US2993784A - Aluminium alloys - Google Patents

Description

nited States 2,993,784 AL ALLOYS Roy Alfred Ulfketel Huddle, East llsley, Nigel John Murray Wilkins, Hermitage, and Colin Frederick Britton, Abingdon, England (all of United Kingdom Atomic Energy Authority, Patents Branch, 11 Charles II St., London SW. 1, England) No Drawing. Filed Apr. 27, 1959, Ser. No. 808,951 Claims priority, application Great Britain June 21, 1956 2 Claims. (Cl. 75-148) This invention relates to aluminium alloys having good corrosion resistance to Water at high temperatures and pressures.

Aluminium is potentially a very suitable material for use in nuclear reactors, since it has a reasonably low capture cross-section for neutrons, it is readily available and comparatively inexpensive, and its methods of fabrication are well known. For water-cooled reactors, its practicability is limited by its corrosion resistance. The most suitable operating conditions for a water-cooled reactor are those in which the coolant operates at a high temperature, eg at 300 C. to 350 C. However, the mechanism of corrosion of aluminium by water at high temperatures has been found to be quite different from that which operates at low temperatures.

At low temperatures, that is at temperatures below about 200 C., corrosion in water leads to the formation of a protective, hydrated oxide film which grows only very slowly, at a rate which increases with temperature. A penetration rate of only 0.3 micron per year has been observed for commercially pure aluminium in static distilled water at 50 0., the rate increasing to 4.8 microns per year at 150 C.,'and about 30 microns a year at 200 C.

At temperatures above about 200 C. another corrosion mechanism occurs. After a period of corrosion similar to that at lower temperatures, blisters appear which grow larger as time goes on, the surface of the metal roughens, and the metal is penetrated and attacked much more rapidly, eventually disintegrating altogether. This type of corrosion is believed to be due to hydrogen diffusing into the aluminium through the protective oxide layer.

The time which elapses before this type of corrosion occurs, varies with the temperature, the impurity content of the aluminium, and the metallurgical condition of the aluminium. At temperatures of 300 C. and above, commercially pure, rolled aluminium disintegrates completely in a few hours. This type of corrosion, however, does not occur at all for most aluminiumrich alloys at temperatures below about 200 C.

It has previously been found that aluminium alloys containing from 0.5 to 5.0 weight percent nickel have much improved resistance to corrosion of the blistering type by water at temperatures above 200 C. It has also previously been found that the presence of small amounts of iron and silicon, such as are present in commercially pure aluminium, delays the onset of blistering corrosion even further. For example, it has been found that alloys of high purity (99.995 aluminium with 2.5% nickel are resistant to blistering corrosion at 300 C. for periods varying from 4 to 50 days, whereas alloys of commercially pure aluminium, containing 0.5% iron and 0.25% silicon as major impurities, with 2.5% nickel are resistant to blistering corrosion at 325 C. for periods varying from 100 days to over 300 days.

A characteristic feature of these aluminium-nickel alloys, however, has been the variable corrosion resistance of specimens of identical chemical composition, even if fabricated from the same stock. None of the alloys hitherto described, therefore, had a sutficiently re- Patented July 25, 1961 ICC liable resistance to blistering corrosion in water at high temperatures, and many had a very poor resistance to such corrosion.

It is an object of the invention to improve the resistance of aluminium-nickel alloys to corrosion in water at high temperatures, particularly at temperatures above 200 C.

It is a further object of the invention to improve the consistency of resistance of different samples of such alloys to corrosion in water at such temperatures.

It is a still further object of the invention to improve the resistance of aluminium-nickel alloys to corrosion by water at temperatures of about 325 C.

According to the invention, there is added to aluminium-nickel alloys, containing from 0.5 to 5% nickel by Weight, about 0.1 to 0.3% by weight of titanium.

Also according to the invention, there is added to aluminium-nickel alloys containing from 0.5 to 5% nickel by weight, about 0.1 to 0.3 by weight of titanium and about 0.005 to 0.10% by weight of beryllium.

The aluminium-nickel alloys, to which titanium, with or without the addition of beryllium, has been added, advantageously contain also iron and silicon, in the proportions normally present in commercially pure aluminium, mainly for the purpose of improving the mechanical strength of the aluminium, namely 0.1 to 1% iron and 0.05 to 0.5% silicon.

We have found that the addition of titanium, with or without beryllium, in the above-mentioned proportions to aluminium-nickel alloys prolongs greatly the period during which the alloys are resistant to blistering corrosion, and that the addition of titanium and beryllium together leads to consistently good resistance to such corrosion, regardless of the method of fabrication of the alloy specimens, at temperatures up to about 325 C.

It is generally believed, that nickel affords some protection to aluminium against corrosion in water at temperatures above 200 C. by reducing the hydrogen diffusion damage to the aluminium. Nickel has a low solubility in aluminium, and in amounts of from 0.5 to 5% by weight forms a separate phase, uniformly distributed as small particles throughout the aluminium. Nickel has a low hydrogen overpotential compared with aluminium, and it is therefore probable that hydrogen produced cathodically as a result of the corrosion process is evolved freely from the nickel-containing particles on the surface of the metal, and less diffusion of hydrogen into the metal occurs.

The additions of titanium and beryllium, which we have discovered to be effective in delaying or preventing blistering corrosion of aluminium, are believed to improve the protective properties of the oxide film itself, formed by normal corrosive attack on the aluminium surface. In this way, greater resistance to hydrogen diffusion is provided. In order to be effective by improving the properties of the oxide film, alloying additions should be uniformly distributed through the film, and therefore, the alloying elements are preferably in true solid solution in the aluminium base. Thus we have found that the preferred additions of titanium and beryllium are less than the solubility limits for these elements in solid aluminium, namely, 0.3 weight percent for titanium and 0.1 weight percent for beryllium.

Although it has been known in the past to add titanium in proportions of 0.01 to 1% to aluminium for grain-refining purposes, and it has also been known that corrosion-resistance in general is improved by grain-refinement, there has been no teaching, before our discovery, that addition of titanium to aluminium-nickel alloys (which consist of a nickel-containing phase as well as the aluminium base) improvestheir resistance to the particular type of corrosion (referred to herein as blister- Example 1 An alloy was prepared from a commercially pure aluminium, with addition of titanium, and had the following nominal composition:

Percent Nickel 1.75 Iron 0.75 Silicon 0.15 Titanium 0.15

The alloy was cast in the form of a /2 inch diameter ingot in a copper mould, rolled to inch thickness at about 500 C. and then cold-rolled to 16 S.W.G. sheet. After annealing at 620 C. for 24 hours, specimens were cut from the sheet and exposed to pressurised water at 325 C. After 12 months at this temperature none of the specimens showed blistering corrosion.

Example 2 An alloy was prepared from a commercially pure aluminium, with additions of titanium and also beryllium, and had the following nominal composition:

Percent Nickel 1.75 Iron 0.75 Silicon 0.20 Beryllium 0.05 Titanium 0.20

The alloy was cast in the form of a /2 inch diameter ingot in a copper mould, rolled to 4 inch thickness at about 500 C. and then cold-rolled to 16 S.W.G. sheet. After annealing at 620 C. for 24 hours, specimens were cut from the sheet and exposed to pressurised water at 325 C. After 18 months at this temperature none of the specimens showed blistering corrosion. A smooth, dark grey film was formed on the surfaces of the specimens which, after an initial period while the protective film was formed, showed a weight gain of only 0.65 milligram per sq. cm. per month, corresponding to an average corrosion rate of only 29 microns per year. This film was stripped oft" some of the specimens, by chemical- 1y dissolving the oxide layer, after the specimens had been exposed for 12 months. The stripped specimens showed a weight loss of 11.2 milligrams per sq. cm. compared with their original weight, corresponding to an average true penetration rate by corrosion of only 43 microns per year.

An alloy having the same nominal composition as that of Example 2 was also formed by semi-continuous casting as 4- /8 inch diameter bar. Sections of A inch and /2 inch thickness were cold-rolled to 16 S.W.G. sheet. Specimens were tested in the same way and showed similar resistance to blistering corrosion and a similar low corrosion rate.

The alloys of the invention may be employed for cladding fuel elements, or as the metal phase in cermet fuels, or as a constituent of fuel alloys, in nuclear reactors using high-temperature pressurised water as coolant and/ or moderator.

This is a continuation-in-part of our application Serial No. 667,297, filed on June 21, 1957, now abandoned.

We claim:

1. An alloy of aluminium oonsisting essentially of 0.5 to 5 weight percent nickel, 0.1 to 0.3 weight percent titanium, 0.005 to 0.10 weight percent beryllium, 0.1 to 1.0 weight percent iron and 0.05 to 0.5 weight percent silicon, the balance being pure aluminium.

2. An alloy of aluminium consisting essentially of 1.75 weight percent nickel, 0.2 weight percent titanium, 0.05 weight percent beryllium, 0.75 weight percent iron, and 0.2 weight percent silicon, the balance being pure aluminium.

References Cited in the file of this patent UNITED STATES PATENTS 1,579,481 Hybinette Apr. 6, 1926 1,704,252 Hybinette Mar. 5, 1929 1,912,382 Nook June 6, 1933 1,941,230 Smith Dec. 26, 1933 2,823,995 Blackmun Feb. 18, 1958 2,826,518 Anderson Mar. 11, 1958 2,826,519 Anderson Mar. 11, 1958 FOREIGN PATENTS 558,023 Great Britain Dec. 15, 1943 529,289 Canada Aug. 21, 1956 OTHER REFERENCES Aluminum All'oy Casting, *Carrington, Charles Griflin and Co., Ltd., London, page 290.

Claims (1)

1. A CURRENT LEAD-IN DEVICE FOR ELECTRIC GLOW DISCHARGE CHAMBERS COMPRISING TWO ELECTRODES ARRANGED ON WITHIN THE OTHER AND SPACED FROM EACH OTHER, AT LEAST ONE OF THE ELECTRODES BEING ADAPTED FOR CONNECTION TO A SOURCE OF ELECTRIC POTENTIAL, AN INSULATOR BETWEEN THE ELECTRODES, ONE OF THE ELECTRODES BEING ADJUSTABLE IN AT LEAST ONE OF THE AXIAL AND RADIAL DIRECTIONS WITH RESPECT TO THE OTHER ELECTRODE, SAID ELECTRODES FORMING A GAP BETWEEN THEM WHICH IS DISPOSED IN ADVANCE OF THE INSULATOR VIEWED FROM THE INTERIOR OF THE CHAMBER, SAID ADJUSTABLE ELECTRODE BEING MOVABLE TO MAKE THE WIDTH OF THE GAP THE SAME THROUGHOUT ITS WHOLE EXTENT.