Passivation Overview
New Austenitic stainless steel is formed largely from recycled sources of lower alloys. Surface Finishing, welding, and other common processes add to the impurities impregnated in the surface. The R.W. Evans and Associates process, which employs non-toxic, non-corrosive, and biodegradable substances, utilizes a three-stage procedure to remove impurities and protect the system.
Corrosive mediums, such as Water-For-Injection and Clean Steam, accelerate formation of corrosion products in systems when surfaces are not properly prepared. Corrosion, once begun, is self-catalyzing and continuous (as shown below).
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| (Above left) W.F.I. System Red Rough - S/S Surface pit and corrosion debris at 500X; (Above right) Debris field magnified to 7500X |
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Nitic and/or Phospohoric acid "passivation," normally used in a 2-stage process, is a simple mineral acid treatment that removes only some free iron, leaving other contaminants in place. In addition, these 2-stage processes may damage the stainless steel substrate and generate considerable hazardous waste. Chelants, as used in the R.W. Evans and Associates proprietary three-stage process, however, are highly versatile. They "coordinate" around metallic ions and inactivate their ability to further react chemically, then carry them off and away from the stainless surface. Additionally, these are in the main, non-toxic and non-corrosive.
All project are individually tailored to encountered conditions. Each system is unique and so is the employed chemistry. While component chemicals may be similar, proportions, durations and temperatures will vary in each case. No two applications will be identical.
No single "cookbook" method will suffice. A "one-size-fits-all" approach cannot succeed because of the wide variations encountered in the "real world." This extends to weld qualities and degree of heat tint; the ferrite content of welds and system components; surface finishing techniques and other conditions. For rouge removal, there are many different concerns (including the causal sources). Each application must be customized.
It should be noted that these procedures are applicable for pre-operational mode, as well as for corrosion removal. In all cases only contaminant ions and corrosion products are addressed. The stainless itself and surface finish is undamaged (only contaminants and anomalies are removed). We do not utilize destructive mineral acid "pickling" approaches, hydrofluoric acid or ammonium biflouride which deplete chromium from stainless surfaces seriously affecting its corrosion resistance.
The first step of our 3-stage process uses a staged alkaline chelation with concurrent detergency. This step removes dust, debris, and oils as well as inclusions within the microstructure of the stainless steel. The second step includes a staged mild acidic chelation using applied reductants. Free iron, extraneous nickel and their oxides are removed. In addition, this step removes the heat-tint from welds.
The final step is the passivation stage. While the first two steps remove impurities and contaminants, the third step, not only engenders the formation of a viable passive film, it also promotes and nurtures the ongoing dynamic mechanism of maintaining its integrity.
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