EP0602680A1

EP0602680A1 - Potassium silicate-containing sealant

Info

Publication number: EP0602680A1
Application number: EP93120451A
Authority: EP
Inventors: Sasaki Satoshi
Original assignee: Praxair ST Technology Inc; Praxair Technology Inc
Current assignee: Praxair ST Technology Inc; Praxair Technology Inc
Priority date: 1992-12-18
Filing date: 1993-12-17
Publication date: 1994-06-22
Also published as: JPH06228722A

Abstract

A potassium silicate-containing sealant having good resistance to attack by molten metal, such as molten zinc and a method for coating the sealant on a substrate.

Description

Field of the Invention

The invention relates to a potassium silicate-containing sealant having excellent resistance to molten metal, specially molten zinc and its manufacturing method, said sealant preferably applied to a component which contacts or is immersed in molten zinc in a hot-dip zinc plating line.

Background of the Invention

Molten zinc attacks metals such as a steel and the like and easily penetrates into small holes or gaps in the micron meter range because of its low surface tension and viscosity. Some protecting methods are proposed or are in use for improving the resistance to molten zinc, as for example, specially developed steels as disclosed in Laid Open Japanese Patent Application No. S56-112117 and applying a thermally sprayed WC-Co coating as disclosed in Japan Patent Application Laid Open No. 111-225761. However, these approaches are not sufficiently resistant to molten zinc attack.
Molten zinc resistant steels are basically iron base alloys and do not have enough resistance to molten zinc attack. The cost of those alloys are much higher than normal structural steels. Coatings such as self fluxing alloys and WC-Co are used as thermally sprayed coatings to protect substrates from the attack by molten zinc but sufficient resistance has not been achieved due to the permeation of molten zinc through interconnected porosity and selective attack on the metal binders.
Even when MoB, which is resistant to molten zinc attack and disclosed in Japanese Patent Application 4-14821J, is used, molten zinc tends to mechanically adhere to the roughened surface or to the walls of the pores on the surface.
It is well known that sealing a coated surface by water glass or colloidal silica (silicon dioxide) is effective to improve the resistance of the thermally sprayed coating to attack by molten zinc. However, the durability of these sealing materials is not sufficient and a thick thermally sprayed undercoat having excellent durability is required.

Summary of the Invention

The invention relates to a sealing material having an improved resistance to molten zinc attack and anti-wettability thus making it ideally suitable for structural materials or thermally sprayed coatings that are intended to be used in molten metals.
It was found that anhydrous potassium silicate is a sealant which has higher durability than existing water glass and colloidal anhydrous silicate. A suitable sealant for this invention which showed excellent durability in molten zinc bath is formed by a process comprising the following steps:

(a) preparing a water solution containing potassium silicate as an element;
(b) applying the solution on the surface of an article to be sealed; and
(c) baking the article under an appropriate temperature range.

Accordingly, the present invention is to provide firstly a sealant having an excellent resistance to molten metal, specially to molten zinc, comprising anhydrous potassium silicate as a main element which favorably consist of 30 to 70 wt% silic acid anhydride and 70 to 30 wt% anhydrous potassium oxide, secondly an article resistant to molten metal, specially to molten zinc, made of metal substrate and a dense layer on its surface comprising anhydrous potassium silicate as a main element and thirdly a process for forming a sealant resistant to molten metal, specially to molten zinc, comprising the steps of:

(a) formulating a mixed aqueous solution containing potassium silicate as an original material;
(b) applying the solution on the surface of a component to be sealed; and
(c) heat treating the component coated with the solution at 150° to 600°C.

Anhydrous potassium silicate has better durability structurally against molten zinc than silica and anhydrous sodium silicate specially when it is used at over 400°C. It also has a good matching coefficient of thermal expansion with metal made substrates to be sealed as well as low wettability with molten zinc and molten aluminum.
The reasons for limiting the baking temperature in the range of 150 to 600°C is that water in the solution can not be vaporized well below 150°C which can result in cracks in the sealant layer when it is rapidly heated up to the molten zinc temperature (approx. 470^oC) during the operation while the substrate or thermally sprayed coating will generally start to be oxidized over 600°C.

Description of the Drawings:

The sole drawing shows a cross sectional view of a furnace used for zinc immersion testing of coated samples. Specifically, bar type test specimens coated with the solution of the sealant of this invention are baked at 450°C before being tested. A sealant prepared by the water solution of potassium silicate is applied on a bar type specimen 1 and baked at 450°C to obtain the sealant layer 2. The specimen is immersed into molten zinc 3 in a graphite pot 4 placed in a furnace 6 kept at 450°C by a heater 5.
Table 1 shows results of the immersion test in which no degradation was observed after 1000 hours duration and cracks take place after 1100 hours with some small partial spalling but the spalling does not appear to extend after 1400 hours.
Evaluation is made by ratings A to E based on the microscopic observation of degree of zinc adhesion and degradation of the scaled layer after the immersion test.

A:: No Change.
B:: Cracks with very small defects.
C:: Larger defects with no exposure of the substrate.
D:: Substrate is partially exposed and zinc strongly adhere to the exposed surface of the substrate.
E:: Zinc strongly adhere all over the substrate and hard to remove.

A sealant prepared by the water solution of silic acid anhydride is applied on a bar type specimen and the immersion test is conducted in accordance with the same procedure as recited above. As shown in Table 1, cracks are observed after 300 hours, small defects extend after 500 hours and after 700 hour testing substrate is exposed and zinc strongly adheres.
The same test is conducted for water glass sealant. Zinc adheres to all areas of the specimen that was sealed with water glass after 300 hours as shown in Table 1.

Claims

A sealing material having good resistance to attack by molten metal comprising anhydrous potassium silicate as a main element.
A metal made article resistant to molten metal having a dense layer comprising anhydrous potassium silicate as a main element on its surface.
A method for forming a sealing material resistant to molten metal comprising the steps of:
(a) formulating a mixed aqueous solution containing potassium silicate as an original material;

(b) applying the solution on the surface of a component to be sealed; and

(c) heat treating the component coated with the solution at 150 to 600°C.
The method of Claim 4 where the molten metal is molten zinc.