Background:
Stainless steel is an alloy of iron and chrome with more than 50% iron and more than 10.5% chromium with small amounts of other alloying elements. The key to stainless steel being stainless is that when the minimum levels of chromium has been reached or exceeded, a thin and tenacious passive layer will form on the surface acting as inert protective layer. This layer is not indestructible and do get damaged during handling, use and fabrication. It is important to repair a damaged passive layer and to maintain it.
The following discussion is based on the best practices described by the ASTM Standard A380 for cleaning and restoring passivity of stainless steel surfaces.
According to the standard “Passivation is the process by which a stainless steel will spontaneously form a chemically inactive surface when exposed to air or other oxygen-containing environments. It was at one time considered that an oxidizing treatment was necessary to establish this passive film, but it is now accepted that this film will form spontaneously in an oxygen-containing environment providing that the surface has been thoroughly cleaned or descaled”. The key is therefore to clean the material to such an extent that no contaminants are present and to allow the passive layer to reform and repair.
The standard is very comprehensive in describing different chemical and other treatments to clean all scenarios of surface contamination and dirt. The standard also identifies specific conditions and it is advised that the full standard be studied for high risk applications and complicated cleaning of stainless steel process systems and equipment. In this discussion the focus will be on cleaning stainless steel surfaces exposed to contamination and dirt during normal handling, fabrication and operating environments. The discussion will also address general chemical cleaning of stainless steel surfaces.
The removal of soluble salts, corrosion products, and free iron and other metallic contamination resulting from handling, fabrication, or exposure to contaminated atmospheres.
In the annexures of the standard reference is made to the cleaning-passivation of stainless steel with nitric acid solutions with the purpose to enable the removal of soluble salts, corrosion products, and free iron and other metallic contamination resulting from handling, fabrication, or exposure to contaminated atmospheres. The standard prescribe the following procedures for the different grades to this purpose.
Table 1 is a summary of the information contained in the annexures of the standard. Take note that this is a simplified version of the procedures described in the standard and should be regarded as a guideline only. The table relates different grades of stainless steel in specific conditions to different chemical solutions with its respective process temperature and time. Following these procedures should render the material free of metallic and chemical contaminants after general handling and fabrication.
| Material Grade | Condition | Solution | Temp. (C) | Time (Min) |
|---|---|---|---|---|
| 200, 300, and 400 Series containing Cr 16 % or more (except free-machining alloys) | annealed, cold-rolled, thermally hardened, or work-hardened, with dull or non-reflective surfaces | HNO3 20–50 % | 50–70 _______ 20–40 | 10–30 ______ 30-60 |
| 200, 300, and 400 Series containing Cr 16 % or more (except free-machining alloys) | annealed, cold-rolled, thermally hardened, or work-hardened, with bright-machined or polished surface | HNO3 20–40 % plus Na2Cr2O7· 2H2O, 2–6 weight % | 50–70 _______ 20–40 | 10–30 _______ 30-60 |
| 400 Series containing Cr 16 % or less (except free-machining alloys) | annealed or hardened with dull or non-reflective surface | HNO3, 20–50 % | 45-55 _______ 20-40 | 20-30 ______ 60 |
| 400 Series containing Cr 16 % or less (except free-machining alloys | annealed or hardened with bright machined or polished surfaces | HNO3 20–25 % plus Na2Cr2O7· 2H2O, 2–6 weight % | 50-55 ______ 20-40 | 15-30 ______ 30-60 |
| All free-machining alloys in the 200, 300 and 400 Series | annealed or hardened, with bright-machined or polished surfaces | HNO3 20–50 % plus Na2Cr2O7· 2H2O, 2–6 weight % | 20-50 | 25-40 |
Table 1 Procedures for cleaning stainless steel surfaces after handling, fabrication and environmental exposure.
Refer to the full standard for exclusions and other specialised and critical applications.
The general cleaning of stainless steel surfaces and assemblies.
The standard also describes procedures of chemical cleaning with solutions other than the nitric acid solutions discussed above. The purpose of this is aimed at general cleaning of stainless steel surfaces, but also the cleaning of assemblies containing components made from stainless steel as well as components from carbon steel. Table 2 mainly takes condition into account. It contains chemical cleaning solutions for annealed components or welded assemblies that might be sensitised.
| Material Grade | Condition | Solution | Temp. (C) | Time (Min) |
|---|---|---|---|---|
| 200, 300, and 400 Series (except free-machining alloys | Fully annealed only | citric acid, 1 weight % plus, NaNO3, 1 weight % | 20 | 60 |
| 200, 300, and 400 Series 200, 300, and 400 Series (except free-machining alloys) | annealed, cold-rolled, Fully annealed only | ammonium citrate, 5–10 weight % | 50–70 | 10-60 |
| Assemblies of stainless and carbon steel (for example, heat exchanger with stainless steel tubes and carbon steel shell) | sensitised | inhibited solution of hydroxyacetic acid, 2 weight % and formic acid, 1 weight % | 93 | 6h |
| Assemblies of stainless and carbon steel (for example, heat exchanger with stainless steel tubes and carbon steel shell) | sensitised | inhibited ammonia-neutralized solution of EDTA (ethylene-diamene-tetraacetic acid) followed by hot-water rinse and dip in solution of 10 ppm ammonium hydroxide plus 100 ppm hydrazine | Up to 120 | 6h |
Table 2 Procedures for general cleaning stainless steel surfaces and assemblies.
Conclusion:
Passivity of the stainless steel surface is of utmost importance in ensuring the required corrosion protection and thus longevity of any stainless steel product. It is therefore important to clean surfaces with various methods to ensure the effective removal of metallic and chemical corrosion agents and contaminants after handling, fabrication or as routine general cleaning. A wide range of methods are described in the standard. It would seem that chemical cleaning offers the most effective solution. The discussion above summarised the information contained in the standard concerning chemical cleaning as to act as broad guideline for use in general engineering applications. However, for more specialised applications and requirements it will be beneficial to study the complete standard for all conditions and special processes. It is also important to remember that all chemicals are potential safety and health hazards. Please ensure that all safety measures are in place when applying these methods.
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