JPH06955B2 - Chemical cleaning method for stainless steel surface - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an improved chemical cleaning method for a stainless steel surface for dissolving and removing oxide scale formed on the stainless steel surface.

[Conventional technology]

Considering the cause of rust and corrosion on the surface of chemical tanker lead stainless steel in the construction process, (1) The structure of stainless steel partially changes by cutting, bending or welding.

(2) Almost all of the spatter slag during welding causes rust, and leaving it may lead to pitting (pinhole).

(3) Paint splashes from the hatch and manhole, and adheres to the stainless steel surface to contaminate it.

(4) Iron dust, sea breeze, and dirt from a grinder enter the tanker, causing "corrosion".

It is of course not only for quality control of the product that the stainless steel surface of the onboard tank constructed in this way is left and the product is loaded while the stainless steel surface is dirty, but it is also passivated by acid cleaning not only for suppressing the subsequent corrosion. It is necessary to add a film and return to stainless steel that can exhibit corrosion resistance on its own. Its role is acid cleaning.

[Problems to be solved by the invention]

For acid cleaning for the purpose of scale removal as a part of the acid cleaning, a mixed acid solution containing nitric acid, oxalic acid and hydrofluoric acid as main components has been conventionally used. However, such an acid solution has the following drawbacks. I got it.

(1) The structure of the welded part of stainless steel changes due to the effect of heat, but especially the part that is affected by heat at around 650 ° C is highly sensitized, so acid corrosion due to acid solution is severe and it is a practical problem.

(2) Common iron and foreign matter can be removed relatively easily,
Since the effect of removing the spatter-slag is poor and it may remain locally, it is necessary to remove it beforehand with a grinder or the like.

Therefore, there is no need to worry about acid corrosion of the heat-affected portion due to welding, and it is necessary to provide an acid cleaning method that can also remove sputter slag. As a result of repeated studies by the present inventors,
The present invention has arrived at the present invention by finding an excellent cleaning method as follows.

[Means for solving problems]

That is, the combined use of hydrogen peroxide, phosphoric acid and hydrofluoric acid does not add an acid corrosion inhibitor, and the acid corrosion is remarkably suppressed even in the welded part which is affected by heat (sensitization) at about 650 ° C. It also made it possible to remove spatter slag from the weld.

That is, the present invention is an oxide scale produced on the surface of stainless steel by applying a paste-like acid solution obtained by further adding an adhesive agent to a mixed solution of hydrogen peroxide, phosphoric acid and hydrofluoric acid to the surface of the stainless steel. The present invention relates to a chemical cleaning method for a stainless steel surface, which comprises dissolving and removing

Since the dissolving power of the oxide scale by this acid mixed solution depends on the mixing ratio of hydrogen peroxide, phosphoric acid and hydrofluoric acid, hydrogen peroxide (30% 1.5 to 3 volumes, phosphoric acid (as specific gravity 1.7) 4.5 to 7 volumes and hydrofluoric acid (as 30%) 1
A mixing ratio of 3 volumes is preferred.

The pressure-sensitive adhesive in the cleaning liquid used in the method of the present invention is for making the acid solution into a paste, and examples of the pressure-sensitive adhesive that can be used include alkaline earth metal compounds, colloidal silica powder, and activated clay powder.

There are various materials other than the above as the material that gives tackiness, but since the present invention contains hydrogen peroxide as a cleaning agent, the organic tackiness agent is oxidatively decomposed by the hydrogen peroxide. Therefore, it cannot be used because it loses its effectiveness as an adhesive and hydrogen peroxide is consumed as much, so that its effectiveness as a cleaning agent decreases. Therefore, the material that can be used as the adhesive is not decomposed by hydrogen peroxide unlike the above-mentioned materials, and therefore a material that gives viscosity to the cleaning liquid is used.

The above-mentioned pressure-sensitive adhesives, that is, alkaline earth metal compounds, colloidal silica powder, activated clay powder, etc., can be treated by a mixed solution consisting of hydrogen peroxide, hydrochloric acid and a fluorine compound which is a cleaning agent in addition to imparting adhesiveness. However, it is not dissolved or altered to reduce the tackiness.

As an alkaline earth metal compound, magnesium fluoride,
As the colloidal silica powder, for example, calcium fluoride is NIPSEAL VN3 (trade name, manufactured by Nippon Silica Industry Co., Ltd.), and as the activated clay powder, galephone earth (trade name, manufactured by Mizusawa Chemical Industry Co., Ltd.) can be used.

The concentration used should be such that the paste-like acid solution does not drip when applied evenly to the surface of stainless steel.

In the cleaning according to the present invention, a mixed acid solution prepared in a paste form is uniformly applied to a stainless steel surface of a chemical tanker lead having an oxide scale adhered thereto by a spraying acid resistant pump or a brush to dissolve and remove the oxide scale.

The scale dissolution mechanism by the pasty acid solution used in the present invention is not clear, but hydrogen peroxide, phosphoric acid, hydrofluoric acid and the like alone have almost no dissolution ability, but these are mixed solutions. In that case, since it exhibits a strong dissolving ability with respect to the oxide scale, it is presumed that these three factors act synergistically on the oxide scale with some action to enhance the dissolving ability.

〔Example〕

Example-1 The following test was conducted in the chemical cleaning method of the stainless steel surface. This test is an acid corrosion test of a stainless material using the paste-like pickling solution shown in Table 1. The test material for stainless steel is SUS 3
04 Steel sheet delivered (as received) and SUS 304
A corrosion test was carried out in the following manner using a sensitized steel plate (650 ° C. × 2 hr).

Put the paste-like pickling solution in a flat polyethylene dish until the test piece (30 mm × 40 mm × 0.5 mm) sinks and soak it for about 16 hours, then remove the test piece, rinse with water and dry it, then weigh it before and after the test. The corrosion amount was determined from the difference and the corrosion state of the test piece surface was observed, and the results shown in Table 1 were obtained.

For comparison, the nitric acid in paste form shown in Table 1,
Using a mixed acid solution containing hydrofluoric acid and oxalic acid as main components, the amount of corrosion was determined according to the above-mentioned example and the corrosion state of the surface of the test piece was observed. The results are summarized in Table 1.

As is clear from Table 1 of the test results, in the acid solutions of the present invention of the examples, the corrosion amount of the test piece is very small, the corrosion state of the surface of the test piece is uniform and the surface is not corroded at all.

On the other hand, in the conventional acid solution of the comparative example, when the test material (SUS 304 steel plate) was sensitized, the corrosion amount increased about 10 times as compared with the acid solution of the present invention, and the corrosion state of the test piece surface was Since the occurrence of pitching is observed, if there is a sensitized portion in the material to be cleaned, the same corrosion phenomenon occurs, which is a practical problem.

Example-2 The paste-like acid solution shown in Table 2 was sufficiently applied to the welded joint portion of SUS 317L clad steel, and after about 16 hours, the acid solution application portion was washed with water and dried to remove the scale of the welded portion. The corrosion state was observed and the results shown in Table 2 were obtained.

For comparison, the scale removal state and the corrosion state of the welded portion were observed using the paste-like mixed acid solution containing nitric acid, hydrofluoric acid and oxalic acid as shown in Table 2 according to the above examples. The results are summarized in Table 2.

As is clear from Table 2 of the test results, the scale of the welded portion was completely removed by the acid solution of the present invention of the example, and the corrosion state of the acid solution applied portion was uniform corrosion over the entire surface, and there was no rough skin. can not see.

On the other hand, with the conventional acid solution of the comparative example, the removal of spatter slag at the welded portion was incomplete and remained. As for the corrosion state, some local corrosion was observed near the weld.

〔The invention's effect〕

 The method of the present invention has the following effects.

(1) It is not necessary to add an acid corrosion inhibitor because it has an acid-dissolving ability only for oxide scales and has a function of forming a protective film for stainless steel to suppress acid dissolution.

(2) Compared with the conventional acid solution, the acid corrosion of the welded part, which is affected by heat at around 650 ° C, is significantly suppressed.

(3) Even with spatter slag in the weld, which was difficult to dissolve and remove by conventional acid cleaning, the dissolving power was significantly improved and efficient cleaning was possible.

INDUSTRIAL APPLICABILITY The method of the present invention is suitable for a chemical surface treatment that can be applied to stainless steel equipment and devices.

Claims (1)

[Claims] 1. Hydrogen peroxide (as 30%) 1.5-3 volumes,
Phosphoric acid (as specific gravity of 1.7) 4.5 to 7 volume and hydrogen fluoride (3
(0%) to a mixed solution consisting of 1 to 3 volumes, a paste-like acid solution obtained by further adding an adhesive is applied to the stainless steel surface to dissolve and remove the oxide scale formed on the stainless steel surface. Characteristic stainless steel surface chemical cleaning method.