Last Updated 1 month ago by Kenya Engineer
This article examines the relationship between difficulties and failures associated with Duplex Painting Hot Dip Galvanizing (HDG), it discusses a range of issues, some of which are not well understood or widely known and the effects of not using appropriate procedures for either HDG or the Painting Process.
The purpose is to provide coating specifiers, steel fabricators, galvanizers and the painting industry with an understanding to some of the problems that adversely affect painted HDG treated structures.
ABRASIVE BLASTING AND THE “KIRKENDAL EFFECT”
Experience over many years has highlighted a lack of understanding by the participating industries of the failures and damage that occurs to HDG by inappropriate sweep abrasive blasting necessary as surface preparation for painting. The debate over responsibility has largely centred on damage caused by the blasting activity necessary to provide a mechanical key for adhesion. However, when the recommended practices are followed, damage can be sustained to what appears to be sound HDG for reasons not commonly known or understood. Damage appears in the form of either peeling, pin holes, or the creation of voids. Steel Fabricators are the ones faced with this predicament as surface treatment is generally included in their work scope. As a result, the reputation of the painting contractor and all others in the chain of events comes into question, where the facility owner may endure costly project delays and future consequences if premature coating failure occurs.
When abrasive sweep blasting causes removal or damage, the specific cause/s must be identified before any responsibility is attributed to ensure similar situations are not repeated in the future.
THE GALVANIZING (HDG ) PROCESS
It is common industry practice to describe HDG as a “ 98% Zinc Coating “ this description when used is ambiguous and not entirely correct, the reference of 98% only relates to the purity of the zinc used in the production process. The galvanizing process creates an “ Metal Alloy “ consisting of four distinct layers (Gamma, Delta, Zeta, Eta) resulting in a total metal weight mass of approximately 58.5%Zinc, 40% Iron, 1.5%lead (Fig 1) at a given thickness for example, 85-micron thickness equals 600 g/m2, comprising of ( 351grams Zinc / 240 grams Iron/ / 9 grams Lead.)
The Lead component in the process is for two reasons, HDG kettles have a layer of Lead beneath the molten zinc for operational reasons, 1) To protect, insulate and to aid in the removal of Dross ( Ion, ash, flux skimmings ) from the bottom , 2) Prime Weston Zinc ZN5 ( 98% ) contains 1.4% Lead is the feed stock used by the industry. The chemical composition for ZN5 can be found in a separate standard for Zinc Ingots such as ISO752 which clearly outlines the Lead component.
The top Eta layer is a zinc/lead alloy, relatively soft compared to carbon steel. Hardness is measured by the Diamond Pyramid Number (DPN). Zinc is 70DPN compared to steel at 159DPN. Air pressure for sweep blasting HDG in preparation for painting needs to be lowered to 40PSI maximum to reflect the lower malleable hardness of metallic zinc.
When damage to HDG and paint failures occur, the causes are often not clearly identified and, in many cases, unfairly attributed to either the abrasive blasting process, paint application practices or defective paint materials. What is not commonly understood damage and duplex paint failures in the majority of cases are largely associated with the galvanizing process and then, exacerbated by abrasive sweep blast cleaning. The galvanizing process creates what is known as “The “Kirkendall Effect”, (Ref 6).
In 1947, Dr. Ernest O. Kirkendall, of Wayne State University, USA first described the effect that now bears his name. Kirkendall’s experiments showed that at high temperatures inter-diffusion of zinc occurs with other metals at an atomic level and if not controlled, or suppressed, cause voids to form in HDG at the interface between the Eta and Zeta layers .
This effect is caused by a metallurgical reaction between zinc and steel that continues below the melting temperature of zinc (420°C) creating voids (fig1). These voids adversely affect the bonding and adhesion between the top two HDG layers and if disturbed or exposed by abrasive blasting results in exposing pin holes, peeling, and detachment.
The occurs when HDG is slow air cooled or not adequately quenched immediately after the galvanizing treatment, in scientific terms it is referred to as “Atomic Diffusion” between solid metals where Atoms rise or migrate upward under high temperature if cooling is slow, they drop back leaving a void. If no peeling occurs holes/ voids go unnoticed and difficult to detect with the naked eye.
Paint application further compound the problem with the formation of blisters and adhesion failure, paint applied to voids results in solvent entrapment during the curing process.
Localised temperature changes by entrapment and evaporation, form blisters in the still liquid coating. This phenomenon particularly affects two pack materials, such as epoxy, polysiloxane and polyurethane, where the retained solvent acts like a plasticiser on the liquid coating during the curing process developing blisters causing detachment of the coating from the HDG surface (Fig 4). These coatings have high cohesive film strength meaning the exterior surface remains intact, however maximum adhesion is compromised leaving voids and a gap at the interface for the electrolyte ( water, oxygen & soluble salts) to accumulate thereby activating the zinc protective properties.
The HDG industry worldwide have not acknowledge the Kirkendall Effect in any promotional or technical literature to the potential problems associated with this effect, this raises the question why has this not been disclosed ?. When it occurs experience has shown, it is the poor old painting contractor who generally suffers the consequences.
KIRKENDALL EFFECT ON CORROSION OF HDG UNDERPAINT CONDITIONS
On completion of the galvanizing process a protective film (zinc carbonates) forms on the surface as it reacts with oxygen and carbon dioxide which inhibits initial corrosion. In circumstances where this is removed, the corrosion rate of zinc increases substantially, particularly in marine, chemical, damp or humid conditions resulting in zinc corrosion material referred to as “efflorescence bleeding”(Fig 5A) to accumulate at the HDG/Paint interface. This causes adhesion failure between HDG and the paint system. Efflorescence bleeding is the term used to describe a change of a solid zinc metal surface to a powdery substance which in this instance is zinc corrosion material in powder form.
The question therefore is what ‘are the influencing factors for change?’
Whilst abrasive blasting provides the surface profile necessary for quality mechanical adhesion, it removes the protective zinc carbonate film to expose pure reactive zinc of the outer HDG layer. When exposed to an electrolyte (water, oxygen and soluble salts) the galvanic protection property of the underlying zinc is activated. The electrolyte penetrates the paint either as vapour, or in liquid form through pinholes. The result is zinc corrosion material approximately 100 times greater in space volume (Ref 5) than the original zinc surface which in turn, becomes hydroscopic drawing more of the electrolyte through, thereby increasing the corrosion cycle to cause larger volumes of zinc corrosion material to accumulate at the interface between HDG and the paint system (Fig 5).
The rate of corrosion is the time it takes the electrolyte to penetrate a coating film, this is dependent on paint thickness and its moisture vapour transition rate. The principle of vapour transition works equally in both directions. Anti corrosive paint materials are primarily designed to keep the environment (electrolyte) out. adversely it can also keep it in. When vapour or water has penetrated the coating film to the underside, it no longer has access to ‘air circulation’ and remains at the interface as water, oxygen, and soluble salts, making up all the ingredients necessary for zinc corrosion to occur. Once corrosion has started it cannot be stopped and, due to the hydroscopic nature of the resultant corrosion material, it continues unless one of the elements of water, oxygen, or soluble salts is removed which is highly improbable under these circumstances.
It’s worth mentioning the why Inorganic Zinc paints do not suffer from the same dilemma in similar circumstances, Metallic Zinc is in Particle Form spread throughout the film surrounded by an Inorganic Binder. Corrosion product produced accumulates within the paint film and not at the interface, whilst a small quantity may gather at the interface, this is not enough to cause adhesion failure of the topcoat system.
The presence of “Kirkendall Voids”, combined with the removal of the protective zinc carbonate film exposes pure reactive zinc, which contributes to premature paint failure. In view of this explanation, it is not entirely correct to state that “paint failures are usually due to deficient paint application procedures or incorrect choice of paint systems”, (Ref 4), and or inferior faulty paint materials, particularly as there is no mention of the Kirkendall Effect and its possible consequences.
Paint manufacturers are reluctant to recommend and generally will not warrant their products when applied to HDG treated steel because of the increasing number of failures. The situation now no one involved in the chain of events, be it galvanizer, paint manufacturer, or the paint applicator are prepared to provide any form of warranty on the basis that no one organisation has complete control of the overall process and materials used.
HIGHER INCIDENCE OF SURFACE IMPERFECTIONS WITH PAINTED HDG SURFACES
Relevant world HDG Standards only address a product description and a process for Corrosion Protection, they do not cover the requirement for aesthetic appearance. Designers are more often calling for an architectural finish on HDG components, creating in effect a new product which requires specific consideration. Information must be presented for steel procurement so that the additional detailing required for painting is considered when tendering a project and observed on dispatch for painting.
Painting will always highlight imperfections (Fig5 ), many disputes arise because this is not commonly recognised by those people and organizations involved in the chain of events i.e. fabrication contractors, builders, architects and clients. The galvaniser should be made aware at the tendering stage that there is a painting and architectural requirement and that all imperfections such as lumps, bumps, spikes welding slag etc should be removed prior to dispatching items for paint coating application.
Preparing HDG to a standard suitable for an architectural finish comes at an additional cost over and above standard industry charges, HDG is less uniform due to the inherent production process and will always highlight more surface imperfections than a protective paint system applied to steel by conventional spray-painting methods.
STEEL CHEMISTRY, THE EFFECT OF SILICON AND PHOSPHOROUS
It has long been established that steels with high Silicone or Phosphorous content increase the rate of the galvanizing reaction and consequently accelerate the grow of the iron zinc layers. These steels are known as “reactive steels” and are often characterised by an alloy structure consisting entirely of iron- zinc and will be generally thicker than normal and visually will appear either smooth, dull grey or have a rough, sandpaper finish. Where this occurs is not uncommon for HDG to completely detaches or flakes from the steel substrate, ,in extreme cases can result in thickness up to seven times the normal requirements.
Reactivity of a steel can be determined readily by calculation of what is known as the Silicon Equivalent which considers the combined effect of silicon and phosphorus.
The formular is Silicon Equivalent = %Si + 2.5 x % P, a large proportion of steel is produced by electric arc furnace method where there exists a propensity for high concentrations of these elements than steel made by the continuous casting process.
Welding rods with high levels of these ingredients also contribute to excess zinc thickness, If the use of materials high in silicon/phosphorous is unavoidable, the galvanizer should be notified in advance. Whilst the steel composition is beyond their control, there are precautions that can be taken to minimize, or in some cases even eliminate the risk of excess zinc deposition. These precautions if taken can lower the cost of HDG not only to the fabricator but ultimately to the project principal.
Steel fabricators do not always know that some steel or welding rods are at greater risk of having a high silicon or phosphorous content. This should be known before purchase, if not, should be tested for conformance with the relevant specified Steel/Welding Standard prior to galvanizing. Alternatively, the specifier can specify the required steel chemistry for the project and set the maximum limits for silicon and phosphorous. Specific testing for these elements is not common, which is unfortunate given that the cost is relatively minor when compared to, increased cost of HDG, re-work or dispute resolution. Accordingly, it is recommended that the silicon and phosphorous content is known before the galvanizing contract is put out for tender, particularly where a high aesthetic value is required. If not, the steel fabricator and ultimately the project owner will simply pay more to cover the extra zinc consumed, and or any possible repair costs. The costing practice by the galvanizing industry is to charge by weight on completion, regardless of the silicon phosphorous content. Unless the galvanizer knows the steel chemistry in advance, the problem of over thickness only becomes evident after the galvanizing process is completed.
OVER THICKNESS ON HDG COATING
There are precautions the galvanizing contractor can take to avoid or minimize the effects of over thickness if steel is high in silicon and phosphorous for example,
- Operate at the lowest possible galvanizing temperature 435°C
- Pre-heat steel prior to galvanizing
- Fast entry and exit from the bath
- The addition of alloy materials such as Nickel to the bath
Items 1, 2 and 3 are relatively easy and inexpensive whereas the use of nickel is very costly; unfortunately, not all galvanizers use alloying metals such as nickel or employ the other measures listed, whilst all precautions come at a cost it would nevertheless still reduce the charges for HDG to the project.
The use of nickel and the other measures to mitigate over thickness can be justified, given that thicker HDG is highly likely. It is in the interest of the specifying authority and fabricators to determine and document the silicon and phosphorous content so that measures against thicker and defective HDG can be taken prior to abrasive blasting and coating application.
Specifying steel and welding rods with the proper chemistry goes a long way to alleviating the problem, however it must be emphasised that composition certification sampling reflects the content at a specific location and statistically implies the makeup of the entire item or batch. Nevertheless, there can be isolated areas of silicon and phosphorous due to the non-homogeneous nature of steel composition. Increased thickness of HDG is not all doom and gloom, providing there is no detachment the greater thickness is a positive advantage as it extends the service life of the product.
RESPONSIBILITY FOR DEFECTS IN HDG AND COATING PRACTICES
Contractually, unless all known measures are taken by the contracting parties to mitigate the potential problems it becomes difficult to justify any charge or claim which may be put forward during any given project. There is a relevant principal called “Tacit Approval”. This applies to contract specifications regarding possible doubt, total conformance or perceived performance, implied but not spoken and therefore, becomes a shared responsibility by all involved if anything untoward occurs. Structural steel is always on the critical path for construction, disruptions not only cause loss to the contracting parties but also may contribute to possible protracted litigation, construction delays to both the project and client and in some cases serious financial loss due to possible premature failure.
The abrasive blast cleaning and coating industry need to recognize that accepting HDG for painting that has not been subjected to the appropriate Quality Assurance means any defects in whatever form will become their responsibility. Equally they have a clear responsibility for their own QA activities and should ensure that the recommended procedures for preparation and painting are strictly adhered to. Other points of responsibility for damage are transportation, unloading, and final steel erection.
Fabrication design is all important, for example, the use of boxed or rolled hollow sections is where considerable savings for treatment can be made. For safety reasons these sections need to be vented or open ended, as a result internal and external surfaces are treated with price based on weight. Alternatively, if the same sections were sealed the cost saving in favour of zinc painting the exterior surface only can be 20 – 25% lower, however this comparison is only worth considering if the volume of boxed sections requitements are considerable. Other comparisons would not show the same difference and would fluctuate between systems, in favour of one or the other.
SUMMARY
Damage of HDG and Paint coating failures will occur when the following conditions arise:
- Incorrect blasting procedures, excessive pressures above 40Psi can remove HDG to expose bare steel. Removal is through abrasion, and not peeling between the layers as with the Kirkendall Effect.
- Removing the protective zinc carbonate film by abrasive blasting to exposes fresh ‘pure reactive zinc’, thereby accelerating the corrosion cycle and increasing zinc corrosion material at the interface, particularly when exposed to chemical, marine, damp, and humid conditions, causes premature adhesion failure between the two materials.
- The presence of “Kirkendall” voids between the Eta and Zeta HDG layers compromises the metallurgical bond, enhances the corrosion cycle, and adds to the formation of paint blisters and detachment.
- Excess HDG thickness due to steel and welds with high levels of silicon or phosphorous can become brittle and detach through normal handling, erection and or abrasive blasting.
- Surface imperfections in HDG finish, if not removed will be highlighted after duplex painting, welding slag needs to be removed prior to articles being delivered for galvanizing.
RECOMMENDATIONS
- The galvanizing industry need to advise the painting industry of the possible effects of Kirkendall Voids.
- The galvanizer should be made aware at tender stage if HDG is to be painted
- All coatings applied to HDG should be defect tested for adhesion, pin holes and holidays on completion.
- Specifiers should be made aware of and specify the minimum levels of silicon and phosphorous in steel and welding rods, together with the removal of imperfections for architectural requirements.
- Steel and welding rod suppliers should be required to supply chemical analysis certification for their materials to ensure conformity with the specification.
- Coatings over HDG should be restricted to those with the highest resistance to oxygen and water vapour, particularly at the zinc interface.
HDG has an outstanding performance record, this is largely due initially to the protective zinc carbonate layer which slows the corrosion rate. That is not to suggest that the volume of metallic zinc and exposure conditions are not influencing factors. Nevertheless, the removal of the protective layer by abrasive blasting exposing pure reactive zinc to under paint coatings where there is no obvious air circulation needs to be seriously questioned as failures between HDG and paint systems are all too frequent, and consequently have developed a poor reputation. Experience has shown HDG is better left uncoated.
If painting is to continue, it is strongly recommended that inspection procedures include those mentioned above, the cost is minor considering the consequences should a project experience disruption or premature failure occur.
A measure of caution needs to be exercised when considering HDG that requires painting to increase anti corrosive performance or for architectural purposes. It is worth considering a proven alternate zinc paint system, where pure solid zinc is not exposed at the interface, where inter-coat adhesion is more reliable and permanent, where degreasing of zinc primer is not required, where abrasive blasting of the anti-corrosive primer is unnecessary, where silicon or phosphorous has no impact, where the aesthetic appearance is better, where the performance to first maintenance is equal, where the cost of protection favours the zinc paint system, and finally where the client can get a warranty.
References:
- AS/NZS4680/ISO1461/ASTMA123M after fabrication hot dip galvanizing
- AS1627.4 Abrasive Blast Cleaning
- Professor Hideo Nakajima, Osaka University – “Kirkendall Voids”
- Galvanizing Association of Australia Handbook
Introduction to AS/NZS4680
- Galvanizers Association of Australia Technical data sheet Gen/5/1
- TPC9 USA Publication users guide for HDG
- E.O Kirkendall: Zinc Diffusion in Alpha Brass – TRANS AIME (1947) pp 130-142
- Corrosion Management, Vol 9 / No 1- July 2000
- Metallic Zinc Based Coating Systems, CSIRO Research Services, PPG Protective and Marine Coatings, A & A Szokolik Consultants Pty Ltd
- Zinc Coating Review 2023/2024 N. Karakasch
- Galvanizing & Zinc Based Paints N Karakasch
- Architectural Galvanizing 2024 N Karakasch
- Corrosion Protection & the Environment N Karakasch
- https://1drv.ms/u/s!Aq79HUD7uYHAgYEwP6vePDgzxY3zfg (photos and video re Kirkendall, and Silicon and Phosphorous effects) – open hyperlink.