 |
|
 |
 |
Codes and Standards |
|
I. ADHESION AND SURFACE PREPARATION The primer is the critical element in most coating systems because it is most responsible for preserving the metallic state of the substrate, and it must also anchor the total system to the steel. This it may do in one of two ways, depending upon the nature of the primer vehicles. Most coatings adhere to metal via purely physical attractions (e.g, hydrogen bonds) that develop when two surfaces are brought closely together? Paint vehicles with polar groups (-OH, -COOH, etc.) have good wetting characteristics and show excellent physical adhesion characteristics (epoxies, oil paints, alkyds, etc.). Much stronger chemically bonded adhesion is possible when the primer can actually react with the metal, as in the case of a WP-1 wash primer5 pretreatment (SSPC-Paint 27), or a phosphate conversion coating. For adhesion to take place, the coating and substrate must not be separated from one another by more than approximately 5 A -- about three times the diameter of an oxygen atom. Any contaminant on the steel will increase the separation and decrease paint film adhesion. Moreover, reactive sites on steel at which adhesion can occur are masked not only by contamination, but also by chemically bound species which may themselves satisfy sites on the steel that would otherwise be available for reaction with the paint vehicle. Thorough surface preparation removes such contamination, and exposes many more reactive sites, thereby dramatically increasing the amount of surface area where adhesion can occur. The removal of surface contamination is important not only for adhesion, but also for good corrosion resistance. The barrier film prevents corrosion by increasing the electrical resistance (RCt) of the path of currents generated by slight differences in electrochemical potential between adjacent areas of the metal surface or be* between the underlying metal and another metal having a different potential. Paint films are not completely impermeable to the concentration of water and oxygen, and transmission of both is normally high enough so that prevention of the cathode reaction is impossible6.7.% Penetration by water and oxygen does not produce a resistance low enough to maintain a corrosion current, and though most paint films take up water relatively quickly, they take up ion solutions only very slowly% This keeps the electrolyte resistance relatively high and the corrosion low since corrosion is dependent upon ionic flow. However, even when the electrical resistance of penetrating moisture is reduced by absorption of ions, the resistance of a good barrier film remains high enough to achieve an important reduction in the magnitude of the corrosion current. Underfilm ionogenic (ion producing) materials (particularly chlorides and sulphates) that are left after poor surface preparation can be dissolved as ion-free water, penetrate the film, form conductive electrolytes, and increase corrosion. Also, under conditions of immersion, differences in ionic concentration between liquids beneath and outside the film give rise to osmotic migrations of water into the film. This promotes blistering and eventual film rupture. Further degradation and loss of protective value can result from electroendosmosis generated by differences in the electrochemical potential on the metal surface at and around the film disruption. Salts may also form from soluble matter within the film. The effect of corrosive salts such as chlorides is obvious. Inhibitive ions, themselves, however, may also cause problems. At the interface, the ionic solution from inhibitive pigments passivates thc metal by increasing the polarization of the anode (Ap in equation 2 in Chapter 1.1). However, such passivation of underfilm surfaces can have a detrimental effect under certain conditions by accelerating the corrosion at bare spots. There wilt be a considerable difference between the potential of the unpassivated metal at a bare spot (PA in equation 2) and the relatively large areas of passivated metal (PC in equation 2) under the paint film. This can result in accelerated corrosion at the bare spot. For this reason the use of inhibitive primers (containing passivating pigments) is avoided by some formulators on surfaces submerged in conductive electrolytes such as salt water. Unlike well cured films; insufficiently cured films allow the penetration of much more ionic materia10.11. Polymer groups such as carboxyls and hydroxyls tend to foster ionic penetration<~2.~3.~4,~s). As pigment volume concentration (p.v.c.)* is increased, these factors are overwhelmed because interstitial penetration dominates. Good barrier films, therefore, are high molecular weight films with uniform crosslink density, cured uniformly, for. formulated well below the critical pigment volume concentration (C.P.V.C.)** with low water solubility pigments. Lamellar pigments (leafing type aluminum) dramatically reduce ionic transmission rates. Care must always be taken in using metal flakes (such as copper or stainless steel) to ensure complete pigment encapsulation to avoid unwanted galvanic effects. Lamellar metallic pigmented barrier films are best used as finish coats, with a nonmetallic primer to improve adhesion. Similarly, tie coats should be used to improve adhesion between such metallic barrier films and zinc-rich primers. AIl things be!ng equal in atmospheric service, thicker barrier systems give better protection, as shown in work by SSPC and the FSCT~6, In general, the more severe the environment, or the longer the requirement for protection, the greater will be the coating dry film thickness required. Care should be taken, however, in the application of high build systems to thin walled structures and other dimensionally unstable substrates. Thick films (particularly those of rigid thermosets) are less able to provide the necessary flexibility to substrate movement (e.g. expansion and contraction) than are thinner films, and can easily undergo adhesive and cohesive failure leading to subsequent disbondment. Such delamination has been found in rail car tank linings, for example. Vehicle choice for barrier primers is also important. High molecular weight thermoplastics (e.g. vinyls and chlorinated rubbers) are effective, particularly at high builds. Thermosetting systems such as epoxy/phenolics and certain polyesters are also effective vehicles, as are the coal tar epoxies. Vehicles with high hydroxyl or car-boxyl contents (oils, alkyds, acrylics, etc) tend to attract water into the film.
High-build vinyl and chlorinated rubber systems of 8 mils and more make excellent barrier systems. Both polymers contain an inherent flexibility. They employ a moderately slow solvent system with an efficient thix-atrope to produce high wet film builds. Organomont-morillonites, pyrogenic silicas, hydrogenated caster oil derivatives or high molecular weight polyolefins are often used as thixatropes. Minimum effect on viscosity is desirable for ease of application. Solvent systems with high boiling aromatics or mono ethyl ether acetate are used. Application of up to 20 wet mils (5-7 dry mils) in one coat is possible. High build epoxy systems are also effec-tire. Such synthetics are more permeable than coat tar enamels applied in super thick films. One hundred mils of coal tar on buried pipelines or immersed structures used in combination with impressed current cathodic protection can reduce current requirements for cathodic protection ten thousandfold as compared to requirements for bare steel,7. High solids thermosets produce good barrier films, but they bring their own problems. Urethanes and epoxies may suffer from an unfavorable potlife/drytime ratio resulting from exotherms that tend to increase reaction rate in the can but which are dissipated from the applied film. High solids urethanes often have the additional problem of hygroscopicity. The successful use of multiple component systems is very dependent on the skill of the applicator. Mixing and application instructions must be followed exactly. In this type of primer, pigments are incorporated to provide a source of corrosion inhibitive ions which can be carried to the metal interface as water penetrates the film. Here they modify anode and/or cathode reactions, driving the steel potential into its passive region~8. There are two principal routes to such inhibition. The first, direct inhibition, relies on a controlled dissolution of ions from the pigment itself. At the interface, the ionic solutions passivate the metal by increasing the polarization-lion of the anode (increasing Ap in equation 2), by increasing the polarization of the cathode (Cp), or by thickening the natural oxide layer and increasing the electrical resistance across anode and cathode (increasing R). Perhaps the most efficient direct inhibitives are the salts of hexavalent chromium. In paint films, chromate inhibition is provided from such pigments as zinc potassium chromate, strontium chromate, etc. Pigment solubility is most important. Highly soluble pigments (calcium chromate) are rapidly depleted, while those with very low solubility (lead chromate) provide too few hexavalent chromate ions for protection. Zinc chromate offers a moderate solubility and is extensively used. Other less toxic inhibitors, the molybdates, phosphates, phospho-silicates, borates, and borosilicates also protect by similar mechanisms. Both type and loading of pigment are important, as are the type of vehicle and its moisture vapor transmission rate (MVTR). The ratio of the primer's pigment volume concentration (p.v.c.) to the critical pigment Volume concentration (c.p.v.c.) of the pigment system is equally important. Very tow ratios (low pigment content) give overly tight films and increase the tendency of the primer to blister. Filiform corrosion of the substrate can occur. Too high a ratio provides rapid dissolution of the inhibitor, and allows corrosive ions such as chlorides from the environment to penetrate the film. Chlorides and other depassivators compete with inhibitive ions for anodic adsorption, and can nullify inhibition, or at least increase the quantities of inhibitor needed. Inhibitive primers are best restricted to environments where the penetration of chloride ions is limited. Their use in immersion conditions is definitely not recommended because of the danger of osmotic blistering. The best pigment Volume concentration: critical pigment volume concentration ratio is determined experimentally, but a level near 0.9:1 is often used. Barrier type vehicles are less effective with inhibitive primers than the more permeable vehicles (oils, alkyds, etc.). Oils and alkyds do not, however, have the alkali resistance of vinyls, etc., and this is a disadvantage. Alkali generated at the cathode can rapidly saponify alkali-sensitive coatings. Alkali attack occurs at cathode areas adjacent to or surrounding corroding areas (anodes). Vehicle saponification can render a film quite water-soluble in such areas, This frequently occurs in alkyd systems where adhesion is destroyed. |
Copyright © Preferred Tank & Tower Inc., 2008