JPS59211576A - Method for removing sprayed al or zn film - Google Patents

Abstract

PURPOSE:To remove locally and easily a sprayed Al or Zn film on steel in a safe and sure manner without scratching the steel by bringing a hygroscopic substance contg. an alkali soln. into contact with the part of the film to be removed. CONSTITUTION:A water absorbing or porous substance contg. an alkai soln. such as >= about 25% aqueous NaOH soln. is brought into contact with the necessary part of a sprayed Al or Zn film formed on the surface of a steel plate, and the plate is washed with water. Said substance is cloth, paper, cotton, sponge or other chemical product having a favorable wettability to said alkali soln. Only the necessary part of the film brought into contact with the alkali soln. is locally removed without scratching the steel plate.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for efficiently and reliably removing a thermally sprayed At or Zn (hereinafter simply referred to as thermally sprayed) coating only in a required range.

Although steel has excellent workability, it is susceptible to corrosion. As a countermeasure against this corrosion, various surface treatments have been carried out in the past, and among these, thermal spraying of At and Zn, which can be applied to a thick film (200 μm or more), is extremely effective as a countermeasure against corrosion of steel. Such thermal spraying is currently widely used for corrosion protection of tanks and other large steel plate structures such as water gates.

Corrosion protection by thermal spraying for tanks, etc. has traditionally been carried out by on-site thermal spraying, in which the tanks, etc. are manually sprayed after assembly has been completed, but recently, thermal sprayed steel sheets with excellent quality have been used in steel factories to improve efficiency. For this reason, the method of using pre-sprayed steel sheets is becoming popular as an alternative to the above-mentioned on-site thermal spraying construction because it is especially advantageous in terms of cost and efficiency.

When assembling the sprayed steel plates into a tank, etc., welding is mainly used as a joining method. For this welding, it is necessary to smoothly remove the sprayed coating near the area where the sprayed steel plates are to be sprayed. Must be. If welding is performed in the presence of a thermally sprayed coating, there is a risk of welding defects occurring.

Also, in magnetic particle flaw detection carried out to inspect tanks, etc., if the thermal spray coating is too thick, the defect detection ability will be poor, so the thermal spray coating must be removed from the inspection area.

Mechanical methods such as (1) scraping with a disk sander or belt sander, (2) removal by sand or grid blasting are generally used to remove such thermal sprayed coatings, but these methods have the following drawbacks. In other words, all of these mechanical methods are inefficient; in method (2), finer abrasive grains cause clogging of the core, making continuous work impossible and requiring frequent tool replacement; If the inverted V abrasive grains are made coarser, the grinding will extend to the base steel sheet under the advisory coating, resulting in scratches on the base surface. In addition, the production of dust due to grinding significantly deteriorates the working environment and poses health and safety problems. The planuto treatment method (2) is also practically impossible to remove a thermally sprayed coating with good adhesion within the limited number of steps 11J. In addition, when conducting open inspections required by various standards, etc., it may be necessary to remove the thermally sprayed coating on the welded part, but as shown in Figure 1, the thermally sprayed coating on the close contact area ( Since the weld bead (2) has an irregularly curved shape that protrudes along the weld bead (3), if it is removed by the mechanical method described above, the close contact bead will be significantly damaged.

The present invention solves the above-mentioned problems all at once, and can reliably remove the sprayed coating without causing any damage to the base (steel plate base, close bead), and is highly efficient and safe. , provides a cost-effective method for removing thermal spray coatings.

That is, the gist of the present invention is to bring a hygroscopic substance containing an alkaline solution into force contact only with the area of the At'i or Zn bath coating to be removed, and to remove the thermal sprayed coating from that area. A method for removing an At or Zn74 spray coating is characterized in that only the At or Zn74 spray coating is removed locally.

In the first place, At or Zn molten skin is a film formed by these particles tightly bonding with the steel sheet base.
Chemically, for example, if an alkali such as NaOH is reacted, it can be dissolved and peeled off. For example, in the case of A2 lytic film, it releases hydrogen and dissolves, forming an aluminate (:tMxo-y
Atxo3-zHso (Mek7)b mirate),
MAAo, 2. M3AtO3 (O/I/1-aluminate)) and becomes exfoliated.

Therefore, one possible method for removing the thermally sprayed coating is to apply and bring it into contact with an alkaline aqueous solution. Such chemical methods are obviously more advantageous than the mechanical methods mentioned above in terms of efficiency, cost, safety, etc., and can be said to be more unique in that respect, but in reality, the following The problem is that. In other words, the removal of the thermal spray coating actually needs to be carried out only in a very defined area, but since the alkaline aqueous solution has poor viscosity, simply applying it will cause the alkaline aqueous solution to remain on the thermal spray coating. It spreads easily, making it impossible to determine the area to be removed.

The first possible solution to this problem is a physical method of setting embankments around the area to be removed.
This is inefficient and economically difficult to implement1.

However, according to the present invention, if an alkaline water bath solution is included in a moisture-absorbing substance having a shape corresponding to the area to be removed from the thermal spray coating and brought into contact with the target area, the target bathing liquid ( The experiments conducted by the present inventors have shown that it is possible to prevent the alkaline solution from spreading outside the target range and to limit the dissolution and peeling of the sprayed coating to the target range under any conditions regardless of the direction of the drawing. became.

In the method of the present invention, it is possible to selectively peel off only the thermal sprayed coating on the target area, and then perform a removal process such as water washing or tawashing to remove the target area. This makes it possible to reliably remove the film locally.

In the method of the present invention, the alkaline aqueous solution to be included in the hygroscopic substance is firstly an aqueous solution of caustic soda (NaOH), and other examples include aqueous solution of ammonia = 7 (
NHJ), hydroxylation power)v Shiv) Mu(Ca(OH)x
), potassium hydroxide (KOH), and other aqueous solutions. The concentration of this alkaline aqueous solution cannot be uniformly defined because the activity varies depending on the type of alkali. In the case of NaOH, it is preferable to use an aqueous solution having a content of 25% or more. The degree of dissolution (removal) of the sprayed coating by this alkaline aqueous solution is not unrelated to the pH value of the alkaline solution. There is a relationship between these as shown in Figure 2. Therefore, when the target is an At solvent film, the pH of the alkaline aqueous solution should be set to 9.
That's all, ′-! The same is true when targeting Zni spray coatings (adjusting the pH to 12 or higher is effective).

Next, as the moisture-absorbing substance, a water-absorbing or porous substance that has good wettability with the alkali is suitable, and specifically, cloth, paper, cotton, and other chemical products such as sponge can be used.

Next, the effects of the present invention will be explained using examples.

A 150μ AA4 spray coating is applied to the steel plate using a gas wire method.
This was used as the test material. This sample material was tested under the same conditions (J
Tensile strength (adhesive force) of At sprayed steel plate obtained in 5H8301)
From the test results, it was confirmed that it had a 170'4 sea. This value is equivalent to or even better than that normally used.

This thermal sprayed steel plate was placed in a horizontal position, and as shown in Figure 3, the solvent film (2) was coated with prepared paper, cloth, cotton, and 03 types of hygroscopic materials (4/) (4 pieces x 4 J) (dimensions: 300 The Na0I-T aqueous solution (6) (saturated aqueous solution at room temperature) was absorbed by the stirring rod (5), and the mixture was left for 30 minutes.

Further, as shown in FIG. 4, the thermal spray coating (2) of the above thermal sprayed steel plate held vertically was subjected to the same natural treatment using cloth as the hygroscopic material (4). The hygroscopic substance (4) is fixed to the vertical sprayed coating surface by using adhesive tape (8) using epoxy adhesive (7) on its four sides.
There were three types: fixed with a permanent magnet (9).

After the experiment, all of the moisture-absorbing substances were removed, the sprayed coating was washed with water, and the status of the coating removal was investigated. As a result, in all of the above examples, the thermal spray coating in the area where the hygroscopic material came into contact was completely removed, and the Id and solvent coatings in other areas remained intact and intact. was.

As is clear from the above description, the method of the present invention can remove A on the steel plate base or weld bead without causing any damage.
It is possible to completely remove the thermal sprayed coating of T+Zn, and it can be used regardless of whether the thermal sprayed coating is oriented horizontally or vertically, and the contact with the aqueous solution can be kept within the desired range. No matter what shape the film is to be removed, the method of the present invention has the ability to consistently and stably remove the film in the desired direction. In addition to being much more efficient than conventional methods, the method requires only moisture-absorbing materials such as cloth and paper and an alkaline aqueous solution, so it does not require any special equipment and is cheaper to implement. The present invention has many advantages, and it can be said that the present invention greatly contributes to reducing the number of applications for anti-corrosion illumination for tanks, water gates, etc. and improving the efficiency and safety of the work.

[Brief explanation of the drawings] Figure 1 is a cross-sectional view showing the state of the solvent film on the welded part of a tank, etc. during welding assembly, and Figure 2 shows the relationship between the solubility of thermal spraying in an alkaline water bath and the OpH value of an alkaline aqueous solution. FIG. 3.4 is a diagram showing the relationship, and is a perspective view for explaining the natural method carried out as an embodiment of the method of the present invention. In the figure, ■ = base metal, 2: solvent film, 3: weld bead Applicant Sumitomo Metal 1m Industries Co., Ltd. Figure 1 Figure 3 C Figure 4

Claims (1)

[Claims] (1) A method characterized in that a hygroscopic substance impregnated with an alkaline solution is brought into contact with the part of the At or Zn sprayed coating to be removed, and the sprayed coating is locally removed at that part.
Method for removing t or Zn thermal spray coating.