Rinsing phosphate coatings

The appHcation of this type of conversion coating can be by spray or immersion and is easily tailored to the needs of the user (see Coating processes). The number of stages may vary from two, ie, clean and phosphate then rinse, to as many as five, ie, clean, rinse, phosphate, rinse, and post-treat. The performance requirements dictate the number of operation stages as well as the need for a post-treatment. 

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. 

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 protective value of a phosphate coating is enhanced by a dip or rinse in an acid chromate solution. Joint Service Specification DEF-29 makes such a rinse mandatory for steel parts treated by an accelerated process, and optional after treatment by a non-accelerated process. Details of rinses are given in Section 15.2 (Table 15.10, p. 15 30). 

Alternating Current Impedance Utility in Evaluating Phosphate Coating, Phosphorus-Chromium Rinse, and Paint Performance... 

Uses Non-chrome seal rinse for use in final rinse stage of metal deaning and phosphatizing operation seals conventional Iron phosphate coatings by forming additional layer ot coating containing zinc Features Does not contain chromates... 

To rinse, wash the surface in clear, continuous running water to remove all traces of soluble salts which may cause blistering on the surface. The tank can be similar to the phosphating lank. It may, however, be coated with an anti-corrosive paint to extend its life. 

Fitzpatrick et al. [41] used small-spot XPS to determine the failure mechanism of adhesively bonded, phosphated hot-dipped galvanized steel (HDGS) upon exposure to a humid environment. Substrates were prepared by applying a phosphate conversion coating and then a chromate rinse to HDGS. Lap joints were prepared from substrates having dimensions of 110 x 20 x 1.2 mm using a polybutadiene (PBD) adhesive with a bond line thickness of 250 p,m. The Joints were exposed to 95% RH at 35 C for 12 months and then pulled to failure. 

A patchy form of rust that attacks paint films from underneath, can be caused by sweaty hands, residues from fluxes, etc. Examples of the latter include residues from phosphating and soluble salts (including those from unsuitable rinsing water) and they can manifest themselves on steel in the form of a creeping filiform corrosion, i.e. as progressing threads of rust which loosen the coating. This can be followed visually through transparent films. It occurs, however, only when the relative humidity of the surround-... 

After major surface contaminants have been removed, e.g. by wet abrasive blasting of hot-rolled structural steel, application of a thin coat of an etch primer greatly reduces the incidence of underfilm corrosion, presumably by eleminating localised areas of poor adhesion. Phosphate pretreatments followed by effective rinsing have a similar effect over cold reduced sheets. 

Radiometric Evaluation of the Effectiveness of the Chromic Acid Rinse Treatment for Phosphated Work , US Department of Commerce, Office of Technical Services, Rep. No. PB 111,397 (1952) A Study of the Effect of Chromic Acid and Chromic-phosphoric Acid Rinse Solutions upon the Subsequently Applied Paint Coatings , US Dept, of Commerce, Office of Technical Services, Rep. No. PB III, 578 (1954)... 

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