Corrosion chromate conversion coatings

Chromate conversion coatings perform poorly in environments containing acidified chloride. In salt/S02 spray tests the substrate metal is heavily pitted after three to four days of exposure. In this work, a new coating was developed which itiproved the corrosion resistance of the conventional chromate coating remarkably. 

Almost all aluminum structures are painted with organic polymers for corrosion protection. The purpose of incorporating an inhibitor interface (chromate conversion coating) between the substrate and tlie paint film is to ensure protection vAien paints fail to perform( ). It has been generally acc ted that no matter what Icind of paint system, and how well it is applied, it always will have some porosity defects and will degrade with tine during service. 

Salt Spray Test Panels coated with the standard chromate conversion coating and CMT were compared with each other in their corrosion resistant properties in several ways. The conventional 5% NaCl/S02 fog chamber tests showed excessive corrosion and pitting within one week on chromate conversion coated (COC) 7075-T6 A1 alloy panels. The CMT coated panels were almost uncorroded and without any pits. The plates in Figure 1 show the conditions of the panels after 7 and 14 days exposure in this environment. Even after 14 days exposure the CMT panels were still far better than COC panels. 

The mechanism of developing corrosion protective properties in an inorganic coating principally consists of forming insoluble oxides on the netal surface. Additionally, oxides must have certain corrosion inhibition (redox) properties which can protect the nnetal substrate from corrosive species like Cl and 804 . In the case of chromate conversion coating, OCC, the oxides of aluminum and chromium have been responsible for their corrosion inhibitive properties which were derived from their soluble and insoluble portions of the... 

Post-Treatments. Although many post-treatments have been used over plated metals, chromate conversion coatings remain as the most popular. Chromates are used to improve corrosion resistance, provide good paint and adhesive base properties, or to produce brighter or colored finishes. Formulations are usually proprietary, and variations are marketed for use on zinc, zinc alloys, cadmium, copper and copper alloys, and silver (157). Chromates are also used on aluminum and magnesium alloys (158,159). More recently, chromate passivation has been used to extend salt spray resistance of autocatalytic nickel plated parts. 

Chromate conversion coatings are used widely on aluminum alloys as a pretreatment for painting, though in some applications, where noncondensing atmospheric exposure is expected, they may be used as the primary means of corrosion protection. Chromate conversion coatings are used on magnesium, cadmium, and zinc, and on galvanized steel to suppress the formation of white rust. 

A corrosion protection system should provide protection of the oxides and, in addition, should provide a good adhesive base for subsequent coating. The conventional corrosion protection system consists of alkaline cleaning and deoxidization of the surface followed by the application of a chromate conversion coating. 

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. 

Figure 28.2 shows the typical images of three Alclad panels (an aluminum alloy clad with nearly pure aluminum) tested with SO2 salt spray. The panels are (1) chromate conversion coated followed by priming with E-coat (2) chromate conversion coated followed by priming with Deft (chromated spray paint) and (3) plasma coated followed by priming with E-coat. The corroded area and corrosion width are also given in the figure. 

Although the panel with the plasma deposited film followed by priming with E-coat is visually better, the use of the corrosion width provides a method for quantifying the improvement in the corrosion performance. Also the factor of about 2 difference in corrosion width between the two chromate conversion-coated panels is difficult to obtain from the qualitative difference observed from the scanned images. It can be seen from this comparison of three panels that the use of the measured corrosion width makes the differentiation of corrosion performance much easier. This method of evaluating corrosion test results is used to determine if the combination of the two bests could indeed yield the better corrosion protection of aluminum alloys. 

Figures 28.3 and 28.4 depict corrosion tests results obtained for 2024-T3 and 7075-T-6 respectively. The first picture (from left), CC Deft, represents the control sample prepared by chromate conversion coating and chromated spray primer (Deft). The second sample, CC E-coat, is prepared by the same procedure for the CC Deft except that E-coat replaced Deft primer. The third sample is prepared by applying the same E-coat directly (without chromate conversion coating) on the...

Pitting corrosion away from the scribe was observed on almost all panels of both controls, which have chromate conversion coatings, after the corrosion tests. Pitting... 

Adhesion of the cathodic E-coat to the plasma polymer surfaces is an important parameter in the corrosion protection of A1 alloys. In general, the adhesion performance of E-coat applied to plasma polymers was found to be far superior to that of the control panels. A-Methylpyrrolidinone (NMP) paint delamination was not observed after 120 min for E-coat on plasma polymer surfaces as compared to a maximal time for complete delamination of 5 min for E-coat on chromate conversion coating [2B] CC/E panels. The adhesion performance of cathodic E-coat on the plasma polymer surfaces could not be differentiated by the conventional tape test (ASTM D3359-93B), since E-coat on all of the combinations... 

Figure 28.7 Schematic of corrosion process on chromated primer coated on chromate conversion coated A1 alloys surfaces.

In order to compare the relative extent of the damaged surface corrosion and of the pitting corrosion, the percentage of the corroded area in the tested surface is used, and summarized in Table 28.2. It is important to note that pitting corrosion was found only on chromate conversion-coated controls (one with chromated spray primer and another with E-coat). It is also important to note that E-coat directly... 

Figure 31.14 shows the Rp values of [2A] with three different chemical pretreatments and with a TMS plasma polymer on each of the three pretreated surfaces, as well as on the control [2A]CC surfaces (chromate conversion-coated 2A). It can be seen that the Rp values of [2A] were decreased to some extent by pretreatment of alkaline cleaning and were drastically reduced by alkaline cleaning plus deoxidization. As observed in the XPS results, the accumulation of Cu elements and removal of oxide layer on [2A] surfaces were presumed responsible for the reduction in corrosion resistance of these chemically pretreated [2A] panels. 

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