Some aspects of passivation are as simple as walking around the block.
Milt Stevenson Jr.Others, however, are part of a wild, uncharted region in the world of metal finishing. Consider the well-known facts first, then move on to less familiar ground.
Passivation, a metal-finishing process, usually refers to treating stainless steel. Its main purpose is commonly seen as removing surface-imbedded metallic contaminants, which would reduce the part’s corrosion resistance or interfere with its function. Most people recognize that passivation makes stainless steel parts more corrosion-resistant, and that’s about it.
The Main Theories
Passivation is a very complicated theoretical issue. The subject has been debated since Faraday proposed the initial theories in 1844 and more than one exists today.
The generalized film theory, according to Evans, states: “Most cases of passivity … appear to be attributable directly or indirectly to a protective film, although not always an oxide film.”
The electron configuration theory, also widely accepted, relates to the energy levels of metals (iron, chromium, nickel, and others) whose inner electron shells are incomplete and which have unfilled energy bands in the metallic state. It’s pretty complicated stuff!
Another important theory, proposed by Uhlig, states that “passivity is not an absolute property like melting point or heat of fusion. A metal may possess variable degrees of passivity.” (For details on all three theories, see H. H. Uhlig and R.B. Mears, “Passivity,” in their indispensable Corrosion Handbook.)
Finding Practical Applications
What happens in the research and theoretical spheres can have important down-to-earth ramifications. For instance, Uhlig’s proposal of degrees of passivity points to possible problems if care is not taken to apply the correct passivation process. In this regard, metal finishers need to know specification details, plus alloy and heat treatment.
It’s crucial to specify heat treatment because of the variables that exist. The solutions used to passivate stainless steel are usually nitric acid, varying in concentration from 20% to 50%, ranging in temperatures from 60°F to 150°F, and either containing or not containing sodium dichromate.
Unfortunately, one solution won’t do it all. Different solutions and processing parameters are required for different alloys. To be adequately informed, the finisher needs to know the alloy, the applicable specs, and the heat treatment condition if it applies. This applies particularly to the more troublesome alloys commonly in use, such as 303, 430, and 440, the 400 series as a class, and 17-4PH investment castings.
Troublesome Alloys
As a class, 400 series stainless must always, at the very least, be processed in dichromate-inhibited nitric acid. In the case of the higher carbon content 430 and 440, a much higher concentration (up to 50%) is needed.
Heat treaters should be warned to use close controls and consistency since the higher carbon content may significantly lower corrosion resistance. Due to the high carbon content, some metallurgists reportedly consider the 440s primarily a high-alloyed steel (rather than a stainless steel).
Most free-machining stainless steels contain sulfur or selenium, which improves their machining characteristics but may cause problems for finishers. Type 303 stainless is a good example: unless a higher concentration of nitric acid with inhibitors is used, dulling or etching can occur. Some dimensional loss is possible, and more severe cosmetic degradation is possible.
Experience and Communication are Critical
Recent increased use of investment castings has created·a new set of problems. To make passivated components pass the final humidity and salt-spray tests as required by military and ASTM specifications, engineers have found it necessary to pre-pickle with fluoride or permanganate-type solutions before the passivation treatment.
The problem is that some of these pre-pickles are potentially harmful to dimensions and can also have a detrimental effect on metallurgical grain size. Stringent internal controls help minimize these problems, and all pre-pickle treatments should occur before final machining. A seasoned, technically competent finisher will know these things, but communication between customer and finisher is important to prevent delays and wasted effort.
Sometimes, all the parts must be tested, and it may be necessary to reclean and repassivate several times. This is costly, especially when second or third process cycles are required. Machinists may be able to require the casting house to certify that 100% of the parts pass the required testing. This will ensure that no “harsh” pre-pickle steps are performed on precision machined parts.
Several recognized passivation tests are now widely used, including water immersion, high humidity, salt spray, copper sulfate, and ferroxyl. These tests determine the presence of free iron, while other tests, such as atomizer and solvent-ring testing, check for other surface contaminants.
While no single test can be applied to all stainless types, generally, the specifier dictates which tests are required to determine acceptable levels of passivity. The customer and finisher must communicate openly and thoroughly so that the finished product meets all performance requirements.
Milt Stevenson Jr. is Vice President and Chief Environmental Officer at Anoplate in Syracuse, New York. His father founded Anoplate in 1960, operating in a 104,000-square-foot facility. Milt is an active member of the finishing industry and its association and was named an NASF Fellow in 2019 for his lifelong work and dedication to the industry. Visit www.Anoplate.com.
