Stainless steel contains a minimum of approximately 11% chromium composition, The chromium forms an extremely thin and hard chrome-oxide film to protect the surface of the stainless steel. The surface of stainless steel will be influenced by oil stains, scratches, rust, impurities, low melting point metal pollutants, paint, welding slag, spatter, lining plate, stainless steel chemicals, equipment, assembly, welding, weld inspection and the processed material.
Pickling and passivation of Stainless steel both are chemical treatment processes used for surface treatment of stainless steel after welding or exposure to high temperatures to clean the oxides and any other contamination.
Pickling and Passivation of Stainless steel can corrode in service if there is contamination of the surface. Both pickling and passivation are chemical treatments applied to the surface of stainless steel to remove contaminants and assist in the formation of a continuous chromium-oxide, passive film. Pickling and passivation are both acid treatments and neither will remove dirty material, grease, or oil.
Pickling of Stainless Steel
Stainless Steel 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.
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
The pickling process is used to remove a thin layer of metal from the stainless steel part. This is different from the descaling process, which removes visibly thick oxide scale from the stainless steel part.
The pickling process can be done in a tank or vat, or it can be done by immersing the stainless steel in a bath of acid. The most common acids used for pickling are hydrochloric acid and sulfuric acid.
Passivation of Stainless Steel
Stainless Steel Passivation is a process that is separate from pickling, which can be performed on its own or after pickling. Unlike pickling, the passivation process does not remove any metal. Instead, the surface of the stainless steel is treated with an oxidizing acid, to dissolve carbon steel, and sulfide inclusions and remove iron and other surface contaminants from the stainless. The acid further works to promote the formation of the chromium-rich passive film, which imparts the corrosion resistance quality. Whereas pickled steel will appear dull or etched, passivation that is done correctly does not affect the metal’s appearance.
Passivation is formed on the material by the formation of an extremely thin and complex surface oxide layer. The layer itself is vulnerable to damage from manipulation of the material during processing, fabrication, and even transport. The result is a loss of passivity in damaged areas, with a reduction in corrosion resistance as a consequence.
Pickling vs. Passivation
| Pickling | Passivation |
|---|---|
| Highly aggressive cleaning | Least aggressive & uses weak chemical acids. |
| Rate of material erosion is high & gives deep cleaning | Pickling removes dirt, grease, oil, & oxides. |
| The rate of material erosion is high & gives deep cleaning | Passivation does not remove any surface impurities. |
| Pickling is the first process for cleaning. | Passivation is generally carried out after pickling |
Pickling and Passivation Chemicals/Acids for Stainless Steel
The main chemicals for pickling & passivation of stainless steel (Austenitic Stainless Steel and duplex Stainless Steel) are:
- H2SO4 (Sulfuric acids)
- HNO3 (Nitric acid)
- HF (Hydrofluoric acid)
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 chemicals, temperature limit, and treatment time are given in the table from the ASTM A380 standard.
Similarly, for the passivation process- the chemical, temperature limit, and treatment time are given in the table from the ASTM A380 standard.
Passivation treatment is carried out using the HNO3 (Nitric acid) solution.
Stainless steel has a unique feature: It is self-healing. Due to the alloying elements in the stainless steel, a thin, transparent ‘passive layer’ is formed on the surface.
Even if the stainless steel surface is scratched or damaged otherwise, this passive layer, which is only a few atoms thick, instantaneously reforms under the influence of oxygen from air or water.
This explains why stainless steel does not require any coating or other corrosion protection to remain bright and shiny even after decades of use.
Codes and Standards for Pickling and Passivation
- ASTM A380 Standard Practice For Cleaning, Descaling, And Passivation Of Stainless Steel Parts, Equipment, And Systems
- ASTM A967 Standard Specification For Chemical Passivation Treatments For Stainless Steel Parts
- ISO 16048 specifies the methods most often used for passivation of corrosion-resistant stainless steel fasteners
- BS EN 2516 Passivation Of Corrosion Resisting Steels And Decontamination Of Nickel Base Alloys
- AMS 2700 This specification defines the engineering requirements for a process to assure the removal of free iron or other less noble contaminants from the surfaces of corrosion-resistant steel parts