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Surface Treatment of Stainless Steels: Acid Pickling, Passivation and Polishing

The pickling, passivation, mechanical polishing, chemical polishing and electrochemical polishing processes of some stainless steel products developed in recent years, and the corresponding surface treatment formulas and process schemes are given for different stainless steel grades and parts specifications.

Stainless steel is widely used because of its excellent corrosion resistance and good appearance. Stainless steel in the production process, inevitably go through annealing, normalizing, quenching, welding and other processing, the surface will often produce black oxide skin. Oxidation not only affects the appearance of stainless steel quality, but also on the subsequent processing of the product will have a negative impact, so the subsequent processing must be used before the pickling, polishing and other surface treatment methods to remove it. Stainless steel oxide NiO2, Cr2O3, FeO-Cr2O3, FeO-Cr2O-3Fe2O3 and other dense oxide, the removal of the difficulty of stainless steel grades and part specifications, manufacturing conditions vary, if the surface treatment is not appropriate, not only a waste of manpower, but also may cause stainless steel pitting, scrap. Stainless steel pickling, passivation currently see less research reports, the author on some in the production of simple and feasible stainless steel surface treatment methods are described below.

1. Stainless steel pickling

The main purpose of pickling is to remove stainless steel by high temperature operations (such as heat treatment, welding, forging, casting, etc.) produced by the oxide film, revealing the original surface of stainless steel. Stainless steel parts with different shapes and specifications, sometimes require different pickling methods, such as dipping, scrubbing, pickling paste pickling, etc..
Stainless steel pickling, passivation, including the following steps: pickling before acceptance – oil removal – water washing – pickling – water washing – in addition to hanging ash – water washing – passivation -Water washing-neutralization-water washing-drying-hydrogen removal.

1.1 Oil removal

When the surface oil is serious, should use organic solvents such as gasoline and other cleaning. Less oil can be used when chemical degreasing, electrochemical degreasing, etc.

  • (1) Chemical oil removal: sodium hydroxide 60-80g/L, sodium phosphate 20-40g/L, sodium carbonate 20-40g/L, sodium silicate 3-10g/L. Total alkalinity 65-100g/L, temperature 70-90 ℃, time to oil removal until.
  • (2) Electrochemical oil removal: sodium hydroxide 30-50g/L, sodium phosphate 20-30g/L, sodium carbonate 20-30g/L, water glass 3-5g/L. Total alkalinity 40-60g/L, temperature 60-80℃, current density 3-10A/dm2, time: cathode 3-5Min, anode 1-5Min, electrode material with steel plate or nickel-plated steel plate.

Large stainless steel products, such as composite plates, large pressure vessels, etc., can not be put into the oil removal tank, you can use a rag or mop dipped in room temperature degreaser, laundry detergent and other water solutions, wipe to remove oil. Batch of large, small size and light weight parts, the use of roller degreasing better. Put these parts into the roller with appropriate abrasives and degreasing agent for degreasing, good effect, low cost, easy to operate. However, it is not applicable to the easily deformed sheet parts.
The standard of clean oil removal is: after oil removal, water washing, water film for 30 seconds without cracking.

1.2 Pickling

According to the stainless steel material selection pickling solution. Austenitic stainless steel using a more aggressive solution pickling effect is better, and martensitic stainless steel using a less aggressive solution pickling effect is better. Stainless steel parts with different shapes and specifications, sometimes need to use different pickling methods.

1.2.1 Dip cleaning

Smaller parts can be immersed in the pickling tank dip. Austenitic stainless steel pickling process is shown in Table 1.
Table.1 Austenitic stainless steel pickling process

Recipe No. Sulphuric acid/g·L-1 Ferric sulfate/g·L-1/g·L-1 Nitric acid/g·L-1 Sodium nitrate/g·L-1 Hydrofluoric acid/g·L-1 Hydrochloric acid Sodium chloride/g·L-1 Sulfonated coal/g·L-1 Water Temperature/ Time/Min
1 150180 4050 1020 Allowance 6080 58
21) 80100 7080 4050 1.01.5 Allowance 4060 1530
3 300330 130140 Allowance 6570 2030
4 5(%) 5(%) 25(%) Allowance 3040 1015

Note:1) Sandblasted parts time is 1-3Min. 
Martensitic stainless steel pickling can use the following process:

  • (1) Nitric acid 20% (φ), hydrofluoric acid 8%-10% (φ), the balance of water, 3-10Min at room temperature.
  • (2) Nitric acid 140-150g/L, phosphoric acid 110-120g/L, water balance, 5-10Min at room temperature.

1.2.2 Scrubbing

Larger stainless steel workpiece, such as composite plates, large pressure vessels, etc., can not be put into the pickling tank pickling, you can use acid-resistant cloth dipped in Table 1 solution, repeatedly wipe for pickling. The weld seam and heat-affected zone, can be coated with the solution, scrubbing with water sandpaper. Weld inclusions of flux and slag can be used to pick up tools such as steel needles.

1.2.3 Pickling with pickling paste

For composite plates, pressure vessels and other large-size workpiece, can also be coated with pickling paste for pickling. Commonly used formulas are as follows:

  • (1) Nitric acid 10%-15% (φ), hydrofluoric acid 15%-20% (φ), water 60%-70% (φ).
  • (2) Nitric acid 5%(φ), sulfuric acid 10%(φ), hydrochloric acid 40%(φ), water 45%(φ).

Add clay to the two solutions and mix into a paste.

1.2.4 Other pickling methods

Larger cylinders and tubes can be pickled by applying a small amount of pickling solution on the surface and making it rotate. Some large parts, such as pressure vessel welds, can be used steel wire wheel, grinding wheel grinding, and then coated with pickling paste or pickling liquid pickling.
It is necessary to pay special attention to the following points of pickling:

  • (1) Martensitic stainless steel can not be pickled with a more corrosive formula, otherwise local corrosion will occur. But austenitic stainless steel can be acid pickled with the process of pickling martensitic stainless steel.
  • (2) Formulation 4 of Table 1 contains chloride ions, pickling speed is faster, only for the initial pickling of thick oxide film. After the initial pickling and then the second pickling with other pickling process. Chlorine ion is easy to cause stainless steel pitting corrosion. When preparing pickling solution try to use distilled water to prevent the mixing of chlorine ions.
  • (3) Scrubbing, rolling, pickling paste pickling, the pickling surface should always be kept wet, not dry, otherwise the local acid concentration is too high, will produce a cloudy pattern.
  • (4) In a variety of pickling process must pay attention to strict control of the pickling temperature, time and other process conditions, otherwise it will be under pickling, over pickling, and even the occurrence of workpiece corrosion scrap.

1.3 In addition to hanging ash

Stainless steel pickling surface is generally residual layer of hanging ash. There are two methods to remove the hanging ash as follows:

  • (1) Electrochemical removal of hanging ash. That is, with electrochemical degreasing tank, anodic treatment. Anode current density of 3-5A/dm2, time 5-10Min.
  • (2) Air / water gun flushing hanging ash. That is, with a pressure of 2-5kg/cm2 of compressed air and tap water mixed with high-speed water to rinse the workpiece.

1.4 Lowering activity

When pickled with a solution containing hydrochloric acid, chloride ions are adsorbed on the surface of stainless steel, especially the inclusions, hairline and other defects, even with distilled water washing is difficult to clean. Chlorine ions make stainless steel susceptible to corrosion, it is difficult to passivate, so the chlorine ions must be removed, the process is called down live processing.
Can be cleaned with hot distilled water 2-3 times, and then hot 20%-50% sodium sulfate solution dip wash. Even after these treatments, the effect is often less than ideal, so the high corrosion resistance requirements of stainless steel products, as far as possible without the solution containing hydrochloric acid pickling.

1.5 Hydrogen removal

Tensile strength of stainless steel greater than 100kg/mm2, pickling should be in the air circulation furnace at 180-200 ℃ for hydrogen removal treatment 2-3h, to prevent hydrogen embrittlement.

2. Passivation

Passivation is a certain solution to the surface of stainless steel treatment, so that the surface of stainless steel to form a corrosion-resistant passivation film, dissolve the surface of iron and other metal impurities to improve corrosion resistance. After pickling, stainless steel should be passivated according to the following process [3,4,5,6].

  • (1) Nitric acid 300-500g / L, 30-60Min at room temperature.
  • (2) Nitric acid 300-500g/L, sodium dichromate 20-30g/L, 30-60Min at room temperature.

In order to avoid residual passivation solution on the surface of stainless steel, resulting in future corrosion, passivation should be in 30-50g/L sodium carbonate solution at room temperature and neutralize the treatment for 1Min.

3. Polishing

Commonly used polishing are mechanical polishing, chemical polishing, electrochemical polishing 3 kinds.

3.1 Mechanical polishing [2]

Mechanical polishing refers to polishing and batch finishing with polishing wheels, polishing belts, etc. Batch finishing is with the polishing agent in the abrasive on the surface of the parts of the polishing action, so that the surface is smooth, to achieve the polishing effect. Polishing can be obtained after the surface roughness of 0.4μm below the mirror bright.
The shape of simple parts can be used hard polishing wheel or polishing belt, the shape of complex parts with soft polishing wheel polishing. Batch of small parts in large quantities with batch finishing method. Batch finishing and roller roller polishing, vibration machine vibration finishing, centrifugal machine centrifugal finishing and rotary finishing methods.
Mechanical polishing of the surface grinding amount is very small. It is difficult to polish the rough surface. This requires pre-polishing, using grinding wheel, grinding belt with grinding paste for grinding, divided into rough grinding, medium grinding and fine grinding. After fine grinding, the surface roughness can reach 0.4μm.
In order to achieve some other requirements, such as de-oxidation, deburring, de-slagging, matting and other purposes, sometimes sandblasting, shot blasting and brush light with steel wire wheel surface treatment, polished surface with stainless steel wire wheel can better avoid iron pollution.

3.2 Chemical polishing [7,11]

Chemical polishing is to immerse the parts in a suitable solution, because the solution is faster than the concave parts of the surface protrusion dissolved, so that the surface flattening, to achieve the purpose of polishing. Generally speaking, the polishing ability of chemical polishing is poor, only a small amount of brightness can be improved. But the mechanical polishing than the labor-saving, time-saving, and can be polished on the inner surface of small parts. Recently, it has also been reported that the use of the addition of brightening agent can make 18-8 austenitic stainless steel surface polishing to mirror brightness. Chemical polishing process is shown in Table 2, but the following points should be noted.

  • (1) Formula 2 polishing speed, low finish, should be used for pre-polishing.
  • (2) Chemical polishing produces an active surface, the workpiece must be passivated to ensure corrosion resistance.
  • (3) For vascular stents, screws and other high-volume small parts, should take mechanical stirring to make polishing uniform.
  • (4) For stainless steel composite plate and other products with a large surface polishing treatment, pay special attention to make the polished surface to keep wet, and should be fully washed after polishing, to prevent uneven surface brightness.

Table.2 Chemical polishing process

Recipe No. 1 2 3 4 5 6 7
Sulphuric acid (98%)/ml·L-1 2 300
Hydrochloric acid (37%)/ml·L-1 5 60(g/L) 120180
Nitric acid (65%)/ml·L-1 10 132(g/L) 65(g/L) 100 75 1535 6.5(%)
Phosphoric acid (85%)/ml·L-1 10 250(g/L) 600 2550 25(%)
Hydrofluoric acid(40%)/ml·L-1 25(g/L) 175
Oxalate/g·L-1 40
Polyethylene glycol (M≥6000)/g·L-1 2 20
Sulfosalicylic acid /g·L-1 0.3 10 1
Hexamethylenetetramine/g·L-1 2 3
Thiourea/g·L-1 4 2
OP-21/ml·L-1
Temperature/ 95 40 8090 90110 6070 2428 8090
Time/min 5 310 35 310 23 3036h 35
Applicable materials Austenitic stainless steel Austenitic stainless steel pre polishing Austenitic stainless steel Carbon steel, martensitic stainless steel, austenitic stainless steel Carbon steel, martensitic stainless steel, austenitic stainless steel Austenitic stainless steel Austenitic stainless steel

3.3 Electrochemical polishing [11,12,13,14,15,16,17,18]

Electrochemical polishing improves the reflective properties of the parts; improves corrosion resistance; reduces the surface hardness of machined parts; and reduces the coefficient of friction due to reduced surface roughness. Electrochemical polishing can also be used to remove parts burrs, etc. Compared with mechanical polishing, electrochemical polishing has the following characteristics.

  • (1) Mechanical polishing will produce a surface hardening layer and abrasive inclusions, reducing the corrosion resistance of stainless steel, while electrochemical polishing produces a passivated surface, increasing the corrosion resistance of stainless steel.
  • (2) Electrochemical polishing of the substrate has certain requirements, such as uneven metallographic organization will produce an uneven polished surface, deep scratches can not be polished flat. And mechanical polishing of the substrate requirements are much lower.
  • (3) On the shape of complex parts, wire, thin plates and small parts, electrochemical polishing is much easier than mechanical polishing.
  • (4) Electrochemical polishing than mechanical polishing productivity, but the large workpiece can not be put into the polishing tank, and requires a particularly large current, it is difficult to electrochemical polishing, the process is shown in Table 3.
  • (5) Electrochemical polishing of the workpiece surface current density must be uniform, if necessary, the need to use the shaped cathode, otherwise the surface brightness is not uniform.
  • (6) Electrochemical polishing current is relatively large, the fixture and the workpiece needs to have a large enough contact area and good contact, or local overheating will burn the workpiece.
  • (7) Some polishing processes for austenitic stainless steel can not be used to polish martensitic stainless steel, easy to produce corrosion.

Table.3 Stainless steel electrochemical polishing process

Recipe No. 1 2 3 4 5 6 7
Phosphoric acid (85%)/g·L-1 7080 5010 5060 42 4045 11 500(mL/L)
Sulphuric acid (98%)/g·L-1 1540 2030 3437 36 400(mL/L)
Chromic anhydride/g·L-1 34 10 50
Glycerol/g·L-1 1245 47 25 78
Water/g 3020 235 1520 11 2017 18 Allowance
Density/g·cm-3 1.64 1.65 >1.46 1.761.82
Temperature/ 35100 5070 5080 90140 7080 4080 5565
Current density/A·dm-2 35100 20100 20100 2080 4070 1030 2050
Voltage/V 68 1530 1020
Time/min 510 28 10 816 515 310 45
Applicable materials Carbon steel, martensitic stainless steel Carbon steel, martensitic stainless steel Austenitic stainless steel Austenitic stainless steel Martensitic stainless steel Austenitic stainless steel Austenitic stainless steel

4. Conclusion

Although there are many stainless steel pickling formulas, but the use must be careful. For different stainless steel products to choose their own suitable formula, if necessary, should be in the pickling before a small sample test, the effect is good and then put into production. This will ensure the quality of pickling, to avoid pitting and other defects. Corrosion resistance requirements of stainless steel, pickling must be carefully passivated, neutralization, to ensure quality. After mechanical polishing of stainless steel, should be pickled, passivation, remove surface impurities, in order to ensure that the surface does not change color and does not produce patterns. Chemical polishing to produce an active surface, should also be passivation treatment. Electrochemical polishing, the surface of the workpiece oxidation, stainless steel for the passivation state, no passivation treatment. Polishing, according to the requirements of the surface quality and the existing equipment, tooling conditions, choose the appropriate polishing method.

Author: Lin Baichun

Source: China Stainless Steel Pipes Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at [email protected]

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