IL35730A

IL35730A - A method of producing protective diffusion layers on the surface of cobalt-based alloys

Info

Publication number: IL35730A
Application number: IL35730A
Authority: IL
Original assignee: Deutsche Edelstahlwerke Ag
Priority date: 1969-12-05
Filing date: 1970-11-26
Publication date: 1973-11-28
Also published as: CA924970A; AT296715B; SE358420B; IL35730A0; NL7017774A; BE759275A; ZA708014B; DE1961047B2; US3692554A; CH552682A; GB1282530A; DE1961047A1; FR2072284A5; JPS4931609B1

Description

A method of producing protective diffusion layers on the surface of cobalt-based alloys This invention relates to a method of producing on the surface of parts made of cobalt-based alloys protective, diffusion layers that are resistant at high temperatures to oxidation, scaling and corrosion.

It is a known practice to protect objects, for example turbine blades for jet propulsion engines which usually consist of high temperature metal alloys, from oxidation, scaling and corrosion, by diffusing aluminium into their surfaces thereby to provide a protective layer thereon.

The object of the present invention is to provide on parts made of high temperature metals, particularly cobalt-based alloys, a protective layer which imparts improved resistance to oxidation, scaling and corrosion at high temperatures.

For achieving this object the present invention consists in applying to the parts, possibly after they have been previously cleaned and degreased, nickel, one or more metals of the platinum group of metals of the Periodic System of Elements, (preferably platinum), and aluminium, either simultaneously or in succession, and to diffuse the said metals into the surface of the parts .

In a useful embodiment of the method of the invention, layers of nickel, of metal(s) of the platinum group and of aluminium may be consecutively applied to the surface of the part, for example by mechanical or electrolytic means, dipping, spraying formed diffused severally or jointly into the surface of the part by a diffusion heat treatment.

In an alternative method of carrying out the invention a layer of nickel and then a layer of one or more metals of the platinum group may be electro-lytically deposited and the aluminium then deposited from a gas phase on the surface of the part and diffused into the surface of the part together with the nickel and the metal(s) of the platinum group. With advantage the part that has first been provided with a nickel layer and then with a layer of metal (s) of the platinum group, may be embedded in a powder mixture containing metallic aluminium possibly associated with a small proportion of a metal halide and an inert filler (e.g. alumina), the aluminium being then deposited on the part from a gas phase and finally diffused into the surface of the part together with the nickel and the metal(s) of the platinum group. Such a mixture may for example comprise about 5 of aluminium and about 95 of the inert filler.

Cobalt-based alloys that have been provided with a protective diffusion layer according to the invention last about twice as long as similar parts that have been provided with a protective aluminium diffusion layer without nickel and platinum. The graph of the accompanying drawing having time on the abscissa and change in weight on the ordinate shows, as a function of operating been aluminised by a conventional method, and that have been protected by the method according to the invention. It will be seen that the reduction in weight which indicates the rate at which material is lost from the surface of the tested part by oxidation, scaling and corrosion, is less for parts protected according to the invention than parts that have been merely aluminised, and that their useful life of 600 to 700 hours in scaling test when exposed to burnt natural gas at 1100°C with an air factor of I.2, is twice as long. The material of all the tested parts was a cobalt alloy containing 0, 5 carbon, less than 0.5$ silicon, less than 0.5$ manganese, 21.0$ chromium, less than 1.0$ nickel, II.0$ tungsten, 2.0$ niobium tantalum, 1.75$ iron, balance cobalt. The protective treatment of the parts according to the invention was as follows Preliminary surface treatment Wet sand blasting and degreasing in a cyanide, copper-free degreasing bath cathodically for 5 seconds and anodically for 15 seconds, at a temperature of 20°C and current density of 5 amps/sq.dm., followed by pickling in a 10$ sulphuric acid for 10 seconds at 40°C, without the application of current, then washing in warm water at 45°C.

Nickel deposition Electrolytic composition of bath:- 240 g/litre of nickel chloride (NiCl2.H20) 120 g/litre of 36$ hydrochloric acid. 45°C and current density 10 - 11 amps/sq.dm.

Thickness of nickel deposit 20 )i Heat treatment 2 hours at 260°C followed by 3 hours at 450°C.

Preliminary surface treatment of the nickel before Platinisation Degreasing in a cyanide degreasing bath as hereinbefore described cathodically 10 seconds, anodically 20 seconds and cathodically 10 seconds at a temperature of 20°C and a current density of amps/sq.dm., followed by pickling in 10$ by weight caustic soda cathodically 3 seconds and anodically 10 seconds at a temperature of 50°C and a current density of 10 amps/sq.dm., followed by pickling in 4- o hydrochloric acid cathodically 2 seconds and anodically 30 seconds at a temperature of 40°C and a current density of 2.5 amps/sq.dm., followed by washing in warm water at 75°C.

Platinum deposition Electrolytic composition of bath:- 13 g/litre of hexachloroplatinic acid, !^PtClg. 45 g/litre of triammonium phosphate, (NH^)^PO^. 240 g/litre of dis odium phosphate, NagHPO^.

Conditions of electrolysis:- temperature 75°C and current density 5.5 amps/sq.dm.

Thickness of platinum layer 6 - 8 ^um.

Heat treatment as hereinbefore described.

Aluminising from the gas phase ■¾wm the gas-.pka&s.iomposition of powder mixture of aluminium and 95 "by weight of alumina.

Diffusion heat treatment was effected at a temperature of 1100°C for 5 hours.

The result of this treatment was an overall thickness of the protective diffusion layer of Similar results are obtainable with palladium, rhodium and/or ruthenium when used instead of platinum. 35730/2

Claims

1. A method of producing protective diffusion layers that are resistant to scaling and corrosion at high temperatures on the surface of objects consisting of alloys into which aluminium is characterized in that in addition to nickel and one or metals of the group of rhodium platinum are diffused into the surface of the A method according to Claim in which successive of one or several metals the group of rhodium and and finall of aluminium are applied to the surface of the objects electrolytically or by dipping or and from the layers the metals are diffused singly or in common into the surface of the objects by a diffusion heat A method according Claim in which first a layer of nickel and on the latter a layer of one or more metals of the group platinum are deposited and aluminium then deposited from the gas phase and diffused into the surface of the object together with the nickel and the platinum metal or A method according to 1 or in which the object plated first with a nickel layer on the with a of one or more metals of the group rhodium or embedded in a powder mixture containing of metallic email with aluminium oxide the balance up to 100 and the object thus plated and embedded is sub to the diffusion For the Applicants PARTNERS insufficientOCRQuality