Stainless Steel is known for its corrosion resistance due to a passive layer on chromium oxide on the surface which is “self-healing” in the presence of oxygen. It is an extremely versatile product because of this however after welding and fabrication, because there is a heat affected zone, there are steps that need to be taken in order to restore the passive layer.
A Closer Look at Pickling and Passivating Stainless Steel
In this blog we discuss the pickling and passivating of stainless steel.
1. Pickling Stainless Steel
When steel is heated by welding or other means to the extent that a heat tint or oxide scale layer is visible, the layer below that has been depleted of chromium, thereby making the steel less resistant to corrosion. Pickling steel is the process of applying an acid solution to remove heat affected zones along with the underlying chromium reduced layer from the stainless steel. Pickled steel is free from surface carbon steel contamination and embedded iron particles. It typically leaves a dull, matte grey finish. Simply stated, pickling removes the heat affected layer of stainless steel and prepares the surface for passivation.
2. Passivating Stainless Steel
Passivation is a chemical treatment process where the stainless steel is treated with an oxidizing acid. Passivation dissolves carbon steel, and sulphide inclusions and removes iron and other surface contaminants from the stainless steel surface. At the same time, the acid promotes a chromium-rich thin but dense passive film (oxide protective layer) formation. This passive film imparts corrosion resistance quality. Passivation of stainless steel is performed using nitric acid. Similar to pickled steel, passivated steel does not affect the metal’s appearance.
3. Stainless Steel Passive Layer
It is a well-known fact that the passive layer of stainless steel will naturally return in the presence of oxygen so many fabricators do not passivate. One must realise though that is a process that occurs over a certain time period and should the environment be a highly corrosive one it might be best to passivate to ensure this passive layer is restored timeously.
Contact Euro Steel
Some of our branches stock and supply pickling and passivating acids so be sure to contact your nearest branch for more information and prices www.eurosteel.co.za/contact Please note pickling and passivating is dangerous and should always be done in a well-ventilated area with the correct PPE procedures and equipment. For expert advice on correct procedures and information for pickling and passivating please visit www.sassda.co.za