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Passivation of Stainless Steel
Passivation of Stainless Steel
Permanent corrosion resistance and passivity in production processes are the main reasons for using stainless steel. In addition to the decorative aspects, this can be influenced by the selection of alloy and surface treatment used.
Stainless steel is a composite material, consisting of the metallic substrate with a non-metallic surface, the passive layer, consisting mainly of chromium oxide.
The passive layer is formed on its own accord through the reaction of the chrome in the stainless steel with oxygen from the environment. In the case of intact stainless steel the passive layer covers the entire surface area completely and densely. It alone protects the stainless steel, which is otherwise not resistant to corrosion.
Passivation is a chemical process that forms and optimizes the passive layer without attacking the basic material. There are two common processes:
- POLINOX Passiv contains a strong oxidating agent and results in a rapid and complete formation of the passive layer
- POLINOX Protect and Protect TC are patented processes for optimizing an existing passive layer in its composition and structure, thereby considerably improving the corrosion resistance of stainless steel, or reinstating it within the framework of a renovation process.
POLINOX Passiv based on nitric acid with additives to improve the effect. It achieves a rapid formation of the passive layer following pickling or mechanical processing of the surface. The duration of the processing is about 30 minutes at room temperature. POLINOX Passiv is available as a liquid (POLINOX B Passiv) for immersion, sprinkling, wiping or through-pumping, and as a sprayable paste with good adhesive properties (POLINOX FL Passiv) for the treatment of large free-standing surfaces.
POLINOX Protect gives stainless steel surfaces superior corrosion resistance which cannot be achieved through other processes. The surface finish is unaffected. The application can be carried out on new surfaces and also for maintenance and renovation of corroded surfaces. Seam welds and scaled surfaces can be passivated by pickling or spraying without prior treatment.
POLINOX Protect TC is the combination of chemical treatment with a subsequent short heat teatment at temperatures in the range of 140°C – 250°C. This leads to a considerable strengthening of the passive layer, both in depth and structure. POLINOX Protect is a non hazardous substance; it is biologically degradable. Its usage is not subject to official approval or regulation. Processing takes place over a period of approx. 4 hours in an immersion bath (POLINOX Protect B) or by application to a surface or wiping (POLINOX Protect Cleaner).
POLINOX Protect and POLINOX Protect TC:
- can be used on any alloy with a chrome content of over 15% (lower Cr content requires modification)
- can be used on all surface finishes
- improves the resistance to pitting
- doubles the resistance to stress corrosion
- reduces the formation of crevice corrosion
- increase resistance to thermic discolouration to around 150°C
- removes ferritic contamination
- removes rust and corrosion products
- reinstates the corrosion resistance of surfaces which have been attacked
- is not a dangerous substance and is not subject to any restrictions of use
- can be used directly on weld discolouration and welds and in many cases replaces pickling.
- Vehicle and boat construction
- Machine cladding
- Switch boxes
- Plants in the chemical, pharmaceutical, food and drinks industries
- Swimming pool construction
- Mounting elements
- Medical technology
Mounting bracket of low alloy stainless steel, passivated with POLINOX Protect to ensure maximum corrosion resistance
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