ongoing
5	dimetcote 4	Colloidal Silica	usa	in use since early 1960’s
6	rustban 191	lithium silicate	usa	in use since early 1970’s
7	dimetcote 6	ethyl silicate (alkali hydrolysed)	usa	1965-1975’s
8	Dimetcote 5 (renamed resist 5)	water based	aus	1965 ongoing
9	zincalate 120 (renamed resist 86)	ethyl silicate (acid hydrolysed)	aus	1966 ongoing
10	ic531	water based (high ratio)	usa	1980 ongoing