Corrosion protection phosphatizing

Zinc phosphate, Zn2(P0 2> forms the basis of a group of dental cements. Chromium and zinc phosphates are utilized in some metal-treating appHcations to provide corrosion protection and improved paint adhesion. Cobalt(II) phosphate octahydrate [10294-50-5] Co2(P0 2 8H20, is a lavender-colored substance used as a pigment in certain paints and ceramics. Copper phosphates exhibit bioactivity and are used as insecticides and fungicides. Zinc, lead, and silver phosphates are utilized in the production of specialty glasses. The phosphate salts of heavy metals such as Pb, Cr, and Cu, are extremely water insoluble. 

The protection of steel surfaces by paint depends significantly on the chemistry of the paint-metal interface. The system has many variables, because the metal surface is usually pretreated in a variety of ways, including galvanizing and phosphating. Indeed, there are probably several interfaces of importance, and corrosion protection might be a function of the conditions at all of them. 

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. 

The corrosion process can be inhibited by the addition of phosphate or polyphosphate ions [344], inorganic inhibitors as, for example, chromate ions [336], adsorbed alcohols [345], adsorbed amines, competing with anions for adsorption sites [339,] as well as saturated linear aliphatic mono-carboxylate anions, CH3(CH2)n-2COO , n = 7 — 11, [24]. In the latter case, the formation of the passive layer requires Pb oxidation to Pb + by dissolved oxygen and then precipitation of hardly soluble lead carboxylate on the metal surface. The corrosion protection can also be related to the hydrophobic character of carboxylate anions, which reduce the wetting of the metal surface. 

Chromium phosphate has a low solubility. It is therefore nearly always used in combination with other anticorrosive pigments. It is an extremely good long-term inhibitor, but is less effective during the initial phase of corrosion protection. 

The chemical treatment employed is a combination/derivation of an Alkaline Zinc Polymer Program and a Stabilized Phosphate Program. It operates at an alkaline pH and uses zinc to synergize with phosphate to enhance corrosion protection and reduce the amount of PO4 required (this is Chemical 1). It requires a PO4 stabilizer polymer (Chemical 2). 

It is important not to leave the system empty of water for any long period, as rapid surface rusting will take place. As soon as the closed-loop system is declared free of contamination, sufficient corrosion inhibitor is added to provide long-term corrosion protection. The corrosion inhibitor is usually an anodic, passivating formulation, typically based on nitrite or tannin (and often in combination with phosphate, silicate, borate, or molybdate, etc.). Finally, after confirmation that the entire system is adequately treated (which usually requires the inhibited water in the system to be recirculated for a further 16 to 24 hours), the system is signed off and handed over. 

The corrosion resistance of the Si3N4 ceramics in H3P04 solutions differs from that in H2S04 and HN03 because a protective phosphate layer is formed [507, 513]. 

Using the HSAB principle, one can rationalize the corrosion inhibition of iron and aluminum by phosphate in which iron phosphate and aluminum phosphate are produced. Ferric and Al3+ are hard acids, and they react with phosphate, a hard inhibitor and give corrosion protection. Corrosion inhibition of Cu2+ and Zn2+ by amines can be rationalized by the formation of amine complexes of Cu2+ and Zn2+, and this is in accord with the principle that Cu2+ and Zn2+ are borderline acids reacting with amines which belong to borderline inhibitors. Corrosion protection of copper (soft acid) by mercapto-benzothiazole (soft inhibitor) is also in keeping with the HSAB principle. 

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]. 

As mentioned in Section 5.2.5.1, zinc phosphate, while having many desirable properties as an anticorrosive pigment, does not demonstrate the degree of corrosion protection offered by lead and chromate pigments [5.55]. Therefore the pigment industry has concentrated on developing phosphate-based pigments with improved... 

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... 

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