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. Stainless steel surfaces can be affected by oil stains, scratches, rust, impurities, low melting point metal contaminants, paint, weld slag, spatter, linings, stainless steel chemicals, equipment, assembly, welding, weld testing, and process materials.
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 the use of acidic solutions to treat the surface of stainless steel, the main purpose is to remove the surface of the oxide skin, welding slag, oil and other impurities, to make the surface of the stainless steel more polished, uniform, and improve the quality of its appearance and assemblability. In the process of pickling, the commonly used acids include sulfuric acid, hydrochloric acid, phosphoric acid, etc. The specific selection depends on the composition of the stainless steel and the requirements of the treatment.
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.
Neutralization is required after pickling to remove residual acid to avoid damage to subsequent processes or the environment.
Pickling Features
- Dirt and scale removal: effectively remove surface dirt and oxides, and purify the surface.
- Improve surface quality: Increase surface finish and brightness.
- Enhances assemblability: cleaned surfaces are easier to machine and assemble.
Passivation of Stainless Steel
Stainless Steel Passivation is a process that improves the corrosion resistance of stainless steel by forming a dense oxide film on its surface. Passivation generally uses compounds containing chromium, cadmium, ferrous, manganese, zinc and other metals, these metal ions are deposited on the surface of stainless steel, forming a dense, stable, not easily dissolved oxide film. This film can effectively isolate the stainless steel surface from external media, delay the corrosion process and improve its corrosion resistance.
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.
Passivation Features:
- Increase corrosion resistance: the formation of a dense oxide film, effectively resisting the erosion of external corrosive media.
- Maintain surface finish: the surface of stainless steel after passivation maintains a high degree of finish and appearance quality.
- Colorless and transparent: most of the passivated stainless steel surface presents a colorless and transparent appearance without changing their basic tone.
Pickling vs. Passivation
- Principle is different: pickling is mainly through chemical corrosion to remove surface contaminants, while passivation is through the metal deposition to form a protective film to increase corrosion resistance.
- Application is different: pickling is suitable for removing surface dirt, oxide skin, etc., to improve the surface quality; passivation is suitable for improving the corrosion resistance of stainless steel, especially in the wet, corrosive environment is more widely used.
- The effect is different: the stainless steel surface is smoother and cleaner after pickling, while the stainless steel surface after passivation maintains the original appearance of the tone, and can extend its service life.
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