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Stainless Steel Pickling Passivation Method And Process

Writer: Kangding metal Pubdate:2018-04-06 Source:CNC Machining Factory
Description:Passivation of stainless steel Product name Passivation of stainless steel, Stainless steel processing, stainless steel surface treatment, stainless steel finishing 1, stainless steel pickling passivation method comparison Stainless steel e...

Passivation of stainless steel

Product name
Passivation of stainless steel,
Stainless steel processing, stainless steel surface treatment, stainless steel finishing

1, stainless steel pickling passivation method comparison
Stainless steel equipment and components pickling passivation treatment based on different methods of breeding a variety of methods, its scope and characteristics in Table 1

Table 1
Methods Scope of application Advantages and disadvantages
Dipping method For parts that can be placed in pickling tanks or passivation tanks, But not suitable for large equipment The pickling solution can be used for a longer period of time, resulting in higher production efficiency and lower cost; the large volume of equipment is filled with acid impregnation and the consumption of liquid is too large.
Spraying method Applicable to large and medium surface and local treatment Manual operation, poor working conditions, acid cannot be recycled
Paste method For installation or overhaul of the site, especially for welding positions Manual operation, poor working conditions, high production costs Spray method For the installation site, The liquid volume of the large container inner wall is low, Low cost, fast speed, but need to configure spray gun
System circulation method For large equipment, such as heat exchangers, shell processing Easy to use, acid can be reused, but piping and pumping are required
Circulatory system electrochemical method Can be used for parts,
Brush method can also be used to surface treatment of field equipment
The technology is more complex and requires a DC power supply or potentiostat

2, pickling passivation formulations exemplifypickling passivation formulations exemplify
2.1 General processing
(1) Pickling

Pharmacy HNO3 6% ~ 25% + HF 0.5% ~ 8% (volume fraction);
Temperature 21 °C ~ 60 °C; time according to need;
Or Pharmacy ammonium citrate 5% to 10% (mass fraction);
Temperature 49 °C ~ 71 °C; time 10 min ~ 60min.

(2) passivation
Drug HNO3 20% to 50% (volume fraction);
Temperature 49 ~ 71 °C; time 10 min ~ 30min;
Or temperature 2l ~ 38 °C; time 30 min ~ 60min;
Or Drug HNO3 20% ~ 50% + Na2Cr207H2O 22% ~ 6% (mass fraction);
Temperature 49 °C ~ 54 °C; Time 15 min ~ 30min;
Or temperature 21 °C ~ 38 °C; time 30 min ~ 60min.

(3) descaled pickling
Agent H2SO4 8% to 11% (volume fraction);
Temperature 66 °C ~ 82 °C; 6 inches between 5 min ~ 45min;
And agents HNO3 6% ~ 25% + HF 0.5% ~ 8% (volume fraction);
Temperature 21 °C ~ 60 °C;
Or HNO3 15% ~ 25% + HF l% ~ 8% (volume fraction).

2.2 Paste method paste
Pickling cream:
25% HNO3~+4%HF+7l% condensed water (volume fraction) and BaSO4, adjusted to paste.
Passivation paste:
30% HNO3 or 25% HNO3+1% (mass fraction) K2Cr2O7 and BaSO4 were adjusted to a paste. The surface is coated for 5 to 30 minutes and flushed to pH=7 with condensed water. For a single device, chemical passivation by hydrogen peroxide may also be used.
(2) Take the patent m of Shanghai Daming Iron Factory as an example.
Pickling passivation paste:
HNO38% ~ 14% (for passivation agent);
HFl0% ~ 15% (for corrosives);
Hard Moon S Magnesium 2.2%~2.7% (for thickener)
Magnesium nitrate 60% ~ 70% (as filler, improve adhesion and permeability);
Sodium polyphosphate 2.3% to 2.8% (as corrosion inhibitor);
Water (adjust viscosity).

2.3 Electrochemical treatment
The stainless steel workpiece to be treated as an anode, Potentiostatic control anodized, Or the stainless steel workpiece is first made cathode,Control the potentiostatic potential for cathodic treatment, The stainless steel workpiece is then used as an anode to control the potential for anodizing. And continue to change its constant potential for passivation treatment, the electrolyte solution using HNO3. After such treatment, the properties of the stainless steel passivation film are improved, and the corrosion resistance is greatly improved. The pitting critical potential (Eb) increased by about 1000 mV (in 3% NaCl) and the uniform corrosion resistance improved by three orders of magnitude (20% to 30% H2SO4 at 45°C).

3. Application range of stainless steel pickling passivation
3.1 Pickling and passivation of stainless steel equipment during manufacturing
3.1.1 Cleaning and Pickling After Machining Passivation Stainless steel workpieces often have iron chips on the surface after cutting. End of steel and cooling emulsions and other contaminants, Causes stains and rust on stainless steel surfaces. Therefore, degreasing and degreasing should be carried out, and then nitric acid cleaning should be used to remove both iron chips and passivation.

3.1.2 cleaning and pickling passivation before and after welding
Since fats and oils are the source of hydrogen, Oils that have not been removed form pores in the weld. Low-melting metal contamination (such as zinc-rich paint) can cause cracking after welding. Therefore, the surface of the groove and the sides within 20mm must be cleaned before stainless steel welding. Oil can scrub with acetone, Paint rust should be removed first with an emery cloth or a stainless steel wire brush, and then wiped with acetone.

Stainless steel equipment manufacturing No matter what kind of welding technology is used, it must be cleaned after welding. All welding slag, spatters, stains, and oxidation color should be removed. Cleaning methods include mechanical cleaning and chemical cleaning. The mechanical cleaning includes sanding, polishing and shot blasting. Carbon steel brushes should be avoided to prevent rust on the surface. To achieve the best corrosion resistance, dip it in a mixture of HNO3 and HF, or use a pickling paste. In fact, the combination of mechanical cleaning and chemical cleaning is commonly used.

3.1.3 Cleaning of Forging Castings
After stainless steel workpieces, such as forging and heat processing, the surface often has a layer of oxide skin, lubricant or oxide pollution, including graphite, molybdenum disulfide and carbon dioxide. It should be shot blasted, salt bathed, and multi-passed. For example, the US stainless steel turbine blade processing technology is:
Salt bath (10min) → water quench (2.5min) → sulfuric acid wash (2min) → cold water wash (2min) → alkaline permanganate bath (10min) → cold water wash (2min) → sulfuric acid wash (1rain) → cold water Washing (1 min) → Nitric acid washing (1.5 min) → Cold water washing (1 min) → Hot water washing (1 min) → Air drying.

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