REQUIREMENTS FOR CORROSION PROTECTION

16.3 REQUIREMENTS FOR CORROSION PROTECTION

To protect against corrosion, a good paint should meet the following requirements:

1. Provide a Good Vapor Barrier. All paints are permeable, in some degree, to water and oxygen. Some vehicles are less permeable than others, but their better performance as a diffusion barrier applies only to well - adhering multiple -

coat applications that effectively seal pores and other defects. The diffusion path through a paint fi lm is normally increased by incorporating pigments. Particularly effective in this regard are pigments having the shape of fl akes oriented parallel (e.g., by brushing) to the metal surface (e.g., micaceous or fl aky hematite, alumi- num powder). Diffusion, on the other hand, tends to be accelerated by electro -

osmosis whenever the coated metal is made the cathode of a galvanic couple or is cathodically protected.

2. Inhibit against Corrosion. Pigments incorporated into the prime coat (the coat immediately adjacent to the metal) should be effective corrosion inhibitors. Water reaching the metal surface then dissolves a certain amount of pigment, making the water less corrosive. Corrosion - inhibiting pigments must be soluble enough to supply the minimum concentration of inhibiting ions necessary to reduce the corrosion rate, yet not soluble to a degree that they are soon leached out of the paint.

Among pigments that have been recommended for prime coats, only rela- tively few actually do the job that is required. Effective pigments for which per- formance has been established in many service tests include (1) red lead (Pb 3 O 4 ) having the structure of plumbous ortho - plumbate (Pb 2 PbO 4 ) and (2) zinc chro- mate (ZnCrO 4 ) and basic zinc chromate or zinc tetroxychromate. The inhibiting ion in the case of red lead is probably PbO 4 4 − , which is released in just suffi cient amounts to passivate steel, protecting it against rusting by water reaching the metal surface. It is likely that lead oxides and hydroxides of other compositions also have inhibiting properties in this regard, but red lead appears to be best of the lead compounds.

For zinc chromate, the inhibiting ion is CrO 2 4 − , solubility relations being just right to release at least the minimum concentration of the ion ( > 10 −4 mole/liter) for optimum inhibition of steel. The solubility of zinc tetroxychromate is reported to be 2 × 10 −4 mole/liter [3] . Lead chromate, on the other hand, is not nearly

ORGANIC COATINGS

soluble enough (solubility = 1.4 × 10 −8 mole/liter) and acts only as an inert pigment. Commercial formulations of lead chromate sometimes contain lead oxides, present either inadvertently or intentionally, which may impart a degree of inhibition.

Zinc molybdate has been suggested as an inhibiting pigment for paints [4] , being white instead of the characteristic yellow of chromates. It is less toxic than chromates. The sulfate and chloride content of a commercial ZnCrO 4

(or ZnMoO 4 ) pigment must be low so that it can passivate the metal surface. Because inhibiting pigments passivate steel, they are relatively ineffective for this purpose in the presence of high concentrations of chlorides, such as in seawater.

Paints pigmented with zinc dust using essentially an aqueous sodium silicate vehicle (called inorganic zinc - rich paint), or an organic vehicle, are also useful as prime coats, the function of the zinc being to cathodically protect the steel in the same manner as galvanized coatings. Such paints are sometimes used over partly rusted galvanized surfaces because they also adhere well to zinc, but rust should fi rst be removed. Sacrifi cial protection by zinc - rich coatings requires intimate contact with the substrate, and so these coatings are always used as primers. It has been reported [5] that in order to ensure good electrical contact between zinc particles and with the base metal, the amount of pigment in the dried paint fi lm should account for 95% of its weight. A coating of this kind protected steel in seawater against rusting at a scratch for 1 – 2 years, whereas with 86% and 91% Zn, rust appeared after 1 – 2 days and 10 – 20 days, respectively. Vehicles for zinc -

rich paints to accommodate such a large fraction of pigment include chlorinated rubber, polystyrene, epoxy, and polyurethane. The minimum amount of zinc dust pigment required to provide cathodic protection depends on several factors, including Zn particle size, nature of the vehicle, and the amount of ZnO and other pigments that may be present [6] . It probably also depends on the extent to which insulating coatings form on zinc particles before the paint is applied (age of paint).

3. Provide Long Life at Low Cost. A reasonable cost of paint should be gauged by its performance. A paint system lasting 5 years justifi es double the cost of paint if the more expensive paint provides 35% longer life or lasts short of 7 years (labor to paint cost ratio of 2 : 1).

The rate of deterioration of a paint depends on the particular atmosphere to which it is exposed, which, in turn, depends on the amount of atmospheric pol- lution and on the amount of rain and sunshine. The color of the top coat (i.e., its ability to refl ect infrared and ultraviolet radiation) and the type of vehicle used play some part. Other things being equal, the performance of good - quality paints used for corrosion protection is largely determined by the thickness of the fi nal paint fi lm. In achieving a given coating thickness, it is advantageous to apply several coats rather than one, probably because pores are better covered by several applications and also because evaporation or dimensional changes during polymerization are better accommodated by thin fi lms.

METAL SURFACE PREPAR ATION