technical case study
The ‘acid test’ when it comes to effective passivation
Post-fabrication treatment of stainless steel products, or damaged and contaminated surfaces remains critical to ensure that the material lives up to customer expectations. Stainless steel surfaces remain in pristine condition due to a very thin, tenacious chrome-oxide film that develops uniformly and continues when the material is exposed to oxygen in the environment. This film is stable and, most importantly, passive.
T he passivity makes stainless steel surfaces inert and, as such, it does not readily react with the environment, even when in a corrosive environment. This property comes from the chrome content in the material since the more chrome stainless steel contains, the stronger the passivity and the corrosion resistance. The passive film is not a coating that needs to be maintained. It forms naturally and all it requires to remain strong, is unimpeded access to oxygen, simply meaning that the surfaces must be kept clean. Pickle we must When stainless steel is produced at a mill and ready for dispatch, the passive film will be in optimum condition and pristine. However, along the process of the manufacturing value chain, a lot of things will happen to the material that will damage and impair the passive layer. This includes multiple instances of transport and handling, exposure to ferrous and carbonaceous contamination; exposure to polluted atmospheres and the
activities of fabrication such as cutting and forming. It is important
to treat the surfaces after fabrication to ensure the removal of all embedded iron and metallic debris and dirt. This will allow chrome to have free access to oxygen to build an effective passive film. Some fabrication activities do not only impact the integrity of the passive film but also affect the material underneath the passive film and close to the surface. Heat treatment and welding are such examples. At high temperatures, heat oxide layers will form near the surface of the material. These layers are chrome depleted and cannot build and sustain a protective passive film. It needs to be removed to expose fresh material that can form a proper passive film. Heat scale and most other surface defects can be removed by either mechanical methods or chemical
21 Issue 2 – 2023
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