一周年庆典－－PAINT COATING FILM DEFECTS AND THEIR CORRECTION

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PAINT COATING FILM DEFECTS AND THEIR CORRECTIONThere are numerous defects, which may arise, in a paint coating film during application and after, and the most frequently occurring ones are discussed below.
Runs,
Sags and
Curtains:
Runs are narrow, downward movements of paint coating when excess material continues to flow after the surrounding surface has set. Sags are a downward movement of paint coating in a “curtain” or sheet. The most common cause of runs and sags is applying too much material per coat.

Every material has what is known as a sag point. This is the maximum thickness of wet coating that can be applied to a vertical surface. The coating will sag in a curtain if more material is applied. Thinning a coating lowers the sag point, or the thickness at which the paint coating can hold its own weight. It is also possible to blow a sag into a coating when using conventional spray. This occurs when the gun is held too close to the surface. The atomizing air blows the paint coating away from the spot where the gun is pointed, and sag occurs around this spot.

These three defects are very similar in nature and the main difference is the relative sizes of the areas involved.

Runs occur most frequently around detail areas such as nuts and bolts, where controlling film thickness while covering each surface is difficult, especially when spraying. Runs are narrow, downwards movements of paint coating and may be caused by:

---Collection of excess amounts of paint coating in surface irregularities like, e.g. nuts, bolts, pits, holes, cracks and pores.

Correctional action should follow the lines outlined below for curtaining and sagging.

Curtaining affects a larger, continuous area than sagging. This fault is characterized by a downward movement of the paint coating film in the period of time between application and setting, resulting in an uneven coating having a thick lower edge.

The cause(s) of this may be one or more of the following:


---Localized too heavy film build up due to the spray gun having been held too close to the surface, or moved too slowly across.

---Too heavy overlapping of coats.

---The surface or the paint coating itself is too warm affecting the thixotropy of the paint coating.

---The surface or the paint coating is too cold affecting the evaporation rate of the solvents in the paint coating, giving too long drying times.

---The thixotropic mechanism of the paint coating is insufficient.

---The paint coating has been excessively thinned.

---Fresh paint coating applied over very glossy, existing paint coating.

If the sagging or curtaining has set or cured, it must be removed by scraping and/or sandpapering. If the paint coating is still liquid, it should be brushed out. Actions should be taken to ensure that the problem does not recur looking at the causes listed above and taking corrective action.

Any good painter carries a small brush when spraying to correct runs and sags. Brushing out the runs or sags while the paint coating is still wet is a lot easier than the alternative, which is sanding them out after the paint coating has dried. Runs and sags can be avoided or minimized by monitoring the wet film thickness so that too much paint coating is not put on, by using only the minimum amount of thinner to control viscosity, and by not getting too close to the surface when spray paint coating.
Drop
Formation: Both dipping and spray application may result in drops forming at the lower ends of surfaces. The causes for this may be:

---Too low steel temperature during application.

---Insufficient time taken for letting paint coating drip off after dipping.

Drops may easily be corrected by brushing as described above while still wet. Watch out for reformation of drops! When dry they will have to be chipped off or sanded.
Orange
Peel:
Orange peel is a condition of hills and valleys in the coating resembling the skin of an orange. This effect is also called pockmarking. Causes may be sought among the below:

---Too low an air atomizing pressure.

---The gun being held too close to the surface.

---Too rapid evaporation of the solvent(s) in the film.

---Unsuitable thinners used.

---The airless spray pressure was too high.

---The spray nozzle was too small.

---The ambient temperature or the temperature of the atomizing air was too low.

If the coating has set or cured the surface must be rubbed down with emery paper followed by spraying with a solvent or a thin coat of the same paint coating. If the paint coating is re-dissolvable paint coating, the rubbing down with emery paper may be omitted as the subsequent wet coat will dissolve the dry coat thus (usually) as with runs and sags, the orange peel can be brushed out if the paint is still wet or sanded out if the paint has dried.

The cause of the orange peel effect should be determined and corrective action taken. The cure for orange peel depends upon the cause of the problem. The first things to check are equipment set-up and spray technique. Check the spray pattern, and make sure that enough atomizing air is being used. Be careful that you are not using too much air, which will cause dry spray. In the case of airless spray, please try lowering the air pressure to the pump.

Next, check the spray technique and make sure that you are not holding the gun too close to the surface. If these types of adjustments do not correct the problem, consult with the coatings manufacturer about use of thinner to correct the problem. A slower evaporating solvent may be needed.
Dry spray:
Dry spray is a condition caused by the partial drying of the liquid coating prior to reaching the surface. The paint particles do not flow out to form a smooth film. Dry spray results in a rough, sandpaper texture on the surface. Dry spray is not a continuous film and can result in early corrosion or loss of adhesion between coats. Dry spray must be removed by sanding, and the coating must be re-applied. (Except for re-dissolvable paint coatings)

The most common causes of dry spray are holding the gun too far from the surface, using too much atomizing air, and spraying when the wind velocity is too high. Note that the major causes of dry spray are conditions that allow the paint particles to partially dry before hitting the surface. These causes are detailed below.

Proper spray technique requires the gun to be held perpendicular to the surface and the correct distance away. This distance is 15 to 20 cm (6-8”) for conventional air spray and 25 to 30 cm (10-12”) for airless spray. Using too much atomizing air with a conventional air spray gun will also cause dry spray.

Proper set-up of a conventional air spray requires first adjusting the fluid flow to get a steady stream of paint. Then, the atomizing air is turned on and increased until the proper fan pattern is obtained. Increasing the air regulator beyond this point will introduce too much atomizing air and result in a greater tendency for dry spray.

When it is too windy during the painting operation, the air blowing through the spray pattern dries out the paint particles. When the wind velocity gets too high (generally 7-9 m/sec. Or 24-32 km/hr), spray painting cannot be used. You then must brush and roll, or wait for another day.
De-
Lamination:Delamination is a clean break between coats of paint or between the primer and the substrate. It means that the paint is not sticking to the surface. The major causes of delamination are painting over a contaminated surface or allowing too long a dry time between coats. When delamination occurs, the cause must be determined so that proper corrective actions are taken. A main cause of delamination is painting over a contaminated surface. In some cases, this may be gross contamination such as dirt, dust, or other particulate matter. The surface should be checked prior to application of the coating for gross contamination. This is especially important when applying an intermediate or a topcoat to a previously applied coat. Rubbing a rag or clean glove over the surface will show whether dirt or dust is present. Other contaminants that may be present include oil, grease, or chemical fall-out from nearby industrial plants.

If you are painting in an industrial plant, be aware of any contaminants that may be blowing in the direction of your work. Another example is diesel exhaust on an overpass bridge. Truck traffic beneath a bridge may deposit a thin, invisible layer of exhaust on a primer surface. This layer will cause delamination if the surface is not cleaned prior to the application of the next coat.

Another common cause of delamination is exceeding the recoat interval for the material. This is especially true for coatings that cure by cross-linking, such as epoxies or polyurethanes. The application data sheet will give the maximum recoat time for the material. Be aware that the recoat interval is given at a specific temperature. If the weather is warmer than that stated on the data sheet, the recoat interval would be less. So consult with the coatings manufacturer to determine the maximum recoat time for the temperature conditions under which you are working.

If a delamination occurs, the material must be removed and a new coat applied. If dirt, dust, or particles are the cause, cleaning the surface will be sufficient. Oil and grease contamination require solvent cleaning. Exceeding the recoat interval can usually be addressed by brush blasting or otherwise roughening the surface prior to re-application of the coating. Check with the coatings manufacturer to determine if this will be sufficient. Other causes of delamination may require other corrective actions.
Pinholes:
Pinholes are small, deep holes that are visible to the naked eye. Pinholes usually appear in clusters. Pinholing is a common type of defect that can result from several causes, including holding the spray gun too close to the surface with excessive atomizing pressure, insufficient spray atomization with too high a fluid pressure, improper paint formulation, or the condition of the surface being painted.

Please note that both too high or too low an atomization can cause pinholing. This points to the fact that proper set-up of spray equipment is essential for obtaining a smooth paint film.

The formulation of the coating may be the cause of the problem. An example is improper solvent balance, which can result in one solvent evaporating too rapidly at one stage of the drying process. An improper solvent balance can result from the use of the wrong thinner. Always use thinners that are recommended by the coatings manufacturer.


The substrate surface may be responsible for the pinholing. Concrete, for example, has voids, pores and other small imperfections that may result in pinholes when the coating is applied. These features in the concrete must be filled prior to application of the coating.

Another surface to which coatings are applied where pinholing can occur are inorganic zinc-rich primers. Inorganic zinc-rich primers are quite porous and remain that way for a long period of time. When an organic coating is applied on top, the air and solvents in the pore try to escape through the wet film. This pressure creates small blisters or bubbles that form pinholes when they break. Blistering is usually worse in warm weather. One technique to minimize blistering when top coating inorganic zincs is to apply a mist coat passes, or light layer of paint, over the surface. This will aid in filling in the pores while allowing the air and solvents to escape. In essence, it seals the surface. As soon as the mist coat has been applied to the area, the full thickness of topcoat can be applied.

No matter what the cause, pinholing is caused by the applied wet film not flowing together properly thus closing the micro channels in the film.

Pinholed areas can be corrected by application of more material. However, mechanical force will be needed to fill in the pinholes. This can be accomplished by brushing. Several passes over the affected area may be needed to fill in all the pinholes. This is the formation of minute holes in a paint coating film during application and drying, sometime caused by air or gas bubbles in the wet film which burst, forming small craters that fail to flow out before the film sets. Air in the pores of a zinc silicate coating will often cause pinholes in the subsequent paint coating film.
Cratering:
The appearance is like a Moon crates (hence the name). Cratering is the next stage in developments, which at an earlier stage in the drying process of the applied wet film will lead to pinholing. Air or solvent vapor bubbles working their way out of the applied wet film will meet resistance when they get to the surface of the film, causing temporary blisters to form. Before the film is dry, these blisters, which consist of gas or vapor bubbles inside a stretched, flexible film, may burst (or the gas/vapor may dissipate), whereupon the blister will collapse forming the crater.
Blistering:
This is yet another stage of the same process where the gas/vapor cannot get through the top layer of the paint film and due to build up of vapor pressure, a blister is formed. This type of blistering must not be confused with osmotic blistering.
Vacuoles:
Vacuoles are void spaces inside the paint film. These void spaces may be filled with solvent vapor, but can also be completely empty, just filled with air. Vacuoles can (normally) not be seen from the outside of the film, but only becomes apparent when the film is cut open. Vacuoles may become focus points for water molecules forming liquid water, and certainly represent a weakening of the film where present.

Pinholes, craters, blisters and vacuoles all represent various stages in partly or wholly entrapment of air/solvents.
Fish eyes:
These may be seen as small bowl-shaped depressions in a paint coating of varnish film. Normally the reason is oil, grease or silicon contamination on the surface or in the atomization air.

Fish eyes are a separation or pulling apart of a wet film to expose the underlying substrate or paint layer. They get their name because they have the appearance of fish eyes. The most common cause of fish eyes is oil contamination. Indeed, the droplet that forms the fish eye is filled with oil. Silicone on the substrate may also cause fish eyes.

Fish eyes cannot be corrected. The coating must be removed, the surface must be solvent-cleaned, and the coating must be re-applied.

A major cause of fish eyes is oil contamination from a malfunctioning compressor. The oil is carried in the blasting air and deposited on the surface, or it is carried in the atomizing air when using conventional spray. When fish eyes occur, the first thing to check is the cleanliness of the air stream. This requires running the blotter test. For one minute, a white blotter or cloth is held within 60 cm (24”) of the end of an operating air line blast hose (with the abrasive turned off). The blotter or cloth is visually examined for dark spots that indicate the presence of oil. If oil is found, it indicates that the problem is with the compressor, and maintenance is needed. If no oil is found in the air supply, then the oil contamination is on the surface, and solvent or alkaline cleaning is necessary.
Cracked
Blistering:
This is due to localized loss of adhesion caused by entrapped solvents or solvent vapor in or below the paint coating film, or by osmosis of water. Excessively thick films may also exhibit blistering.

Please note that the above three defects are very similar in appearance and sometimes difficult to tell apart.

Pinholing, cratering and blistering are all corrected through removal with emery paper and reapplication of the paint coating in a thin coat.
Uneven
Appearance:Uneven appearance covers many different conditions, including skips, misses, uneven thickness, uneven gloss, or rough surface. Uneven appearance usually results from application deficiencies, including improper mixing, straining, application technique, or worn equipment.

Skips, misses, holidays, thin and thick areas are usually caused by poor application techniques. Proper set-up of equipment is essential for obtaining a uniform coating film.

To minimize application-related problems, do not forget the importance of proper mixing and straining of the coating. Improper or insufficient mixing can cause a number of problems, including the inability to apply the desired wet film thickness when pigments are left in the can, uneven cure of two-component materials, uneven gloss, and other related problems. Insufficient mixing and straining will cause lumps to be carried in the fluid line, resulting in gun plugs. The plugs will give an uneven spray pattern and cause a lot of downtime to fix equipment. Worn spray gun components, such as needles and caps, will give uneven spray patterns such as tails. This pattern will result in thin and thick areas in the applied coating.
Blushing:
Blushing is characterized by the surface of the coating turning white. This surface effect is normally caused by:

---Moisture in the form of rain, fog or condensation settling on the still wet painted surfaces.

---Water mixed into the wet paint coating.

---Improper thinning.

---Excess of amine curing agent causing excess to raise to the surface of the paint coating film causing color change and an oil like shimmer on the surface of the film (amine blushing).

Blushing can normally be removed from the surface of the coating film through washing with fresh water and a detergent. In some cases (amine blushing) a solvent wash is necessary and in extreme cases sanding or complete reblasting may be needed.
Lifting:
Lifting describes the attack by a coating on an old or new paint coating film, usually due to solvent action, often resulting in blisters, blowholes and/or the formation of a wrinkled film. The causes may be:

---The solvents in the second coat are too strong for the first coat dissolving and penetrating this.

---The first coat is still not set to the point necessary for over coating, in other words the minimum recoating interval has not been observed.

If lifting has occurred all affected paint coating must be removed completely and fresh coatings applied.
Bleeding:
This refers to the diffusion of a coloring matter from a substrate and to the discoloration resulting from such diffusion. The diffusion could be pigment as certain pigments are prone to this, or it could be a binder like, e.g. the tar component in a coal tar epoxy. Normally bleeding does not affect the subsequent coat(s), however, the effect is affecting the appearance of the object. Further coats of the finish may solve the problem, however, sometimes the entire coating system must be removed and fresh coatings applied.
Flaking:
Flaking always indicates loss of adhesion, either to the substrate or between coats. The reason may be:

---Contamination on the substrate to be over coated.

---Condensation on substrates at time of application.

---The underlying substrate is too hard and glossy.


Corrective action includes removing all poor adhering paint coating, remove contamination (if this is the cause), dry all surfaces (in the case of condensation) or abrade the hard and glossy surface (if this is the cause). Thereafter the coating is reapplied.
Chalking:
Chalking (sometimes called erosion) is the condition of paint coating surface which, having lost most of its gloss, is coated with a white powder (or “chalk”). This powder can be removed on light rubbing and is the result of a photochemical breakdown of the surface layer of binder with consequent release of pigment.

Epoxy paint coatings are especially sensitive to the photochemical effects of ultraviolet radiation, and so are certain pigments used in paint coating formulations. The main causes are:

---The use of pigments, which are sensitive to ultraviolet radiation.

---The paint coating is based on epoxy resins.

Remedial action includes high-pressure fresh water washing of all affected areas, followed by the application of a coat of paint coating, which is not susceptible to chalking.
Cracking:
Also termed alligatoring/crocodiling or checking or embrittlement.

Cracking is the type of breakdown shown in paint coating films as a break extending through to the substrate. Where this is difficult to decide, they should only call the break a crack if the underlying substrate is visible.

They recognize three types of cracking:

---Irregular pattern type (no particular pattern).

---Line type (usually in parallel lines).

---Sigmoid type (converging and intersecting lines).

Cracking is often the result of inadequate elasticity of the paint coating system on a dimensionally unstable substrate. It differs from alligatoring/crocodiling and checking in that the cracks extend through the entire paint coating system to the substrate. Subsequently the coating may become detached from the edges of the cracks leading to flaking.

Checking is the term used to describe shallow breaks in a paint coating film. These breaks do not penetrate to the substrate. Three types of checking are recognized:

---Irregular type (no particular pattern).

---Line type (usually in parallel lines).


---Crowsfoot pattern (a definite 3-prong pattern with the breaks running from the center at an angle of approx. 120o between prongs).

Severe irregular checking is often referred to as crazing.

Checking starts at the exposed surface, works progressively deeper into the coating, tends to take on a V-shaped cross-section with the open part at the exposed surface, but gives no sign of widening at the bottom through contraction of the coating. Slight checking is not a serious defect, as it indicates a relieving of stresses in a paint coating film.

Alligatoring/crocodiling is used as a term describing pronounced wide breaks over the entire surface but the breaks do not penetrate to the substrate. They may affect a single layer of film only. The appearance is like alligator or crocodile skin.

Alligatoring/crocodiling may begin as checking or cracking but the break tends to get wider at the bottom as well as at the top. The top coating contracts, thus exposing portions of the undercoat. In the typical extreme cases of alligatoring/crocodiling, the “islands” of coating between interlacing breaks have not only contracted in area but have increased in thickness and consequently often have become wrinkled. The cause of this defect is usually the application of a hard coating on top of a softer one like, e.g. application of alkyd paint coating on top of bitumen-based paint coating.
Mud-
Cracking:
Mudcracking is characterized by a network of cracks in the paint film that resembles a dried mud field. Mudcracking occurs in most generic types of coatings but is most common in fast drying coatings such as inorganic zinc-rich primers and vinyls. The most common causes of mudcracking are high film build, excessive atomization, and application to too hot of a surface or when the temperature is too high.

Mudcracked areas must be removed and the coating re-applied. Acceptable methods to remove mudcracking include sanding or use of aluminum wire screen for inorganic zinc-rich primers. If the mudcracking is caused by overthickness, the underlying material may not have cracks. If so, removal of just the mudcracked material and application of a repair coat in accordance with the coating manufacturer’s instructions may be sufficient.


Mudcracking caused by overthickness or excessive atomization can be avoided by proper equipment set-up and application techniques. Keep an eye on film build, and apply the coating in as uniform a pattern as possible.

If the weather is too hot, mudcracking may be solved by changing the thinner. Many suppliers of inorganic zinc have 2 thinners, a summer thinner and a winter thinner. The summer thinner has solvent evaporating solvents, which help in the application of a wet film under high temperatures. If the problem is too hot a surface, then it may help to shade the surface from direct sunlight or schedule work so that areas in direct sunlight are coated earliest in the day.
Wrinkling:
Wrinkling has the appearance of wrinkled skin. The surface has skinned over while the underlying material is still wet. Wrinkling is most common with oil-based paints.

The common causes of wrinkling are application of too thick a coat of paint or application when the weather is too warm.

If wrinkling occurs, the wet paint underneath will never dry. Therefore, the wrinkled areas will have to be scraped and a thinner coat applied.

Wrinkling can be avoided by applying the coating within the recommended film thickness range. The old saying that there can never be too much of a goods thing does not apply to paint coating films.

If the problem is caused by too warm a surface, shading the area from direct sunlight will solve the problem.
Blistering: It is necessary to distinguish between blistering, which is application related and blistering which is related to service exposure of the paint coating film.

Application related blistering is normally caused by:

---Solvents entrapped in a paint coating film, which has dried/cured on the surface thus, entrapping solvents inside the semi-dry film.

---Air entrapped beneath the paint coating film, typically air in the pores of an underlying coat. Normally such air will force its way through the paint coating film creating “popping”, however, in the case of thick films the air may become entrapped causing blisters.

The service exposure related blisters might be classified as follows:

---Blistering in Immersion Service

Perhaps the most common problem in immersion service is blistering of the lining material. Blistering itself does not necessarily denote failure, and often a blistered coating will provide long-term corrosion protection to the underlying steel or concrete substrate. However, a blistered lining will inevitably provide a lesser corrosion protection than a non-blistered lining, all other factors being constant. In time, many blisters will fill with rust, and active corrosion within the blister cavity may occur.

Alternately, the blister may fill with concentrations of permeating chemical species, leading to aggressive attack of the underlying substrate. When the tank is drained, the blisters dry out, stressing the lining film and causing cracks or adhesion loss. As a consequence, blistering should be avoided whenever possible, and in order to do so, it is necessary to have some understanding about blistering mechanisms.

Four types of blistering mechanisms will be discussed here: osmotic blistering due to entrapped water-soluble salts, osmotic blistering due to retained solvents, cathodic blistering, and cold-wall effect blistering.

The first two types of blistering mechanisms are by far the most common and best understood. Blistering due to cathodic disbonding occurs only under very specialized conditions in conjunction with cathodic protection of a coated surface. Cold-wall effect blistering can be readily demonstrated, but the exact cause of the blistering mechanism is not known.

---Osmotic Blistering Due to Entrapped, Water-Soluble Salts

This classic blistering mechanism requires the presence of a water-soluble salt (such as that coming from perspiration, salt contamination near coastal areas, and chemical contamination from nearby industrial plants) entrapped beneath a coating or between coats of a lining system.

As moisture, in time, permeates the lining after it is placed in immersion service, it dissolves the entrapped water-soluble material, rendering an entrapped solution with a high salt-to-water ratio. In contrast, the bulk liquid within the vessel is relatively free of water-soluble salts, and accordingly has a low salt-to-water ratio. The higher the purity of the bulk liquid within the vessel, the greater the tendency for osmotic blistering. This is a particular problem when high-purity, de-ionized water is stored, as in the nuclear power industry, and in condensate systems in power generation facilities. The difference in salt concentrations (the entrapped solution of high salt content and the bulk liquid of low salt content) leads to what is commonly called an “osmotic driving force”. This very powerful force increases the rate of moisture permeation from the bulk liquid side to the entrapped solution side in order to create equilibrium.

Equilibrium will be accomplished when sufficient moisture from the bulk liquid side permeates the lining such that the salt concentrations on either side of the lining (which acts as a semi-permeable membrane) are equalized. In practice, this never occurs, due to the rapid drop-off of the osmotic driving force after initial permeation occurs, and the inelasticity of the lining material, which holds and contains the entrapped salt concentration within the blister cavity. Thus, the ability of the lining material to hold the entrapped salt solution in the blister cavity (much like the skin of a balloon holds air at higher pressure than the outside air) may allow considerable blister pressure to build up within the blister cavity.

When the lining is in service, such blister pressure is countered in some part by the head pressure of the solution. When the vessel is drained and the blister is punctured, often liquid spurts out in a stream as the blister collapses. Alternately, if the blister is not punctured, water may migrate from the blister cavity to the lining surface and evaporate, causing the blister to collapse as well. Thus, for best results, a lining system should be inspected for blisters shortly after draining. Osmotic blistering sometimes will concentrate on flat or horizontal surfaces, or areas where perspiration or falling airborne contamination can settle.

Paradoxically, however, the very act of draining a tank may cause a blistered lining, which otherwise would have been protective if the tank had not been drained, to crack or be exposed to oxygen from the air, which may cause rusting within the blister cavity. Thus, draining a tank to inspect for blisters may be somewhat self-defeating, as the lining “dries out”, shrinks, perhaps cracks, and is exposed to oxygen.

---Osmotic Blistering Due to Entrapped Solvents

This blister mechanism is quite similar to osmotic blistering by water-soluble salts, except that hydrophilic (water-loving) solvents are entrapped within the film, taking the place of the salts as described above.

Hydrogen bonding attraction between the water molecules in the bulk solution and the entrapped solvent increases the permeation of water through the lining material to the point of solvent entrapment. The amount of water that permeates depends upon the type and polarity of the entrapped solvent, and the permeability and physical nature of the lining material. While, strictly speaking, the driving force is not an osmotic driving force, the effect is the same, and the blister formation is identical. When entrapped solvents are the cause of blistering, the blistering is generally more predominant toward the bottom of the vessel where solvents can accumulate, and solvent volatilization from the lining is retarded. Alternately, in cooler areas or heat sink areas, solvent volatilization from a lining may be retarded, and blister formation may be more prevalent.

---Cathodic Blistering

When coatings are used in conjunction with cathodic protection, either by impressed current or sacrificial anodes, sufficient cathodic protection current must be supplied to protect the metal tank (or pipe), but not so much current to cause cathodic disbonding. As a rule of thumb, a value of –0.85 volts is used to protect steel surfaces, as measured using a standard copper/copper sulfate reference electrode to contact the environment. If other types of reference electrodes are used, the values of the voltage requirement will be different.

If less than –0.85 volts are used, less than complete cathodic protection will be attained on steel. On the other hand, electrode potentials more negative than –0.85 volts will be a waste of energy, as corrosion on steel is stopped at that voltage. In actual practice, however, it will be necessary to maintain a more negative potential at drainage points of cathodic protection current in order to maintain the minimum of –0.85 volts at locations remote from the drainage points (protrusions, edges, and prominences). This is a result of attenuation – voltage drops caused by cathodic protection currents on a pipeline or vessel flowing through the longitudinal resistance of the metal in order to return to the drainage point.

In this respect, large diameter coated pipes or vessels are easier to protect cathodically than are smaller diameter pipes or tanks, because the larger cross-sectional steel area means a lower longitudinal electrical resistance with resulting lower attenuation. However, if a protective over-voltage is applied (approaching the order of –1.2 volts copper/copper sulfate electrode), the possibility of gaseous hydrogen evolution may occur at coating defects on the coating surface. Hydrogen evolution may occur at coating defects on the paint around defects, resulting in a loss of adhesion and exposure of more uncoated steel. If the over-voltage is considerably in excess of –1.2 volts, extensive blistering and disbonding of the coating system may occur. With an impressed current cathodic protective system, increased current density will be required to protect these now-exposed areas. If the protective system is by sacrificial anode, it may not be possible to increase the current density, and corrosion will initiate at areas where the least cathodic protection is available.

---Cold-Wall Effect Blistering

The cold-wall effect results whenever there is a thermal gradient between one side of a vessel and the other side. Often, the interior of a vessel is heated, while the exterior is at a lower or cold temperature.

The water vapor pressure is higher on the warm side of the lining than on the cool side. This vapor pressure gradient causes moisture vapor to pass into the lining more rapidly than it egresses. Condensation occurs on the cold side because of the increased vapor pressure (i.e., saturation or dew point is reached). There is a continuous driving force to transfer and condense water vapor on the cold side because the system cannot reach thermal equilibrium.

On the warm side of the lining, there is greater molecular vibration and higher energy, causing an increased rate of permeation. As permeation proceeds through the lining to the “cold” side, the rate of permeation decreases, but more importantly, cooling and perhaps condensation occurs.

Due to the unequal permeation as a result of the thermal gradient, moisture will ultimately accumulate adjacent to the cold surface beneath the coating. Accumulation will occur at areas of weak adhesion of the coating to the substrate, resulting in liquid-filled blisters. The greater the thermal gradient between the hot and cold sides, the greater will be the size and frequency of the blisters. Additionally, the higher the temperature, the greater the size and frequency of blistering as well.

Studies indicate that coating systems with the lowest permeability are more resistant to cold-wall effect blistering than coating systems with increased permeability. However, the best way to eliminate cold-wall effect blistering is to insulate the vessel, and minimize the thermal gradient across the lining.
The above are just some of the defects which paint coating films can exhibit and the listing is in addition limited to paint coatings used on marine and onshore structures. Many of them are easily corrected while work is being performed. Application of a uniform coating requires knowledge of the equipment, how it operates, and what can go wrong. Minimizing some defects comes with experience and practice. The study of paint coating film defects is a lifelong one and new and exiting defects and causes may develop at any time.