Stainless steel pickling and passivation

Austenitic stainless steel parts, by welding, heat treatment, forging, and other high-temperature processing, the surface will produce a layer of oxide skin, which should be removed by pickling. Stainless steel passivation is generally associated with pickling; pickling must be passivated after forming a protective film, which can effectively extend the service life of the equipment.
Process Flow

Comparison of the effect before and after pickling:

Pickling

(1) Purpose
Stainless steel in the chromium, nickel, and other elements can promote the metal in the heating conditions to generate dense oxide skin. It is a kind of spinel structure (FeO₂, Cr₂O₃, or N₁O₂), and they are practically insoluble in a variety of acids (nitric acid, hydrochloric acid, or sulfuric acid, etc.), so it must be used in a very aggressive pickling solution. This solution should have the role of a solvent, can dissolve the oxide skin, and the role of an oxidizing agent (can oxidize the insoluble low-valent oxides into soluble high-valent oxides).
(2) Formula
General austenitic stainless steel pickling solution can be used in the following formulations:

Formula 1:

• Hydrogen fluoride (HF) 50-60 g / liter
• Iron sulfate (Fe₂(So₄)₃) 200-240 g / liter
• Temperature: 65-70 ° C
• Time: 20-30 minutes

After immersion, take it out and wash it with flowing water to make it neutral.
This formula can effectively remove the parts on the oxide skin without causing excessive corrosion of the base metal because iron sulfate removes the oxide skin after the metal surface has a passivation effect, and Fe₂(So₄)₃ and HF play a role in the formation of a fluorine complex so that the volatility of the solution is reduced for the relative safety of the operator. Still, the use of this pickling solution should be noted that in the process of operation to maintain the Fe₂(So₄)₃ and HF, The ratio of Fe₂(So₄)₃ and HF solution in the operation is 3:5:1 or more is appropriate, can often add Fe₂(So₄)₃ to make up for its consumption through the laboratory, the content of the combined acid is reduced to 10 grams/liter until.

Formula 2:

• Nitric acid (specific gravity 1.42) 20%
• Hydrofluoric acid 5%
• The rest is water
• Soak for 30-40 minutes at room temperature, remove, and wash with running water to neutralize.

Formula 3:

• Nitric acid 25%
• Hydrochloric acid (specific gravity 1.19) 1%.
• The rest is water.
• Soak at room temperature for 20-25 minutes, remove, and wash with running water to neutralize.

Formula 4: Pickling paste formula:

• Hydrochloric acid (specific gravity 1.19) 20 milliliters
• Nitric acid (specific gravity 1.19) 20 ml
• Water 100 ml
• Nitric acid (specific gravity 1.42) 30 ml
• Bentonite 150g
• Wash with flowing water after acid washing.

(3) Operation steps
1) Before pickling, check whether the parts have bruises, scratches, and other serious defects; if so, should be grinding and other methods to eliminate defects.
2) Before pickling, the parts should be cleaned to remove oil and dirt treatment so as not to affect the effect of pickling.
3) For Large and medium-sized parts using the brush method, the pickling solution is repeatedly and evenly brushed on the surface of the parts so that all parts can be evenly removed from the oxide.
Small parts should be immersed in the pickling solution tank; the pickling process should be frequently turned so that all parts can be uniformly removed from the oxide.
4) After pickling, large and medium-sized parts should be rinsed with water to clean the pickling solution and then painted with 3-5% aqueous sodium carbonate solution for neutralization, neutralization and then rinsed with water to clean the surface of the parts.
Small parts from the pickling solution tank should be immediately immersed in the water tank rinse to remove the surface of the acid, rinse into the 3-5% sodium carbonate aqueous solution into the tank for neutralization, neutralization and then rinse the surface of the parts with clean water.
(4) Pickling quality inspection

• 1) The pickled parts should be inspected strictly for appearance, and the surface should not be allowed to have the slightest oxidized skin or rust spots.
• 2) After pickling the surface of the parts, the residual liquid should be neutral and can be checked with PH test paper; the PH value should be 6-8.

Passivation

(1) Purpose
Austenitic stainless steel by 400 – 850 °C heating or welding in the heating zone or welding on both sides of the heated area of the grain boundary will precipitate high chromium content of carbide so that near the grain boundary of a thin layer of solid solution extreme chromium depletion, the potential decreases, become the anode of the microcell effect. The carbide itself and a little farther away from the chromium-containing solid solution constitute the cathode, so the neighboring areas’ grain boundary is susceptible to grain boundary corrosion.
Austenitic stainless steel surface passivation is due to the formation of a passivation film on the anodic surface; the potential rises so that the anodic reaction is blocked, thereby inhibiting the corrosion of the grain boundaries caused by the metal and alloy corrosion resistance to improve the performance of a surface treatment method.
(2) Passivation method
1) Dipping method
The dipping method is suitable for small parts. After pickling the parts submerged in the passivation solution tank, soak them for a certain period, and then take them out and wash them with water.
2) Brushing method
The brush method is suitable for large and medium-sized pieces; the passivation solution several times in a row with a brush or mop brushed on the surface of the pieces after pickling to keep the surface wet after 2-3 hours, and then rinse with water.
3) Passivation paste passivation method
Passivation paste passivation method is applied to a variety of pieces the passivation paste with a brush or mop is applied to the surface of the pieces after pickling 1-2 times to keep the passivation paste wet, after 2-3 hours, and then rinsed with water, and then wiped with a clean back to the wire or cloth, and finally rinsed with water or lukewarm water once.
(3) Passivation formula
1) Passivation solution formula
(a) Nitric acid HNO3 (specific gravity 1.42) 5%
Potassium dichromate K₂Cr₂O 72%
(b) nitric acid 42-50%, the rest is water, room temperature immersion time 15-30 minutes.
2) Passivation paste formula

• Nitric acid HNO₃ (specific gravity 1.42) 5 ml
• Potassium dichromate K₂Cr₂O 72 grams
• Water H₂O 95 ml
• Bentonite 100 grams

(4) passivation film quality inspection
1) passivation film should be a continuous, uniform, and dense compound; it should be silver-white on the surface of austenitic stainless steel without excessive corrosion. The thickness of the passivation film can be calculated by making a specimen measured by the elliptical polarized light analyzer. Generally, 30Å-60Å is better.
Note: Å is a common unit of measurement of light wavelength and molecular diameter, one order of magnitude smaller than a nanometer, i.e., one-tenth of a nanometer, 1/10000000000m.
2) To test the denseness of the passivation film, you can use the blue dot method of checking; that is, the test solution is applied to the clean passivation surface within 30 seconds to observe the surface blue dot situation. There are blue dots for unqualified.
Blue dot method test solution formula: potassium ferricyanide 10 grams + distilled water 50 ml + concentrated nitric acid (65% -85%) 30 ml, diluted with water to 1000 ml.

Precautions

• 1) The pickling solution should be tested and adjusted frequently; if the solution is too thick (due to water evaporation), water should be added in time, while the formula should be strictly controlled. Otherwise, it will be caused by the improper proportion of the composition of the products over corrosion.
• 2) The temperature and time of pickling and passivation operation should be strictly controlled, and the passivation temperature should not exceed 80°C. Otherwise, the passivation effect is poor, and the corrosion is aggravated.
• 3) Products must be rinsed thoroughly after pickling or passivation, and no residual acid or liquid should be left on the surface of the products to avoid corrosion.
• 4) Preparation of passivation paste should be added to water, then potassium dichromate, dissolved and stirred, then nitric acid, and finally bentonite clay.
• 5) Nitric acid, hydrofluoric acid, hydrochloric acid, and other corrosive media; the operation should wear good labor protection protective equipment, at the same time, keep the pickling passivation drugs from being mixed with other chemicals to prevent the chemical reaction caused by the explosion or combustion.
• 6) Passivation should occur within 6 hours after pickling or sandblasting.