JPS63270498A - Anticorrosive treatment of stainless steel - Google Patents

Abstract

PURPOSE:To efficiently provide high corrosion resistance to general-purpose stainless steel by immersing the stainless steel in an aq. soln. contg. a specified amt. of a Cr<3+>-contg. salt and by successively subjecting the steel to electrolytic treatment as the anode and electrolytic treatment as the cathode under specified conditions. CONSTITUTION:Stainless steel is immersed in an aq. soln. contg. 5-20% Cr<3+>- contg. salt such as chromium sulfate and it is subjected to electrolytic treatment as the anode at <=5A/dm<2> current density for <=10min electrolysis time to clean and activate the surface. The steel is then subjected to electrolytic treatment as the cathode in the same soln. at 0.1-10A/dm<2> current density for 1-30min electrolysis time. One or more kinds of ammonium salts such as 0.05-2.0% iron ammonium citrate, 0.01-0.5% ammonium alum and 0.01-2.0% ammonium sulfate may be added to the soln. and 0.1-10% in total of one or more among a Cu ion-contg. salt, an Ni ion-contg. salt and glucose may further be added. Thus, the corrosion resistance of the stainless steel in environment contg. neutral salt compds. is improved.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention prevents local corrosion such as pitting and crevice corrosion that occurs in neutral chloride environments, which is a disadvantage of stainless steel, and is a relatively inexpensive, general-purpose stainless steel. This invention relates to a method of imparting corrosion resistance to stainless steel through surface treatment so that it can withstand use in seawater, etc.

(Conventional technology) As a method for preventing local corrosion of stainless steel, Mg
, Zn, A1, etc. are used as sacrificial anodes, or consumable electrodes, for cathodic protection.However, the life of the electrodes is short due to wear and tear, and maintenance such as replacing the electrodes is time-consuming, and it is difficult to install the electrodes. It has the disadvantage that it is difficult to apply to difficult places, such as inside pipes. In addition, with this method, it is difficult to prevent local rusting of the wet and dry type that occurs due to the adhesion of salt in the atmosphere, seawater droplets, etc.

On the other hand, in order to suppress local corrosion of stainless steel materially, a high-order alloy composition such as high chromium, high nickel, and high molybdenum is required, which is extremely expensive. Furthermore, the environment in which local corrosion occurs is extremely wide-ranging, including seawater, groundwater, industrial water, and wastewater, and it is economically extremely difficult to prevent local corrosion from changing the composition of ingredients. The inventors of the present invention have previously applied
-8195), we have developed a surface treatment method that can be widely used without causing local corrosion even when using relatively inexpensive stainless steel.

The above-mentioned prior invention provides stainless steel with a certain degree of corrosion resistance, but further research has revealed that when anodic electrolytic treatment and cathodic electrolytic treatment are used together in a specific aqueous salt solution containing chromium ions, It was discovered that a significant improvement in corrosion resistance could be obtained, and the present invention was made based on this discovery.

(Object of the Invention) The present invention is a further improvement of the prior invention, and provides a method for efficiently imparting high corrosion resistance to general-purpose stainless steel.

(Structure of the Invention) The present invention involves immersing general stainless steel in an aqueous salt solution containing Cr3+ at a concentration of 0.5 to 20%, and
After cleaning and activating the stainless steel surface by pre-treatment (anodic electrolytic treatment) at a temperature of dm2 or less and an electrolysis time of 10 minutes or less, a current density of 0 is applied in the same treatment bath used for the anodic electrolytic treatment. .1~10A/dm2, electrolysis time 1~
This is an anticorrosion treatment method for stainless steel in which a 30-minute cathodic electrolytic treatment is performed to coat the steel surface with a reaction product compound of a metal contained in an aqueous solution to impart corrosion resistance.

As the aqueous salt solution, those listed below (1) to (2) can be used.

■An aqueous salt solution containing Cr3+ at a concentration of 0.5 to 20%.

■In addition to the above ■, iron ammonium citrate 0.05-2
．． 0%, ammonium alum 0601-0.5%,
Ammonium salts such as 0.01 to 2.0% ammonium sulfate are added singly or in combination.

■ In addition to the surface description ■■, Cu
Salts containing ions, salts containing Ni ions, and glucose added in combination or in combination.

■In addition to the above ■■■, as a trace additive, the concentration is 0.01~
Thiourea, starch, boric acid, glycerin and arsenic acid are added singly or in combination in the range of 0.045%.

The present invention is a method of combining anodic electrolytic treatment and cathodic electrolytic treatment using the aqueous salt solutions described in (1) to (3) above, and the liquid composition, concentration, current density, electrolysis time, etc. are appropriately selected within each predetermined range. can.

Note that even if only cathodic electrolytic treatment is performed without anodic electrolytic treatment, the surface properties will be improved to some extent, but in order to obtain particularly excellent corrosion resistance, it is necessary to use both anodic electrolytic treatment and cathodic electrolytic treatment. .

The reaction product compounds precipitated on stainless steel by the cathodic electrolytic treatment of the present invention are not clearly understood yet, but unlike the metals deposited by so-called plating, the reaction product compounds precipitated on stainless steel by the cathodic electrolytic treatment of the present invention are It is thought to be composed of a complex hydrate, and it is a thin colored film of several micrometers or less that does not dissolve or disappear even in seawater.

Although the theoretical explanation of the anticorrosive effect of the present invention is not clear,
Its effects have been fully confirmed experimentally.

In the known anodic electrolytic treatment, an aqueous solution of chlorides such as tin chloride and lead chloride, as well as other aqueous salts, are freely used, but in the present invention, an aqueous salt solution containing chromium ions and an aqueous salt solution containing chromium ions used in the subsequent cathodic electrolytic treatment are used. This is the most preferable method in terms of workability and economy, since the same aqueous salt solution is used for the anodic electrolytic treatment as a preliminary treatment by changing only the polarity of the stainless steel.

The reason why the liquid composition, concentration, current density, electrolysis time, etc. used in the present invention are limited is that the desired corrosion resistance can be obtained within these ranges.

In the examples, chromium sulfate was used as the Cr3+ salt, but the present invention is not limited to this, and other Cr salts can also be used.

In the present invention, iron ammonium citrate, copper sulfate, boric acid, and starch are effective in improving the hydrate coating properties of the stainless steel surface, ammonium sulfate is effective in stabilizing pH, and glucose is effective in stabilizing Cr3+.

Additionally, thiourea and arsenous acid are effective in low current electrolysis. A more detailed explanation will be given below using examples.

(Example) [Crevice corrosion test by seawater immersion] Since crevice corrosion is the most difficult type of local corrosion of stainless steel to prevent, a crevice corrosion test using seawater was conducted to confirm the effectiveness of the cathodic electrolytic treatment according to the present invention.

The shape of the seawater immersion T, P and its installation method are as shown in Figures 1, 2, and 3, with holes machined at equal intervals (4).
Sandwich stainless steel T, P, (1) (150 x 25 x 1 mm) between two PVC strip plates (3) (3) that have been treated with PVC, between the perforated parts (4) and (4), and insert the PVC bolts.
Insert the nut (5) into the hole part (4) and the 40kgfc
Tighten with a torque of m, stainless steel T, P, (1)
was fixed. In the same manner, PVC strip plates (3) (3) to which stainless steel 'J14T and P were attached were manufactured. T, P
, to which are fixed, are attached to the PVC frame (2) in parallel as shown in Figure 2.

Alternatively, as shown in FIG. 3, the end of the lower plate (3) of the PVC strip plate may be fixed with a PVC angle (2) and a PVC bolt bolt (5). T fixed as shown in Figure 2 or 3
, P, (1) is subjected to an immersion test in seawater.

The test examined the presence or absence of corrosion in the gap set between stainless steel T and P and a PVC strip plate. The main components of the stainless steel used in the test are shown in Table 1.
The electrolytic cell with anti-corrosion treatment is a 1℃ tank (200mm depth x 5
0mm width x 100mm length) and %U (200mm
(depth x 25mm width x 100mm length) was used.

The anticorrosion effect of the present invention is determined by the type and concentration of the electrolytic treatment solution, current density, and electrolysis time, but the electrolytic time at which the anticorrosion effect is saturated varies slightly depending on the type of solution used, the concentration of the solution, the current density, etc. Not necessarily constant. In the examples described later, the pretreatment electrolysis time was 5 minutes, and the cathode electrolysis treatment time was 1 minute.
It was set to 0 minutes.

Table 3 shows the results of the seawater immersion crevice corrosion test of test pieces electrolytically treated with various salt aqueous solutions as described above.

Additionally, Table 2 shows the results of the seawater immersion crevice corrosion test for untreated materials as comparative materials.

As is clear from Tables 2 and 3, the life of the test piece of the present invention, in which the stainless steel surface was treated using a combination of anodic electrolytic treatment and cathodic electrolytic treatment, was significantly different from that of the untreated material, which was within 3 days. Corrosion resistance is significantly improved. Among them are 430
304 shows a lifespan of more than 2 years, and 304 shows a lifespan of more than 3.5 years.
Some products are still undergoing testing, and it is expected that their lifespan will be further extended.

(The following are blank spaces) Table 1 Table 2 (Effects of the invention) According to the present invention, by treating stainless steel with a combination of anodic electrolysis treatment and cathodic electrolysis treatment, the surface of the stainless steel is coated with electrolyte. It is thought that an anti-corrosion coating is formed consisting of complex hydrates of various metal ions, and this anti-corrosion coating can significantly improve the corrosion resistance of stainless steel when used in neutral chloride environments, etc. becomes. Furthermore, since the anodic electrolytic treatment and the cathodic electrolytic treatment are performed using the same Cr ion-containing aqueous solution, it is an extremely efficient method.

[Brief explanation of the drawing]

FIGS. 1 to 3 are explanatory diagrams showing the mounting conditions for crevice corrosion tests T and P. 1. Stainless steel T, P, 4. Hole 2, PVC frame (angle) 5. PVC bolt/nut 3, PVC strip plate

Claims (4)

[Claims] (1) Stainless steel with a concentration of 0.5 to 20% Cr^3^
Immersed in an aqueous salt solution containing +, current density 5A/dm^2
Below, an anodic electrolytic treatment is performed for an electrolysis time of 10 minutes or less,
After cleaning and activating the surface, electrolysis time is 1 to 30 at a current density of 0.1 to 10 A/dm^2 in the salt aqueous solution.
An anticorrosion treatment method for stainless steel characterized by cathodic electrolytic treatment for 30 minutes. (2) The aqueous salt solution contains iron ammonium citrate 0.05
~2.0%, ammonium alum 0.01~0.5
%, ammonium sulfate, 0.01 to 2.0%, or a plurality of ammonium salts. (3) The aqueous salt solution has a concentration of 0.1% to 10%, and
The method according to any one of claims 1 to 2, which contains one or more salts containing U ions, salts containing Ni ions, and glucose. (4) An aqueous salt solution with a concentration of 0.01 as a trace additive
The method according to any one of claims 1 to 3, containing one or more of thiourea, starch, boric acid, glycerin, and arsenic acid in the range of 0.5%.