What’s up guys? It’s Michael with Michael Talks Metal back this Thursday for video 124. Last week we showed how stainless steels might become sensitized … less resistant to corrosion than it might otherwise be. If you missed that video …click here
Today we are talking about passivating stainless steel.
Passivating ensures we get the most out of the corrosion resistance of a given alloy.
So what makes a stainless steel active or passive?
Stainless steel corrosion resistance comes primarily from the Chromium content of the alloy. But it also requires the formation of a durable layer of chromium oxide at the exposed surfaces. If the surface is contaminated during manufacture or the protective oxide coating is not fully formed, it is considered “active” and further chemical reactions with corrosive media can occur.
When the Chromium oxide is fully formed on a clean surface it resists any further corrosion reactions and the surface is described as passive.
Since contamination of a surface can easily occur during machining, welding, and grinding of parts. Where maximum corrosion performance of a part is required, a “Passivation” process is performed.
So, how do we passivate stainless steel?
In order to passivate the component’s surfaces, we have at least two steps in the process.
First is a cleaning to remove any oils or grease from fabrication that can inhibit the chemical reactions in the following step.
Next up is an acid treatment.
Typically a solution containing about 70% Nitric acid that will dissolve any iron or other contamination on the surface and provide for the formation of Chromium oxides. The acid solution has to be maintained and changed at proper intervals to remain effective and prevent discoloration or smutting of parts during the process.
Process temperature and acid concentration are modified depending on the alloy type. Not all parts require passivation and its important to remember that surfaces must be properly protected after passivation to prevent any later contamination.
Detailed part requirements come from specifications, prints, and purchase orders. The devil is in the details …So do not forget too CHECK THE SPECS!!!!
This is Michael with Michael Talks Metal. Thanks for watching, Michlin Metals is a full service, value added supplier and distributor of all things stainless steel, check the website for more info: www.michlinmetals.com. Thanks for tuning in, still here and haven’t subscribed, click here. Missed last weeks’ video, click here. This is Michael with Michael Talks Metal and I will see you in 2 weeks, we’re off next week so I will see you in April! I’m out.