What is the Pickling and Passivation Processes?
Pickling and passivation both are chemical treatment processes used for surface treatment of stainless steel after welding or exposure to high temperature to clean the oxides and any other contamination. This assists in the formation of the new chromium oxide layer and thus helps to regain its corrosion resistance that otherwise would be hampered due to oxides & contamination. Usually picking is followed by passivation treatment. Passivation does not remove any oxide or contamination but it helps to increase the thickness of the chromium oxide layer in an acidic environment (usually nitric acid or citric acid).
Pickling uses very strong acids such as H2SO4 (Sulfuric acids) and HNO3 (Nitric acid) that remove material from the surface of the material for longer periods while passivation is carried out using HNO3 (Nitric acid) only for short durations.
What is Pickling?
Pickling is a chemical treatment process for the removal of scales such as oxides from stainless steel material by immersion in a liquid containing sulfuric acid or hydrochloric acid. It is used to remove any high-temperature scale/ oxides & nearby low chrome from steel surface by the chemical application after welding. Pickling is an efficient process for the removal of scale, dirt, and oxides from materials. The scale that is removed results from hot forming, welding, or heat treating operations and from corrosion.
Hydrochloric and sulfuric acids are commonly used for ferrous surfaces. Hydrochloric acid, which attacks metal rapidly, is used cold. Sulfuric acid, with a slower rate of attack, is heated. Phosphoric acid, the slowest acting of all types, is used where it is important to obtain a steel surface free from carbonaceous smut.
A properly controlled pickling bath is much more efficient for scale and rust removal than mechanical abrasion. However, pickling must be followed by a thorough rinsing and neutralizing. Hydrochloric and sulfuric acids, unless thoroughly removed, can cause organic finishing difficulties. Pickling is applicable to sheet, sand, and die-cast aluminum and its alloys; copper and its alloys; iron and steel; stainless steel; magnesium and its alloys; and nickel and its alloys. Pickling paste is used for welding areas on larger parts where it is not practical to immerse the parts in the pickling solution. For smaller parts, pickling using immersion tanks is the best method.
Why do We need to do Picking & Passivation?
Stainless steel due to the thermal effect of the welding results in the formation of temper colors on the surface by depleting the chromium oxide layer. These areas are prone to have corrosion due to the lost Cr-oxide protective layer. To enhance the natural corrosion resistance and clean the surface of stainless steel material from free iron, oxide, and other contaminations- picking & passivation processes are used.
What are the Pickling Methods?
The main type of pickling methods are:
- Tank immersion pickling treatment- Gives uniform & maximum corrosion protection
- Spray pickling treatment- for special application. require trained personnel due to safety reasons.
- Circulation pickling treatment- Useful for pipe internals treatment
What is Passivation?
Passivation treatment for stainless steel after pickling has been finished. The parts are treated with acid mix (or pastes) or electrochemical processes, to clean the contamination and expedite the formation of the chromium oxide passive film. Thus passivation enhances the corrosion resistance of the stainless steel material after pickling to form a fresh oxide layer.
‘Passive’ means less affected by environmental conditions. Passivation treatment gives back stainless steel to its original state. As you can see in the above figure, it frees the surface from any free iron and expedites the natural Cr-oxide layer formation. The passivation process uses nitric acid (HNO3) chemical/ pastes that have to be applied on the material surface. The chemical reaction by these acids cleans the stainless steel surface from any free iron contamination (if not removed can be an initiation point for the corrosion) left after pickling. Process control is an important factor for successful surface treatment by passivation. Passivation does not remove any material during the treatment while there is material removal from the surface of the stainless steel in pickling.
Which are the Acids & Chemicals used in Pickling & Passivation?
Pickling is carried out using H2SO4 (Sulfuric acids) or HNO3 (Nitric acid) solutions at a temperature of 20 to 80°C (70- 180°F). The time for the treatment varies from 5 minutes to 45 minutes. The complete details about pickling chemical, temperature limit, and treatment time is given in the below table from the ASTM A380 standard.
Similarly, for the passivation process- the chemical, temperature limit, and treatment time are given in the below table from ASTM A380 standard. Passivation treatment is carried out using the HNO3 (Nitric acid) solution at the test conditions as follows:
What are the Benefits of the Pickling & Passivation Process?
- Pickling & Passivation removes surface contamination.
- Enhance the corrosion resistance of the metals.
- Minimize the risk of metal contamination.
- Increased product life.
What are the Standards for Pickling & Passivation?
- ASTM A380
- ASTM A967
- ISO 16048:2003