Chromate-phosphate treatment

Painting zinc and aluminium makes special demands on the pre-treatment of the substrate. Chromate treatment provides a good basis for paint, and it is still in widespread use in spite of the toxicity of chromate. Phosphate treatment or washpriming can also be used. Alternatives for aluminium are light blast cleaning or anodizing. 

Chemical inhibitors, when added in small amounts, reduce corrosion by affecting cathodic and/or anodic processes. A wide variety of treatments may be used, including soluble hydroxides, chromates, phosphates, silicates, carbonates, zinc salts, molybdates, nitrates, and magnesium salts. The exact amount of inhibitor to be used, once again, depends on system parameters such as temperature, flow, water chemistry, and metal composition. For these reasons, experts in water treatment acknowledge that treatment should be fine tuned for a given system. 

Table 15.13. This specification follows good industrial practice, with additional safeguards in rinsing to remove residues to treatment solutions. Nonaccelerated treatments must be followed by a single rinse which may contain chromate accelerated treatments must be followed by three rinses—cold water, hot water and a final chromate rinse. Table 15.14 shows the salt-spray test requirements for phosphate coatings with various finishes without formation of rust the paints and lacquer have the additional requirement that no rust shall be visible beyond 0-2 in (5 mm) from the deliberate scratches and no blistering, lifting or flaking beyond 0-05 in (1-27 mm) from the original boundaries of the scratches.

The best protection is given by paint. An etch-primed paint scheme can be applied directly to the metal for other paints an inorganic treatment must be given to ensure good adhesion. Of the two classes of inorganic treatment, phosphate treatment has little protective value in itself, but chromate passivation gives appreciable protection and in mildly corrosive surroundings may be sufficient in itself. 

While etch primers, also known as pretreatment primers and wash primers, can be regarded as priming paints which promote their own adhesion by etching the metal surface, they may also be regarded as phosphate/chromate etching treatments which leave an organic residue on the surface to form the basis of the subsequent paint scheme. A detailed account of the etch primers has been given by Coleman . 

Arsenites may also be determined by this procedure but must first be oxidised by treatment with nitric acid. Small amounts of antimony and tin do not interfere, but chromates, phosphates, molybdates, tungstates, and vanadates, which precipitate as the silver salts, should be absent. An excessive amount of ammonium salts has a solvent action on the silver arsenate. 

Due to environmental concerns, research efforts have been directed toward the replacement of chromate-based post-treatments. This paper focuses on a new unique chromium free post-treatment based on "Mannich derivatives" of po1y-viny1pheno 1 which have demonstrated excellent performance on both zinc and iron phosphate treatments. 

On zinc phosphate conversion coatings, the "Mannich" derivatives of poly-4-vinylphenol have demonstrated performance equivalent to chromic chromate systems in salt spray, humidity, and physical testing. In addition. Table III illustrates results observed with automotive body paint systems evaluated by the "scab" or "cycle" test which causes failure more typical of actual end use conditions than do salt spray evaluations. Again, results equivalent to chromic-chromate post-treatments were obtained. In addition, the humidity resistance and adhesion tests were essentially equivalent to the chromium controls. 

Chromate in a higher oxidation state, i.e., Cr , is more soluble than its counterpart Cr203 in lower oxidation state Cr. Again, this metal needs to be first reduced to its lower oxidation states and then stabilized using a phosphate treatment. 

As mentioned before, subsequent phosphate treatment does not affect the stable sulfide, and TCLP results show excellent stabihzation of Cr in any oxidation state. Alternatively, a small amount of reductant in the waste will convert chromate into lower oxidation states. Such methods, however, are not preferred, because the reductant may also affect the solubility of other hazardous compounds. The exception is technetium-containing radioactive waste, in which chromate is also a contaminant. As we shall see in Chapter 17, a reductant is essential for stabihzation of technetium, and that will also help in stabilization of chromium. 

Further, amorphous phosphate films are thought to form during the phosphating treatment, in particular in conjunction with oxidizing agents such as nitrate or nitrite. The passivating effect may be further reinforced by a final rinse with chromate solution. 

Clean zinc-coated steel is a suitable base for paint or adhesive systems, but the first treatment may be different from those used on uncoated steel. Chemical conversion coatings (chromate, phosphate, or oxide types) and primers have been specially formulated for first treatment of zinc-coated steel. Some passivated surfaces (e.g., chromated) are not suitable for phosphating... 

A number of electrochemical treatments are used widely to produce inorganic conversion coatings on a metal, the surface layers including oxides, chromates, phosphates and their mixtures. Depending upon the process, these treatments may be carried out under open circuit or they may be driven by an impressed current. 

Makeup fresh water in recirculating water systems must be added to replace losses from entrainment (drift, or windage), evaporation losses, and blowdown. Windage losses can be estimated as 0.1 to 0,3 percent of the recirculation rate for induced-draft towers. If makeup water introduces dissolved salts (hardness) which will otherwise accumulate, a small amount of water is deliberately discarded (blowdown) to keep the salt concentration at some predetermined level. A calculation is demonstrated in Illustration 7.11. Chlorine treatment of the water to control algae and slime and addition of chromate-phosphate mixtures to inhibit corrosion have been common in the past, but restrictions on discharges to the environment by the blowdown have led to the use of nonchromate inhibitors. Many other practical details are available [14, 15]. 

Chromate conversion solution consists of chromic acid H2Cr04 or H2Cr207, chromate salts and certain activator ions such as sulfates, chlorides, fluorides, phosphates and complex cyanides with pH around 1 2. As the solutions for chromate conversion treatment are acidic which cause the dissolution of Mg into the solutions as Mg, there is a local rise in pH in the immediate vicinity of the metal-solution interface. Mg ions combine with chromate ions to form a compound that is insoluble at the locally higher pH region. This compound precipitates on the metal surface as an adherent coating. Chromate conversion treatment is a very fast process (30-60 s) and can... 

Phosphate conversion (surface modification) The production of an electrically conductive metal phosphate on the surface of a metal hy wet chemical reaction. Example The use of zinc or manganese acid phosphate treatment of aluminum for corrosion protection. See also Chromate conversion. 

Applications. Chromate convrasion coaling treatments are used on five principal types of aluminum parts aircraft and aerospace structural conqxnients, coil (for construction applications such as guttering and siding), extrusions (for window and door fiames), heat exchan parts, and containeis (mainly beverage cans). A considoable amount of aluminum is also used in the automotive industry, but most receives a crystalline phosphate treatment because the aluminum is treated at the same time as the steel fiame. 

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