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Stainless steel pickling passivation process control

Jul 25, 2018

1.Pretreatment of pickling passivation

Stainless steel workpiece before pickling passivation, if there are surface dirt, should be through mechanical cleaning, and then oil degreasing.If the acid lotion and passivation solution cannot remove grease, oil surface will affect the quality of the pickling passivation, therefore oil removal cannot omit, lye, emulsifier, organic solvent and steam can be used, etc。

2.Control of CL- in acid lotion and washing water

Some stainless steel acid lotion or pickling paste with hydrochloric acid, perchloric acid,Ferric chloride and sodium chloride, chlorine ion corrosion medium as the main agent or agents to remove surface oxide layer, in addition to the grease with chlorinated organic solvent such as trichloroethylene, thereby preventing stress corrosion cracking is not appropriate。In addition, industrial water can be used for preliminary washing water, but halogen content should be strictly controlled for final cleaning water. Deionized water is usually used. Such as petrochemical austenitic stainless steel pressure vessel for hydrostatic test water, control the content of C1 - no more than 25 mg/L, if cannot meet the requirements, can be added to sodium nitrate in water treatment, make it meet the requirements, C1 - levels, can destroy the passive film of stainless steel, is pitting, crevice corrosion, stress corrosion cracking.

3.Process control of pickling passivation

Nitrate solution separately used to remove free iron and other metal contamination is effective, but to remove oxide, thick corrosion products, tempering membrane is invalid, such as general HNO3 + HF solution should be adopted, in order to convenient and safe in operation and fluoride can substitute for HF。HNO3 solution alone does not contain corrosion inhibitor, but when HNO3+HF acid is washed, Lan-826 is required. Use HNO3+HF pickling to keep the concentration at a ratio of 5:1 to prevent corrosion. High temperature should be lower than 49 ℃, such as, HF will evaporate.

Of passivation solution, HNO3 should be controlled between 20% - 50%, according to the electrochemical tests, HNO3 concentration less than 20% treatment of passivation film quality is not stable, easy to produce pitting, but also should not be greater than 50% HNO3 concentration, to prevent passivation.The one-step process of deoiled pickling passivation and passivation was adopted. Although the operation was simple and the working hours were saved, there would be corrosive HF in the pickling passivation solution (paste), so the final protective film quality was not as good as that of the multi-step method.

During pickling, the concentration, temperature and contact time of acid can be adjusted within a certain range. As the growth of the acid wash time, must pay attention to the change of the acid concentration and metal ion concentration, attention should be paid to avoid acid, titanium ion concentration should be less than 2%, otherwise it will lead to serious pitting. Generally speaking, increasing the pickling temperature will accelerate and improve the cleaning effect, but may also increase the risk of surface contamination or damage.

4.Control of pickling under stainless steel sensitization

Some stainless steel due to poor heat treatment or welding cause sensitization, using HNO3 + HF acid may produce intercrystalline corrosion, caused by the intergranular corrosion cracks at runtime, or cleaning, or in the subsequent processing, can enrichment halide, caused by stress corrosion. These sensitized stainless steels are generally not suitable for scaling or pickling with HNO3+HF solution. If this pickling is necessary after welding, use ultra-low carbon or stabilized stainless stee.

5.Acid cleaning of stainless steel and carbon steel components

For stainless steel and carbon steel (such as composite stainless steel tube and tube plate in heat exchanger with carbon steel shell), pickling passivation if using HNO3 or the serious corrosion of carbon steel, HNO3 + HF should then add appropriate corrosion inhibitors such as Lan - 826.

When the stainless steel and carbon steel assembly in the sensitized status, cannot use HNO3 + HF acid, glycolic acid can be used (2%) + formic acid (2%), corrosion inhibitor, temperature 93 ℃, time of 6 h + corrosion inhibitor or EDTA ammonium neutral solution, temperature: 121 ℃, time: 6 h, then wash with hot water and soaked 10 mg/L ammonium hydroxide + 100 mg/L of hydrazine

6.Post-treatment of pickling passivation

Stainless steel workpiece after pickling and flushing water, available containing 10% (mass fraction) NaOH + 4% (mass fraction) of KMnO4 alkali 1 raw potassium permanganate solution soak 5 in 71 ~ 82 ℃ ~ 60 min, to remove acid residue, and then rinse thoroughly with water, and dry。After the surface of stainless steel is acid-washed and passivated, there are flower spots or stain spots, which can be cleaned by fresh passivating liquid or high concentration nitric acid. Stainless steel equipment or components with final acid pickling passivation shall be protected by polyethylene film covering or wrapping to avoid contact between different metals and non-metals.

The treatment of acid and passivated effluent should comply with the national regulations on environmental protection. For example, fluorinated wastewater can be treated with lime milk or calcium chloride. As far as possible, the passivation solution does not use dichromate. If there is chromium containing wastewater, ferrous sulfate can be added for reduction treatment.

Pickling may cause martensitic stainless steel hydrogen embrittlement, such as the need to go through heat treatment of oxygen (heated to 200 ℃ heat preservation for a period of time).

The quality inspection

7.Quality inspection of stainless steel pickling passivation

The passivation film of the product is usually inspected on the sample because chemical inspection will destroy it. Examples of methods are as follows:

(1)titration test of copper sulfate

8g CuS04+500mL H20+2 ~ 3mLH2SO4 solution was used to drip into the surface of the sample to keep it wet. If no copper was precipitated within 6min, it was qualified.

(2)titration test of potassium cyanide of high iron

2mL HCl+1mL H2SO4+1g K3Fe(CN)6+97mL H20 solution was dropped on the surface of the sample, and the quality of the passivated film was determined by the number of blue spots generated and the length of time they appeared.

8.Application examples

8.1long series of passivated parts

The upper and lower ends of the plating pieces enter and leave the passivated solution successively when the long series of parts are passivated. On the other hand, when the solution is suspended in the air after passivation, the solution flows down from the upper end to the lower end. The surface of the plate at the lower end is more liquid than that at the upper end, and the chemical reaction time between the lower end and the solution is more. All of these make the lower end of the passivating film darker than the upper end. In order to reduce the color difference, it is recommended for such parts to enter and exit the passivated solution horizontally (hook the lower end with a hook) and prevent the anode from being too long.

8.2long strip parts are passivated

Strip parts passivation as passivation tank can hold, can take temporary measures, can use brick or wood processing a hold plating pieces of the box, box lined with plastic sheets, injection can be used after the passivation solution, using this method is convenient and can avoid uneven film layer or create cohesion marks or other product quality problems.

8.3planar parts passivation

Common plane passivation due to swing in the passivation bath edge parts in contact with the passivation solution than the middle part, the colour and lustre of passivation film caused this part uneven phenomenon, this problem is solved to compressed air mixing, the effect is very good. In order to improve the uniformity of the passivation film still need to pay attention to the current distribution uniformity in the process of galvanized, the edge of the plating parts shall be shielded when necessary, to prevent the parts coating roughness by current is too large, affect the colour and lustre of passivation film.

8.4surface passivation

Smooth surface passivated, because the surface is smooth, difficult to adsorption on the surface passivation solution, quickly lost, so the passivation in the solution and the residence time in air should be appropriately extended, otherwise the workpiece passivation membrane appear more light.

8.5easy water pockets passivation

The quality of the passivation film can also be ensured in this part.

8.6passivation of small parts

The whole bundle of the workpieces can be placed in a plastic basket to passivate, so as not to be out of the group due to vibration when passivation, fall into the slot to cause repair.

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