CORROSION FAILURES ASSOCIATED WITH PINHOLES AND HOLIDAYS

Last Updated 2 weeks ago by Kenya Engineer

In the exciting world of atmospheric corrosion, I might start  for those readers not familiar with corrosion principles related to carbon steel. Steel corrosion is a complex electro- chemical reaction, this in simple terms occurs when bare steel is in the presents of an Electrolyte (Water, Oxygen, and Chlorides containing airborne pollutants).The accepted theory of corrosion involves the transportation of the electrolyte through a protective coating film to the steel substrate. Coatings are the principal materials used for steel exposed to atmospheric  conditions. This is almost the case, whether the structure is in a refinery, chemical plant,shipping,offshore platforms, and bridges to name few.

Coating “failure” is best described as the failure of the coating’s ability to withstand the exposure conditions and continue to effectively protect the substrate. The team “failure “also covers the situation when a coating has reached the end of its service life. It is important to differentiate between coating “ failure “ and coating “breakdown”. Identification of whether one is dealing with failure or breakdown is an important factor, so that appropriate remedial measures can be taken.

Protective coatings are complex materials made up of many interacting ingredients, all have a finite service life, in other words ,they gradually degrade on atmospheric exposure largely due to chalking and lose the ability to protect the steel. Chalking can best be described as the formation of a relatively loose organic powder on the surface of a paint coating after exposure to the weather and in particular to the ultraviolet rays of the sun. Modern day coatings are predominately organic materials, as more of the coating is exposed the quicker will be the tenancy to chalk, which is a continuing process as atmospheric conditions remove the powdery substance exposing fresh material for the process to continue over time. Chalking has been known to range from a few microns to 20 microns per annum.

A coating can only provide protect if it forms a continuous film, free of defects, physical stresses caused by handling, transportation, and erection.  Two major contributors to the corrosion cycle and coating failure are pinholes and holidays.

These aspect are largely dictated by coating type, film thickness, water vapour transition rates, and UV resistance. The principal reason for coating failures depends on proper application techniques to eliminate pinholes and holidays. Pinholes can be defined as the formation of minute or micro holes in a film that occur during application or curing, largely due to trapped air or solvent gas displacement  that try to burst through a partially cured film forming small craters and holes when trying to exit the film. The film at this stage has started to plaster size which impedes solvent release resulting in pinholes that fail to flow out before the film has set. Pinholes can range up to a few millimetres in diameter, quite often modern coating materials contain high volatile solvents which accelerate this process as the solvent endeavours to exit the film  through a semi-cured coating which is no longer totally fluid leaving a pinhole in its wake.

Holidays can be described as any discontinuity or uncoated area where pinhole pass through the coating material resulting in contained air bubbles, in extreme circumstances air pockets can represent a substantial portion of a total coating thickness even if the coating appears to be continuous. Since the accepted theory of corrosion relates to the electrolyte it is vital that the correct materials and application procedures are employed. The quality of application and environmental conditions  need to be recognized and controlled to minimize not only pinholes but also, rough dry spray, or over spray all of which contribute to the pinhole effect.

The role of water, with the rare exception is always involved in the corrosion process and is known in the industry as the “universal solvent “ overtime it is capable of migrating into and through most organic coatings carrying oxygen, and soluble chlorides. This depends on film thickness, and coating formulation. The majority of protective coatings are formulated in what’s known in the paint industry as high molecular dense materials, meaning they are tightly structured to resist the vapour transition. The best performers overall are the two pack catalysed materials such as polyurethane and epoxy, polyurethanes in particular also have outstanding UV resistance which reduces chalking the formation of loose powder on the paint surface after exposure to weather.

When these defects occur, for the most part can be resolved by dealing with these applications openly so that the issue of “what the customer expects versus what the customer received can be delt with before :1) The products are specified, 2) The products are sold, 3) The products are applied. The acceptability does vary with the client ( or individual inspector ) and also the structure being coated. The vast majority of complaints are on large steel sections, ship hulls, and tank areas etc, appearance on smaller steel sections is less noticeable. In some cases, pinholes are barely visible to the untrained eye, but highly visible through 8x magnification, larger pinholes can be cause for questioning or rejection and will require correction because appearance is less than expected.

There is no perfect coating system which will satisfy the demands of every environment or set of application conditions . Usually, a system approach of a properly selected primer ,intermediate, and topcoats provide the best answer to the requirements for a specific coating problem. As a wide variety of paint materials are used, it is inevitable solvent and air bubbles will be present to some degree in all systems. In extreme circumstances, air pockets (holidays or small missed areas) can represent a substantial portion of the total film thickness and as such is a weak point and if not patched become focal points for corrosion.

Technological changes have resulted in the development of many coatings of a specialized nature which has enhanced corrosion resistance properties .These include improved resistance to UV, chemical and marine attack,heat,water penetration, abrasion, and gloss retention. They were largely developed as a complete system based on primer, intermediate coat, and finally topcoats. There is no guarantee that a coating system can be considered to be totally pinhole or holiday free, what contributes to this problem is ,coating type,thickness,application method, and the extent of any contaminates that may be left on the surface after surface preparation. Nor will they totally insulate the steel surface from the environment (electrolyte) all coating materials “ over time “are vulnerable to some degree of oxygen and vapour transmission.

Whilst the creation of pine holes and holidays is not all strait forward, in simple terms can for the most part be attributed to shrinkage of the coating as the solvent permeates out of the film, particularly under hot air conditions were contaminates or trapped air/solvent is present, which prevents the paint wetting out the surface.

ZINC COATINGS

Metallic zinc coatings especially inorganic zinc silicate materials are an area that requires special attention, the occurrence for pinholes to occur is highly predicable and common, in most cases top coating is not generally recommended unless applied over an intermediate tie coat. The potential for pin holing or blistering is dependant on both the specific metallic zinc coating and the topcoat system. Pinholes where metallic zinc materials are used as primers are normally not detrimental to coating system performance except in those environments where rapid attack of the zinc can occur, such as acid or alkaline in environments below a pH of 5.5 or above a pH of 11.

Inorganic zinc coatings, by nature, contain varying degrees of porosity in the dry film. This decreases on weathering, as zinc corrosion products if any fill the porosity. The porosity of these materials does have a significant influence on both the selection and application of topcoats. The application quality must be controlled to minimize topcoat pin holing, rough dry spray or over spray all of which will increase topcoat pin holing; however these can be minimized in a number of ways, the most accepted method is the mist coat-full application technique as this can be used effectively over recently applied material to prevent craters, intact blisters, and pinholes. When pinholes are present, the finish coat can reflect the same condition and, in most cases, can be corrected by applying an additional spray pass during finish coat application.

The owner/specifier/fabricator/applicator/ and inspector if not familiar with top coating inorganic zinc should be aware that :1) some pinholes may occur, 2) Application and repair techniques can assist in eliminating pinholes, 3) these items must be considered when estimating material and labour requirements. Organic zinc coatings don’t suffer anywhere near the same problems as Inorganic materials, they are not porous as they incorporate a variety of organic binders such as epoxy, or polyurethane.

HOT DIP GALVANIZING

The other serious area of venerability involves Hot Dip Galvanizing where it requires duplex topcoats, for architectural or extra corrosion resistance. There is no doubt  the performance  of duplex materials has developed a poor reputation largely as those involved in the chain of events have often failed to recognise the corrosion mechanisms associated with duplex coating. Pin holing and the permeability of the electrolyte has a marked impact on overall performance .

To gain mechanical adhesion ( surface profile) requires abrasive sweep blasting. When an item leaves the galvanizing kettle there is a reaction with oxygen which produces a clear durable carbonate film on the surface which is then removed by the sweep blasting process exposing pure solid metallic zinc. When diffusion of the electrolyte occurs, it activates the protective properties of the zinc. The result is the accumulation of zinc corrosion products( zinc oxide, hydroxide, and carbonate) trapped beneath the duplex material at the interface with nowhere to go expect upwards.

These corrosion products are concentrated solutions and are hydroscope meaning, the law of physics dictates you cannot have a concentrated solution on one side of a “ permeable membrane “ therefore more of the electrolyte is absorbed weakening the coating integrity to equalize the solution on both sides. The result is the zinc protective properties continue unabated, increasing the volume of corrosion products which ultimately leads to adhesion failure of the duplex coating system. Whist there’s no corrosion in the early stages, rapid failure can occur due to accelerated corrosion of the at the zinc/paint interface. However, the failure of the duplex topcoats becomes extremely unsightly and very costly to rectify with no guarantee it would not happen again.

A major concern that adds to pin holing and holidays is the effect of surface contamination, paint coatings require good wetting properties so that the molecules can flow freely to achieve chemical adhesion. Chemical affinity occurs when surface atoms swap or share electrons to form a bond, needless to say contact needs to be intermate (at the atomic level). Contamination interferes with sharing electrons, and therefore adhesion is compromised, the major source of contamination comes from the galvanizing process, in the form of chromate deposits.

The last procedure in the galvanizing process is quenching in a solution of sodium dichromate which is necessary to avoid early flash rusting that leaves a chromate deposit on the surface. From a duplex coating perspective must be totally removed prior to abrasive whip blasting which is necessary for mechanical adhesion. Chromate solutions are also water soluble and hydroscopic which is another factor that contributes to the increase of the electrolyte uptake.

Note quenching should be avoided if items are to be duplex coated this should be reflected in the tender/specification documents and reinforced at the preconstruction meeting. In my experience degreasing has not been a common practice largely to the general perception and misunderstanding  that structural members received from the  galvanizer  has no surface contaminants. Painting contactors need to be informed that degreasing is a necessary requirement.

Inorganic zinc coatings consist of fine particles of metallic held in an inorganic matrix, they don’t  suffer the galvanizing problem, degreasing and abrasive whip blasting for adhesion purposes is not necessary, being somewhat porous  zinc corrosion products are absorbed into the film; hence the reason top coating is always more reliable. In my experience it is rare that topcoat materials delaminate, failure is generally associated with applications well over recommended thickness requirements or break down caused by atmospheric exposure, mainly due to chalking.

PERFORMANCE

From a performance perspective over coating any zinc-based material  provides an important feature, a Synergistic Effect meaning working together or a combined action to increase performance. Therefore, its vital to anticipate any possible defects that may occur in advance prior to selection and application, to ensure extra performance as expected is not compromised.

In many cases, most pinholes do not extend completely through the coating since very little or no holidays are indicated when a 100–250-micron dry film containing visible pinholes is tested with a holiday pinhole detector. Although they are generally considered aesthetically not acceptable, they are repaired as part of the normal coating application process. Like inorganic zinc if owner/specifier/fabricator/applicator and inspector are not familiar with top coating galvanized surfaces they need to be made aware that :1) some pinholes are likely to occur, 2) Application and repair techniques can assist in eliminating pinholes and 3) These items must also be considered when estimating material and labour requirements.

INSPECTION

Inspection and knowledge are the key to minimize the possibility of premature failure and ensuring the coatings are applied to the extent and quality implicit in the contracting/client’s specification. Holiday /pinhole detection equipment is able to locate these even on sharp edger’s where a coating is most likely to  be deficient. It is important to understand that a coating can only provide corrosion protection to the underlying steel substrate when it is free of any defects that allow any corrosive agents such as an electrolyte to contact the steel substrate. When the integrity of the coating film is damaged or compromised, the rate of any resultant corrosion is a direct consequence of the corrosive potential for any given service environment. From a pinhole perspective there is no absolute threshold at which a coating can be considered as pinhole free.

Testing for these defects is necessary where coating integrity is paramount, reason being application is so operator dependant therefore a number of inspection measurements taken over a specific area rather than isolated determination is usually more accurate. Electrical testing is essential to ensure a coating is free from these defects, particularly required where Inservice inspection is difficult for example such as immersed conditions for tankage, and pipeline coatings etc where defects of this nature cannot be tolerated.

Quality control throughout the whole process from design to final inspection is essential to ensure the coating film has been applied with film continuity particularly in conditions where the environment is considered server, otherwise this will lead to premature failure, resulting in higher  maintenance costs and the possibility of revenue loss of production. The extent of pin holing to a large degree can be divided into broad categories of  Low build coatings and High build materials. 1) Low build materials range in thickness of 50-100 microns and for the most part tend to be for architectural purposes, in the majority of cases are not pinhole free.

High build systems range from 100 to in some cases 400 microns plus depending on environmental conditions of exposure. Thickness above 300 tend to be high enough to inhibit inital diffusion. Paint systems will not completely insulate the steel substrate from the electrolyte as there is no guarantee that pinholes will not occur in areas of low film thickness. Defects of this nature can be attributed to shrinkage of the coating as solvent permeates out of the film, air temperature, and surface contaminates if any preventing the coating wetting out the sureface. In moderate or benign conditions these defects are of little consequence, however in marine, industrial, immersed, or high humidity conditions they become a point of vulnerability.

For film thicknesses above 250 microns the likely hood of these defects is greatly diminished and will not prevail with all paint systems, but generally only in the following circumstances: 1) where the paint system has a relatively high permeability to the electrolyte ( typically materials below 100 microns).Including low film and high permeability paints such as exterior water-based latex martials or enamel paints where diffusion is high. These generic types should not be recommended in corrosive or high humidity environments.

CONCLUSION

Finally, it is important reminder a coating system can only provide protection to underlying steel substrate when its free of any defects that allow any corrosive agent such as an electrolyte to contact the steel substrate .When the coating  integrity is damaged, or compromised, the rate of any resultant corrosion a direct consequence of the corrosion potential for any given service environment. Pinholes and holidays are critical factors when selecting a coating system fit for purpose in combination with application techniques to ensure value for money is well spent.

Recognising the type of coating failure, can feed back into future specification knowledge to avoid reptation failures, coating systems have to operate effectively to ensure selection is capable of delivering the life cycle performance expected. Coating assessment is important because it assist to understand the cause/s of failure or breakdown as the case may be. This article is not all inclusive, the intent is to provide a general insight into some of the causes related to coating failures particularly in relation to pinholes and holidays.