Metal treatment – Compositions – Heat treating
Patent
1983-02-03
1985-08-20
Miller, Edward A.
Metal treatment
Compositions
Heat treating
148 315, C23F 710
Patent
active
045362289
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to the inhibition of corrosion in sintered stainless steel.
BACKGROUND OF THE INVENTION
Stainless steel generally owes its good corrosion resistance to a passivating film of chromic oxide. It has been found that sintered stainless steel has a lower corrosion resistance than the wrought metal even when passivated, for example by treatment with nitric acid. There may be several reasons for this, including inadequate passivation and an increase in pitting corrosion caused by the porosity of the sintered material. These problems are particularly serious in the case of attack by acid solutions in non-oxidizing conditions or e.g. hot chloride solutions. In normal circumstances, while acid attacks the passivating film of oxide, the film is constantly being replenished. If there is de-aeration of the solution, however, breakdown of the film occurs rapidly.
Sintered stainless steel has therefore been thought unsuitable for use in the presence of such corrosive materials. Where a sintered material is necessary, e.g. for use as a filter, substances such as sintered glass may be used but these present their own problems. In the food industry in particular it may be necessary to ensure that particles or fragments of a sintered filter do not pass into food products--the use of sintered stainless steel filters might therefore be much preferable to the use of sintered glass filters as the detection of stainless steel fragments would be more easily achieved than that of glass fragments.
It has been proposed, in U.K. Patent No. 2004454B to mold a sausage in a sintered mold and to pass acetic or another acid through the wall of the mold to treat the surface of the sausage. In tests it was found that with molds of sintered stainless steel, after using acetic acid, the molds became clogged with a dark deposit and virtually unusable.
It has also been found that when using sintered stainless steel filters in the filtration of whisky, the whisky becomes discolor. Indeed, in a test, when a piece of sintered stainless steel was immersed in whisky, discoloration was rapid. Investigations have shown whisky to have a relatively low pH of say 3.6, with between 80% and 90% of the acid content being acetic. It is thus considered that the problem encountered may correspond to that in the case of sausage molding as described above.
There is thus a distinct problem in that while sintered stainless steel might be considered a useful material in for example the processing of food or drink, it suffers badly from corrosion problems. These have been particularly noted in the case of acetic acid and would be expected with other acids.
The object of the invention is therefore to reduce or eliminate these substantial corrosion problems.
SUMMARY OF THE INVENTION
According to one aspect of the invention there is thus provided a method of producing sintered stainless steel of enhanced corrosion resistance which method comprises the treatment of at least partially activated interior surfaces of the sintered stainless steel with a phosphate containing solution.
The phosphate layer might be produced by treating active sintered stainless steel in a manner known for other steels, for example by immersing the sintered stainless steel in a solution of phosphoric acid containing e.g. iron phosphates. It has however been found that effective results can be obtained by the use of alkaline solutions of for example sodium pyrophosphate or trisodium orthophosphate. Thus readily available food grade alkalis, such as these, can be used in aqueous solution. In the case of already passivated sintered stainless steel it may be necessary to remove the protective oxide layer before treating with phosphate but this presents no problems. Mere treatment with e.g. acetic acid causes depletion of the oxide layer as is known from the corrosion problems which have been encountered.
According to a further aspect of the invention there is provided a sintered stainless steel having a protective surface layer on interior surfaces
REFERENCES:
patent: 2476345 (1949-07-01), Zavarella
patent: 2521580 (1960-09-01), Hornak et al.
patent: 2927873 (1960-03-01), Bengtsson et al.
patent: 4168983 (1979-09-01), Vittands et al.
patent: 4194929 (1980-03-01), Menke
patent: 4362577 (1982-12-01), Jones
patent: 4384900 (1983-05-01), Gotta et al.
patent: 4420336 (1983-12-01), Klar et al.
Ro, D. H. and E. Klar, "Corrosion Behavior of P/M Austentic Stainless Steel", in Modern Developments in Powder Metallurgy, vol. 13, 1981, pp. 247-287.
Tikkanen, M. H. A., "Modern Methods on Studying Corrosion Resistance of Sintered Stainless Steels".
Markva Neil F.
Miller Edward A.
Pemberton Sintermatic S.A.
Thexton Matthew A.
LandOfFree
Corrosion inhibition in sintered stainless steel does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Corrosion inhibition in sintered stainless steel, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Corrosion inhibition in sintered stainless steel will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-558838