Alloys or metallic compositions – Containing over 50 per cent metal but no base metal – Iron containing
Patent
1997-05-05
1999-03-09
Phipps, Margery
Alloys or metallic compositions
Containing over 50 per cent metal but no base metal
Iron containing
420 34, 420 40, 420 41, 148142, C22C 3002, C22C 3802
Patent
active
058796193
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to austenitic high-Cr and high-Ni alloys and more particularly, to high-Cr and high-Ni alloys which exhibit a good corrosion resistance when placed in an aqueous solution having a relatively low hydrogen sulfide concentration at a partial pressure of hydrogen sulfide gas of 1 atm., or below.
TECHNICAL BACKGROUND
Hydrogen sulfide contained in liquids, such as petroleum, is highly corrosive against alloys. Accordingly, the alloys used in liquids containing hydrogen sulfide should have a good corrosion resistance to hydrogen sulfide. The examples of alloys which are employed in environments where they are in contact with hydrogen sulfide-containing liquids include drill pipes, pipes for flow lines from oil wells, oil country tubular goods for oil and natural gas wells, plate members for natural steam power stations, plate members for installation for desulfurization from exhaust gases, and the like. Especially, drilling of oil wells, exploitation and production of natural gas involve corrosive environments which are severe. For an index indicating the corrosive environment of hydrogen sulfide, the usual practice is to use a partial pressure of hydrogen sulfide in a gas phase. This is because the concentration of hydrogen sulfide in an aqueous solution is substantially proportional to the partial pressure of hydrogen sulfide in a gas phase, thus enabling one to simply express the degree of influence on alloys. With production pipes for oil wells, it has been frequently experienced that alloys are exposed to severe environmental conditions such as a partial pressure of hydrogen sulfide of about 10 atm., and a temperature of about 200.degree. C.
The corrosion of an alloy ascribed to the hydrogen sulfide in such an environment as mentioned above results mainly in the cracking of the alloy under stress (stress corrosion cracking). Accordingly, alloys which are to be employed in an environment containing hydrogen sulfide should have a good resistance to stress corrosion cracking.
Known alloys which are used in an environment such as of a hydrogen sulfide-containing oil well wherein a partial pressure of hydrogen sulfide is as high as approximately 10 atm., include Ni--Cr--Mo--Fe Ni-based alloys which contain Ni in amounts as great as 30-50% (Japanese Laid-open Patent Application Nos. 57-131340, 57-134544 and 57-134545).
For instance, Japanese Laid-open Patent Application No. 57-131340 proposes an alloy which comprises, aside from Ni, Cr, Mo and W, Cu and Co, if necessary, in order to improve the resistance to stress corrosion cracking.
These alloys for oil wells are so designed as to improve the corrosion resistance to hydrogen sulfide including a resistance to stress corrosion cracking. More particularly, the corrosion resistance to hydrogen sulfide is greatly influenced by the content of hydrogen sulfide present in raw oil and the temperature of the raw oil. Accordingly, when used in such an environment of hydrogen sulfide as having set out above, the alloy is so designed that a corrosion-resistant film is formed on the surfaces of the alloy. The corrosion-resistant film should have a two-layer structure including an outer layer consisting of a Ni sulfide film and an inner layer consisting of a Cr oxide film. In order to facilitate the growth of the inner layer of the Cr oxide film, at least one of Mo and W is incorporated in the alloy. The reason why the corrosion-resistant film is designed to have a double-layer structure is so that hydrogen sulfide is prevented from entering into the inner layer by means of the outer layer of the Ni sulfide film thereby preventing the breakage of the Cr oxide inner film with the hydrogen sulfide. The Cr oxide inner film is able to suppress the dissolution of the alloy and thus, acts to improve the corrosion resistance, ensuring a good corrosion resistance to hydrogen sulfide.
However, it has been confirmed that in an environment where the partial pressure of hydrogen sulfide is relatively low at a level of about 1 atm., or below and t
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Phipps Margery
Sumitomo Metal Industries Ltd.
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