Phosphate coatings corrosion protection

This is the final neutralizing rinse after the pre-treatment to obtain a better corrosion resistance. The phosphated surfaces are treated with chromic acid-based or acidified sodium dichromate solutions which are not affected by moisture and thus protect the phosphate coating. 

A primer On metal, the purposes of a primer are to enhance corrosion protection and to give excellent adhesion. The primer will contain anticorrosive pigments, such as strontium chromate or zinc phosphate, which will slowly release ions that can repair damage or faults in the underlying conversion coating. 

The corrosion protection provided by phosphate coatings without a sealing treatment is of a low order their value when sealed is considerably greater. Unsealed corrosion tests are therefore of little value except perhaps for studying porosity or efficiency of coatings destined to be sealed only with oil. 

Chemical conversion coatings are applied to previously deposited metal or basis material for increased corrosion protection, lubricity, preparation of the surface for additional coatings, or formulation of a special surface appearance. This operation includes chromating, phosphating, metal coloring, and passivating. 

Salt spray test. The model coatings of Table I are of the high solid type used in automotive top coats. Their primary function is not corrosion protection since this is first of all a matter of phosphate layer, electrocoat and/or primer. However, the topcoats may contribute to corrosion protection by their barrier function for water, oxygen and salts. Therefore their permeability is important as one of the factors in the corrosion protection by the total coating system. We feel that a salt spray test of the model coatings directly applied to a steel surface is of little relevance for their corrosion protection performance in a real system. 

In order to obtain maximum corrosion protection for painted metal articles, the metal parts are pretreated with an inorganic conversion coating prior to the painting operation. These zinc or iron phosphate coatings greatly increase both paint adhesion and corrosion protection. Traditionally, a chromic acid post-treatment has been applied to these phosphatized metal surfaces to further enhance corrosion protection. 

Pure lead has low creep and fatigue resistance, but its physical properties can be improved by the addition of small amounts of silver, copper, antimony, or tellurium. Lead-clad equipment is in common use in many chemical plants. The excellent corrosion-resistance properties of lead are caused by the formation of protective surface coatings. If the coating is one of the highly insoluble lead salts, such as sulfate, carbonate, or phosphate, good corrosion resistance is obtained. Little protection is offered, however, if the coating is a soluble salt, such as nitrate, acetate, or chloride. As a result, lead shows good resistance to sufuric acid and phosphoric acid, but it is susceptible to attack by acetic acid and nitric acid. 

The formation of hydrolysis products, in the case of zinc phosphate, depends on the permeability of the protective coating. The permeability of the protective coating itself is influenced by the type of resin used, and in particular, by the PVC (Pigment Volume Concentration). This means that the choice of resin, pigments and fillers and the complete formulation have an important influence on the corrosion protection behavior of protective coatings containing zinc phosphate [5.69]. 

The electrolytic deposition of a coating that is known as E-coat provides an excellent corrosion protection as evidenced by automotive coating. Today nearly all automobiles are corrosion protected by applying the cathodic E-coat, in which the steel body of a car is used as the cathode of the electrolytic deposition of a primer coat, on the surface of zinc phosphated steel. It is quite logical to consider that if an E-coat is applied to a chromate conversion-coated aluminum alloy surface, a significant improvement of the corrosion protection of aluminum alloys could be realized because such an attempt represents the combination of the two best components, i.e., chromate conversion coating and E-coat. We could find the best example that demonstrates the need of SAIE in such attempts. 

Most commercially available cathodic E-coat paints contain lead in the recipe, and the removal of lead from E-coat generally leads to an appreciably inferior performance on scab test [7]. The effect of lead-free E-coat on the corrosion performance of plasma interface-engineered systems was examined and the results are shown in Table 33.3. The elimination of lead resulted in significant deterioration of the corrosion protection of the phosphated samples. In a strong contrast to the conventional surface preparation, the plasma interface-engineered systems showed... 

Zinc Zn or Zn-Al coatings are widely used for sacrificial corrosion protection of steel [76], Zn and Zn-Al are readily susceptible to formation of white rust during humid-air exposure. Chromium-phosphate and to a lesser extent chromium-chromate formulations are used to suppress white rust formation. CCCs on Zn are believed to be... 

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