Chromating accelerated tests

Salt spray tests, humidity tests, and other accelerated tests, some usiag sulfur dioxide and carbon dioxide, have shown favorable results for tin—2inc ia comparison with 2iac, cadmium, and fin deposits. Chromating improves the performance. 

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.

Accelerated tests such as ASTM B 201 are very useful in evaluating the quality of chromate films on zinc or zinc-coated surfaces. The test proceeds until white corrosion products have formed and should not be allowed to continue until red rust forms. Only the time to formation of "white rust is relevant. The effects of impurities on the corrosion rate of zinc can be determined by humidity testing. 

These designators were determined by a series of tests, including the copper mirror test, a qualitative silver chromate paper test for chlorides and bromides, a qualitative spot test for fluorides, a quantitative test for halides (chloride, bromide, and fluoride), a corrosion test for flux residue activity, and a surface insulation resistance test at accelerated temperatoe and humidity conditions. 

Thus, a significant issue for chromate replacements is the excellent performance of CrCCs in accelerated tests and, even more particularly, in performance tests without paint. The corrosion protection performance of chromate generally... 

Chromate coatings add corrosion resistance and also act as a bond for paint and powder coatings mainly because of a thin, tightly adhering layer on the metal surface. In accelerated corrosion testing, the first sign of failure of the conversion coating is often a blackish appearance locally. 

Fig. 2. 36 Accelerated corrosion tests (for comparison with Fig. 2.3S) on (a) zinc and (b) cadmium I = SO2 gas 2 = sea mist 3 = warmth and humidity solid lines = nonchromated coatings dashed lines = chromated coatings (Strekalov and Be-rukshtis, 1965).

Alkali ferricyanides oxidize acid solutions of diethylaniline and other aromatic amines, as well as certain monoazo dyestuffs a change of color results and ferrocyanide is formed. This reversible redox reaction, proceeds very slowly and incompletely. However, if the ferrocyanide ions are removed as insoluble white zinc ferrocyanide, the oxidation proceeds rapidly. This acceleration is a result of raising the oxidation potential of ferricyanide ions through removal of ferrocyanide ions. The white zinc ferrocyanide is deeply tinted by adsorption of the colored quinoidal oxidation products of the amines, and thus affords a sensitive test for zinc. The test is especially useful in the presence of chromium and aluminum. It can also be used in other instances provided no other cations are present, which form colored precipitates with potassium ferrocyanide (Co+, Ni+, Fe+, Mn+ , Cu+ ). Traces of iron, which in practice are always to be reckoned with, do not interfere with the zinc test. Anions which oxidize the amines must be absent, e.g., permanganate, chromate, vanadate, persulfate, iodate. 

This particular experiment is designed to be an accelerated version of a standard immersion/weight loss test. This has been validated by White et al. [124] who have compared the multichannel approach with 10 channels to a standard immersion for a number of well-researched inhibitors including chromate, cerium nitrate, cerium iodate, and cerium salicylate as listed in Table 9.2. 

Tests for chlorine, bromine, and fluorine content in fluxes are required as per IPC-TM-650, TM 2.3.33 and TM 2.3.35. The presence of these materials cause a reduction in the SIR and can accelerate corrosion of the PCB conductors and assembly solder joints. A test for chlorine and bromine is conducted by placing a droplet of flux on a silver chromate paper, which turns yellow or white if the halides are present. Presence of fluorine is determined by adding a droplet of purple zirconium alizarin to the flux region. A change in color from purple to yellow indicates fluorine is present. 

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