JPS6047347B2 - Method for forming alumina film with peeling resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming an alumina coating on the surface of a metal product by subjecting a metal product such as heat-resistant steel to hot-dip aluminum plating and then heat-treating the product.

Conventionally, as schematically shown in FIG. 1, a hot-dip aluminum plating layer 2 is formed on the surface of a base material 1, an aluminum diffusion layer 3 is formed in the middle thereof, and then heat-treated to coat the surface with N.

O. (indicated by a black circle) is bonded with FeNCr (indicated by a white background), etc. to form an alumina film 4, and
A method of forming an alumina film is well known, which aims to improve the oxidation resistance and corrosion resistance (halogen resistance) of heat-resistant steel by forming an aluminum diffusion layer 3 in which aluminum is diffused into the base material 1. . However, this alumina coating 4 contains N at the initial stage of its formation.

O. N is surrounded by FeNCr, etc., which is a heat-resistant steel component, but as it is exposed to a high-temperature oxidizing atmosphere and oxidation progresses, N becomes N. It is consumed by the formation of 03 and internal diffusion, and A1.

Fe)Cr, etc., which played the role of a binder in 03, started to oxidize (FeNCr).

03, the entire surface is Al.

O. , (Fe, Cr). 03 mixture layer 5, which peels off from the aluminum diffusion layer 3.

Therefore, the properties of the heat-resistant steel, which had strong oxidation resistance and corrosion resistance due to the effect of the alumina layer 4, are greatly reduced, and in particular, there is a problem in that its corrosion resistance is extremely reduced. Therefore, the present invention forms an alumina coating that has superior peeling resistance than conventional ones, and can effectively maintain the oxidation resistance and corrosion resistance of heat-resistant steel etc. over a long period of time. The purpose is to provide a method.

Therefore, in the alumina film forming method according to the present invention, Zn)Mg)Ni, Ca
After forming an aluminum plating layer containing Zn, Mg, etc. on the surface of the heat-resistant steel, primary and secondary heat treatments are performed to form an alumina coating. ZnA120 has adhesion to the interior due to the reaction between Al2O3 and ZnO or MgO on the surface.

, MgAl2O. It is characterized by forming a layer of composite oxide consisting of a spinel structure, which prevents the alumina coating from peeling off, and at the same time further improves oxidation resistance and corrosion resistance due to the spinel structure itself. There is. The method of the present invention is shown in FIG. 2 in comparison with the conventional method shown in FIG.

That is, in the method of the present invention, first, Zn. ．． Mg.
．． Nj. ．． A base material 10 such as a heat-resistant steel is placed in an aluminum alloy bath containing 3 to 2% of at least one of the elements Ca and maintaining the bath temperature at 700 to 950°C.
Hit? By immersion, an A1 diffusion layer 12 and a plating layer 11 in which an additive metal element such as Zn is mixed in A1 are formed. Z
nsMg,. Ni. ．． Among the divalent metal elements of Ca, at least one metal element is added in an amount of 3 to 2%, because if the amount is less than 3% by weight, a surface coating having an effective spinel structure cannot be formed. This is because the alumina layer naturally peels off, and if it exceeds 2% by weight, the melting point of the bath decreases, which adversely affects the diffusion of Al.

Further, the bath temperature is set at 700 to 950°C, as in the conventional method, in order to form an effective diffusion layer.

Next, the A1 plating layer 11 is subjected to primary and secondary heat treatment to form a mixed film 14 of ZnO and ZTIAI, O4 having a spinel structure (when the additive element is Zn) on the surface of the alumina layer 13. . When the mixed film 14 and the alumina layer 13 are exposed to a high-temperature oxidizing atmosphere for a long time, the oxide ZnO, etc. of the added metal element and Al, O3 combine to form a composite oxide metal layer 15 having a spinel structure. It grows and covers the thinned alumina layer 'l'13' due to the bonding of Al and O3.

This composite metal oxide layer 15 exhibits excellent peeling resistance due to its adhesion to the base material 10, and also exhibits excellent oxidation resistance and corrosion resistance due to its spinel structure. The above primary heat treatment is performed on the diffusion layer 12 and the base metal 10.
The alumina layer 13
This is done to form 7.
This is carried out by exposure to an oxidizing atmosphere at 50 to 950°C for 0.5 to 1 hour.

The secondary heat treatment is performed to form a mixed coating 14 having a spinel structure on the surface of the alumina layer 13, and specifically, the secondary heat treatment is carried out in an oxidizing atmosphere at 950 to 1350°C. This is done by exposing it to time. This secondary heat treatment is performed when the base material is ferritic.
The treatment temperature is preferably 1000 to 1100°C, and for austenitic materials, the treatment temperature is preferably around 950°C.

Example As a representative example, the base material of SUS43O is heated to 750°C.
After soaking for 6 hours in a phlegm-Zn bath,
Table 1 shows the results of structural analysis of the surface layer by X-rays when primary heat treatment was performed at 0°C for 1 hour and secondary heat treatment was performed at 110°C for 2 hours.

As is clear from Table 1, according to the method of the present invention,
In the case of the first and second heat-treated samples, no peeling occurred even after the heat treatment at 1150°C, and the surface was almost completely covered with ZnN2O, which has a spinel structure. Proved: Yes.

Note that the present inventors have determined that the amount of Zn in the bath is 0 to 10% by weight.
Create test pieces in 1% increments until
According to the results of a heat resistance test at 1150° C., spontaneous peeling occurred at O to 2% by weight, as in the conventional case.

Next, the results of the corrosion weight loss test are shown in Table 2.

In this corrosion weight loss test, a lead compound 3Pb3(PO4)2/Pb
This was done by applying Cl2 and heat-treating it under the heating conditions shown in Table 2, and then measuring the corrosion loss. As is clear from this test result, according to the present invention, Z
Compared to conventional products in which divalent metal elements such as n are not added, the degree of corrosion loss can be significantly reduced.

This is because the alumina coating produced by the method of the present invention not only improves peeling resistance, but also provides corrosion resistance due to the spinel structure of the composite metal oxide layer covering the surface of the alumina coating. . Note that since the spinel structure has insulating properties, it can also be used as a high-temperature insulating material.
Although the test results for SUS43O were explained in the above embodiments, it goes without saying that the present invention can also be applied to 13 chromium steel, 18-8 stainless steel, cast steel, etc. used as heat-resistant steels.

[Brief explanation of drawings]

FIG. 1 is an explanatory view schematically showing a conventional alumina film forming method, and FIG. 2 is an explanatory view schematically showing an alumina film forming method according to the present invention. 10...Base material, 11...A1-Zn plating layer, 12...N diffusion layer, 13...Alumina layer, 14...Mixture Coating, 15...
Composite metal oxide layer.

Claims (1)

[Claims] 1. A method for forming an alumina film on the surface of a metal product, in which 3 to 20% by weight of at least one of the elements Zn, Mg, Ni, and Ca is added and the bath temperature is 700 to 95%.
A metal product is placed in an alumina alloy bath kept at 0°C.
After immersion treatment for 20 seconds, the metal product was heated to 750-950°C.
A primary heat treatment is performed for 0.5 to 1 hour in an oxidizing atmosphere of 950 to 1350°C, followed by a secondary heat treatment for 1 to 3 hours in an oxidizing atmosphere of 950 to 1350°C to form a composite metal oxide layer with a spinel structure on the coating surface of the metal product. A method for forming an alumina film having peeling resistance, characterized by: