To ensure a fabrication or components corrosion resistance is optimized prior to delivery, it should undergo a passivation stage. Thorough pre-cleaning is essential to ensure grease, soil and scale to not inhibit the natural materials ability to form the inert passive layer. Traditionally carried out with corrosive high oxidising agents, all surfaces are either sprayed or immersed in a solution which dissolves Iron and readily oxidises the Chromium. This rapidly forms the inert layer of the material and creates the critical surface property relied upon in service. Several methods of passivation are employed depending on the alloy, pre-treatment and type of application. Both ISO 15730 and ASTM A967
Stainless steel passivation specification details each process in depth and a summary of the recommendations is shown below.
Several solutions are detailed under varying parameters, to ultimately remove iron contamination and aid the chromium to oxidise, forming the inert passive layer which protect the underlying layers. Generally, higher resistance is obtained by increasing the chromium oxide to iron oxide ratio which may be carried to varying degrees, depending on the alloy, methods, conditions employed, and the demands of the application. The solutions that recommended the ASTM A967 specification are based on meeting test criteria which are also detailed. The processes are categorized as follows:
Nitric Acid: The most commonly used process employing a high oxidising acid which dissolves iron and oxidises chromium.
Citric Acid: Due to health and safety, these processes are becoming more widely employed. Not such an aggressive oxidising agent, the process absorbs iron and reduces the potential of iron salts re-precipitating on the surface following subsequent rinsing and neutralisation.
Alternative Methods: The specification also details acceptable alternatives both chemical and electrochemical which can effectively pass the one of the suggested test methods.
- Water Immersion Test
- High Humidity Test
- Salt Spray Test
- Copper Sulphate Test
- Feroxyl Test
ESCA & Auger Spectrometry
The test methods detailed above are evaluated on effectiveness either to resist a corrosive environment for a predetermined time or as in the Feroxyl test which (as it is very sensitive) detects for iron. As mentioned above by higher levels of resistance are obtained by increasing the chromium oxide to iron oxide at the surface and this is further aided by an increase in the passive layer depth. ESCA spectrograph can give some indications of chromium and iron peaks at the surface, although the ultimate test is performed by Auger Spectrometry profile depth analysis which clearly details specific ratios and oxide depth. The down side to this test method is the cost and the only small test coupon may be analysed.