GB2027751A - Chromium Diffusion Coating of Steel - Google Patents

Abstract

A corrosion resistant coating on steel is formed by depositing a porous layer of chromium metal onto a substrate, preferably by plasma spraying and thereafter exposing the layer to a halide activator at elevated temperatures to react with the chromium layer to form chromium halides and deposit chromium onto the substrate. The deposited chromium diffuses into the substrate, which may be a low carbon steel. The halide activator may be provided in a solid particle pack although it may also be from a vapour source. The coated steel is used in casings of sodium sulphur cells.

Description

SPECIFICATION A Method for Forming a Chromium Alloy on a Ferrocarbon Substrate The present invention relates to a method for protecting steel and other similar ferrocarbon substrates with a chromium corrosion-resistant coating, particularly for use as a component in a sodium-sulfur cell.

The cell casing of a sodium-sulfur cell, especially the casing for the sulfur compartment, is subject to severe corrosive attack. Relatively inexpensive carbon steels cannot be employed as the casing for the sulfur compartment as they form metal sulfide scale which leads to extensive physical degradation and contamination in the sulfur/sodium polysulfide melt under cycling. This interferes with efficient cell operation and causes cell capacity losses, cell resistance increases and degradation of the electrolyte.

Breiter U.S. patent 3,959,013 discloses the use of a sulfur casing portion of metal, including steel, by providing a corrosion resistant and electronically conducting layer to adhere to its inner surface. The disclosed layer is formed of molybdenum or graphite by plasma spraying. In U.S. patent 4,048,390, an iron alloy is coated for protection with aluminum by pack aluminiding.

Corrosion protective layers have been formed on substrate metals for a variety of uses other than the corrosive environment of a component for a sodium-sulfur cell. For example, carbon steel containers have been chromized by a variety of processes including chromium diffusion by pack chromizing for applications such as high temperature turbine blades where severe oxidizing conditions exist at temperatures close to 9000C. The so-called D.A.L. process is described in Samuel, R.L. and Lockington, N.A., Metal Treatm., 18 (1951) 407,440,495. In this process, a steel substrate is submersed in a pack of ammonium iodide ferrochromium and kaolin powder. The pack is heated at an elevated temperature to form chromium iodides. The chromium iodides deposit chromium, which then diffuses into the steel substrate to form a chromized layer.Other procedures such as the socalled B.D.S. process have also been employed in a diffusion process for forming a chromium-rich coating. There, a mixture of ferrochromium and broken pieces of ceramic are heated in a retort with circulating hydrogen chloride gas. The hydrogen chloride reacts with the ferrochromium to form chromus chloride which is absorbed in the ceramic. Thereafter, the chromus chloride is volatilized and' chromizes the article by replacement and reduction reactions. This procedure is described in A.H. Sulley, Chromium, Butterworth Scientific Publication, London, (1954), 197-199.

An improved corrosion resistant coating is formed for steel and other similar ferrocarbon substrates, suitably for use as a component of a sodium-sulfur cell. The coating is formed by first depositing a porous layer of chromium metal onto the substrate, preferably by plasma spraying, and thereafter exposing the layer to a halide activator vapor at elevated temperatures to react with the chromium layer and form chromium halides which deposit chromium onto the substrate. The deposited chromium then diffuses into the substrate to form a continuous, essentially nonporous chrnmium#arbon-irnn alloy surface layer diffusion bonded to the substrate.

The present invention relates to the formation of a low cost corrosion resistant coating onto steel and other similar ferrocarbon substrates, especially useful as a sulfur-container component for a sodium-sulfur cell. The description of a suitable cell for use in conjunction with the present invention is set out in Mitoff et al U.S.

patent 3,960,596.

Briefly summarized, the process of the present invention comprises first depositing a porous layer of chromium metal on the substrate and thereafter forming that layer into a continuous, essentially nonporous chromium-carbon-iron alloy surface layer diffusion bonded to the substrate. A suitable ferrocarbon substrate comprises a low carbon steel.

One preferred technique for depositing the porous layer of chromium metal is by plasma spraying of fine chromium powder. This technique is generally described in Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd Ed., Interscience, N.Y., Viol. 1 3, p. 271, incorporated at this point by reference. The fine. chromium powder, typically on the order of -325 mesh size, is sprayed under high pressure with a plasma spray gun against ferrocarbon substrate. The solid dry powder physically adheres to and coats the substrate to form a porous layer of chromium metal. To facilitate such physical adhesion, it is preferable to roughen the substrate surface, as by sand blasting or the like, prior to spraying. This provides the additional advantage of cleaning the surface of the substrate.The thickness of this initial porous layer typically is from 2 to 10 mils.

Other processes may be employed for forming initial porous layer of chromium metal. For example, such layer may be electroplated onto the surface by conventional techniques such as set out in A.H. Sulley and E.A. Brandes, Chromium, Plenum Press, N.Y. (1967), 211257, incorporated at this point by reference.

Porous chromium layers of the foregoing type usually do not adhere tenaciously to the underlying substrate. Also, such surfaces are relatively porous and are subject to attack by such corrosive materials as molten sulfur and sodium polysulfides in sodium-sulfur cell. This causes the underlying substrate to be attacked forming a heavy subscale underneath the chromium coating which causes the coating to delaminate or peel off.

In the second step of the process, the chromium is diffused into the substrate to form a continuous essentially nonporous chromiumcarboniron alloy protective layer bonded to the exposed surface of the substrate. The chromium component may be in elemental form or alloy form or combined into compounds with other elements in the substrate. For a steel substrate, such compounds include chromium carbides, to a major extent, and chromium nitrides, to a lesser extent.

A preferred method for forming such alloy layer is to expose the porous layer of chromium metal to a halide activator vapor in a retort at elevated temperature. The activator forms chromium halides which deposit chromium that diffuses into the substrate to form said protective alloy layer.

One suitable technique for generating halide activator vapor is by heating a solid halide in a solid particle pack of inert filler such as alphaalumina power with the substrate embedded in the pack. Preferred solid halide activators which vaporize at suitable elevated temperature is ammonium chloride or ammonium bromide.

In the foregoing process, the generated halide gas reacts with the chromium metal on the surface of the substrate which diffuses into the surface of the substrate at elevated temperatures.

Then, the chromium is deposited onto the steel substrate. In general, a suitable temperature range for generating the halide and for causing diffusion of the chromium into the substrate is from about 850-1 2000C. The preferred temperature is from 950-11 000C whereas the optimum temperature is around 10000C. The diffusion step may be performed in about one hour.

It should be understood that the halide activator vapor may be supplied from a source other than vaporization from a solid particle pack.

Thus, for example, a halogen gas or a halide vapor such as HCI may be supplied from a pressurized tank or a similar suitable source. The foregoing continuous, essentially pin-hole free alloy layer may be formed into a desired depth ranging from one to several mils.

A major advantage of the foregoing process in comparison to conventional pack chromizing is that it is readily adaptable to chromizing processes other than pack chromizing because the chromium source remains on the container during processing. Another major advantage is that a significantly lower amount of chromium powder is required in comparison to pack chromizing. Other than the chromium used for coating of the porous layer, no additional chromium is needed in the pack.

A further disclosure of the nature of the present invention is provided by the following specific examples of the practice of the invention. It should be understood that the data disclosed serve only as examples and are not intended to limit the scope of the invention.

Example 1 A low carbon steel substrate, formed of AISI 1015 steel, was first coated with a porous layer of chromium by plasma spraying with a Metco Type 3M Plasma Flame Spray System with argon and hydrogen mixture as plasma gas. The fine chromium powder (-325 mesh particle size) was coated to a thickness of about 5 mils. The container was then embedded in a pack of 2% ammonium chloride powder and the remainder aluminum oxide particles, all in a stainless steel retort. No additional chromium powder was added in the pack itself. The retort was then heated at 1 0000C for one hour in hydrogen.

The above chromium coating was gray in color.

When immersed in 50% nitric acid at 400 C, the coated substrate was found to be inert insofar as no gas evolution due to the localized corrosion was observed. An optical micrograph of the crosssection showed a fully continuous, pore-free coating, which consisted of two distinct layers of chromium-carboniron alloys with a total thickness of about 12 microns.

Example 2~Comparative Example A low carbon steel substrate of the same type as the foregoing one was plasma sprayed by the above technique but not treated thereafter with ammonium chloride activator. When immersed in 50% nitric acid at 400C, the container showed an immediate, violent reaction. The steel substrate was heavily attacked, and the original chromium coating peeled off. This showed that a plasmasprayed coating is by itself unprotective, unless a diffusion-bonded, pore-free coating is obtained through the halide-activated chromizing process.

Example 3~Comparative Example A low carbon steel substrate of the same type as the foregoing one was plasma sprayed by the above technique. The container was embedded in aluminum oxide powder in a stainless steel retort, and then was heated at 1 0000C for one hour in hydrogen. Unlike in Example 1, an activator such as ammonium chloride was not added in the pack.

The original chromium coating peeled off from the container leaving a partially gray color on the surface. When it was immersed in 50% nitric acid at 400C, an immediate reaction was observed on some parts of the surface while some other parts were not attacked. An optical micrograph of the cross-section of the container showed that it had a chromium-coated layer, but the coating was not uniform and was not fully continuous.

Example 4 A cell with container of Example 1 was employed in a sodium-sulfur cell. It showed very good cell performance characteristics when tested for over 9 months and 450 cycles. During the period, the cell capacity showed only a nominal decrease and the cell resistance showed a very steady behavior.

Claims (10)

Claims

1. A method for forming a corrosion resistant layer on ferrocarbon substrates such as steel comprising the steps of: (a) depositing a porous layer of chromium metal on said substrate, and (b) exposing said layer to an atmosphere comprising a halide activator vapour at elevated temperature to react with said chromium layer which forms chromium halide that deposits chromium which diffuses into said substrate to form a continuous essentially non-porous protective chromium-carbon-iron alloy layer diffusion bonded to said substrate.

2. A method as claimed in claim 1 in which said halide activator vapour is generated by heating a solid halide in a pack.

3. A method as claimed in claim 2 wherein the activator is an ammonium halide.

4. A method as claimed in any one of the preceding claims performed by a physical coating of chromium on said substrate.

5. A method as claimed in claim 4 in which said physical coating comprises plasma spraying.

6. A method as claimed in claim 4 performed by chromium electroplating from a bath.

7. A method as claimed in any one of the preceding claims wherein the ferrocarbon substrate comprises a cell casing for a sodiumsulphur cell.

8. A method for forming a corrosion resistant layer on ferrocarbon substrates as claimed in claim 1 substantially as hereinbefore described in Example 1.

9. A ferrocarbon substrate having a corrosion resistant layer when produced by a method as claimed in any one of the preceding claims.

10. A sodium sulphur cell casing produced by a method as claimed in any one of the preceding claims.