Chromate conversion coatings corrosion resistance

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. 

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. 

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. 

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. 

Deoxidized surfaces of [7B] with a plasma polymer coating ([7B] (Dox)/T) showed higher polarization resistance than the chemically deoxidized surfaces without a plasma polymer. This indicates that the added corrosion resistance offered by plasma polymer films is much higher than that of the barrier-type oxides, formed after chemical cleaning, alone. As compared to the chromate conversion-coated surfaces ([7B] CQ, the deoxidized and plasma polymer-coated ([7B] (Dox)/T) surfaces showed higher Rp values, suggesting that these surfaces have higher corrosion resistance. 

Zeolite coatings have been investigated as corrosion-resistant coatings for aluminum alloys in aerospace applications. The currently used chromic acid anodization and chromate conversion coatings are effective but release hexavalent chromium, which is... 

It has been recently shown that as-synthesized template-containing high-silica MFI coatings on aluminum alloys have superior corrosion resistance to chromate conversion coatings in strong acids, bases, as well as pitting aggressive and the in situ... 

From the viewpoint of corrosion resistance, chromate conversion coatings still play an important role [106] even though a replacement of the toxic chromate is desirable. Rare earth metals are promising superseders for the chromium. They can be incorporated in the layers and enhance the corrosion resistance. 

The enhanced corrosion resistance of phosphated steel can be attributed to two phenomena the increased paint adherence and the chemical passivation of the metal surface by an insulating barrier film of phosphates. A difference between phosphate and chromate conversion coatings is that the later are thought to function as... 

After plating, a chromate conversion coating is usually applied which gives the coating its iridescent green/brown appearance and adds to its corrosion resistance. 

Chromate conversion coatings are produced by chemical treatments which— as the name indicates—convert the zinc surface to form a complex surface layer usually O.S-3 p,m thick, containing chromium hydroxide, zinc hydroxy-chromate, and zinc chromate. Films range from thin with a clear and bright appearance to thicker, yellow-iridescent, brown, or drab appearances. Appearance varies with bath formulation, process parameters, film thickness, and substrates. Corrosion resistance increases mainly with thickness. Film characteristics are given in Tables 1.6A and B. 

Environmental conditions, particularly time of wetness and temperature, determine the leaching rate. In natural environments the leaching rate is commonly low. Pollutants in the atmosphere, particularly chloride ions, also increase the rate of deterioration of the film. Chromate conversion coatings provide good corrosion resistance in a mild atmosphere, such as indoor atmospheres, and surface appearance. They also provide a good base for organic films. 

In a mild atmosphere, such as indoors, chromate conversion coatings provide good corrosion resistance and surface appearance. Chromate conversion coatings also provide good bases for organic films. 

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