Corrosion inhibition in sintered stainless steel

A method of enhancing the corrosion resistance of sintered stainless steel, particularly against attack by acids, such as acetic acid, under non-oxidizing conditions. The sintered stainless steel is activated, for example by treatment with acid and is then treated with a phosphate containing solution. The sintered stainless steel has improved resistance to subsequent attack by acid.

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 
thereof generated by treatment with a phosphate-containing solution or the 
like. 
It has surprisingly been found that if, rather than attempting to maintain 
or enhance the oxide protective layer (generally a Cr.sub.2 O.sub.3 
protective layer) conventionally present, sintered stainless steel is at 
least partially activated by removal of oxide protective layer from 
interior surfaces and is subsequently treated with a phosphate containing 
solution, the resistance of the sintered stainless steel to corrosion 
under non-oxidizing conditions, and particularly to corrosion by aqueous 
organic acid solutions, is enhanced. 
While it is well known that phosphate coatings can be applied to ordinary 
steels to improve their corrosion resistance, it has not been proposed 
previously to replace the normally effective passivating oxide layer on 
stainless steel by a phosphate layer. There is however an important 
advantage of a phosphate layer on the interior surfaces of sintered 
stainless steel, this being that it is insoluble in acetic and other 
acids. Thus while previous proposals have involved the use of an 
alternative material for acidic environments it has been found that by 
using an alternative passivating layer, effective corrosion resistance can 
be obtained. 
Thus, sintered stainless steel in accordance with the invention can be used 
in processes such as those outlined above where acetic acid is involved 
and conventionally passivated sintered stainless steel corrodes. The 
sintered stainless steel in accordance with the invention is particularly 
suitably used in the form of filters or molds, for example for use in the 
foodstuffs industry. 
It has been found that sintered stainless steel which has been treated with 
phosphate can be used in the sausage molding process outlined above, 
without clogging with deposits. Furthermore, sintered stainless steel thus 
treated has been immersed in proprietary whisky and no discoloration 
observed after several hours. 
In the case of the molds for sausages as described above, passivated 
stainless steel initially supplied can be subjeced to a preliminary run 
using acetic acid to attack the conventional protective film of oxide 
followed by a run using phosphate. 
To ensure that the phosphate layer remains, further treatment on a periodic 
basis may be desirable. In the case of the manufacture of sausages for 
example, the apparatus may be flushed through with phosphate at the end of 
each day's run. Care must be taken, however, to ensure that phosphate 
deposits do not clog the sintered material. A particular advantage of 
using phosphates is that food grade phosphates are readily available. 
Thus, periodic treatment of the sintered stainless steel can be carried 
out in food and drink processes without great problems. 
According to a yet further aspect of the invention there is provided a 
method of inhibiting corrosion in a sintered stainless steel element 
through which passes an acidic solution, preferably an aqueous acetic acid 
solution, wherein passage of the acidic solution is terminated, and a 
phosphate containing solution is passed through the element, following 
which the passage of the acidic solution is recommenced. 
The use of the phosphate containing solution should be as soon as possible 
after the termination of the acidic solution. If this is not so, corrosion 
products may be accumulated; in the case of acetic acid these can be 
rinsed away by the use of further acetic acid. After use of the phosphate 
containing solution, air may be passed through the sintered stainless 
steel element. It may be desirable however to flush through with acid or 
water. 
It will be appreciated that not all of the interior surfaces of the 
sintered stainless steel may be provided with a phosphate layer. For 
example, if oxide passivated sintered stainless steel is subjected to 
acetic acid, it may be that removal of the oxide layer will only be at 
certain points within the sintered material. If the phosphate treatment 
itself, or any pre-treatment, is not such as to attack the oxide layer 
then a phosphate layer will only be formed at such points. 
It will be appreciated that from one aspect the invention can be considered 
as the use of phosphate treated sintered stainless steel in food or drink 
processing in acidic environments; or as the use of such material in the 
processinag of alcohol, and particularly whisky. The invention is 
applicable in the case of acidic environments where there is acetic acid, 
and is expected to be applicable in the case of other organic, 
particularly carboxylic, acids. 
In the case of normal, i.e. non-sintered stainless steels, phosphates are 
known to provide protective layers which resist a wide range of corrosive 
environments and it is therefore expected that sintered stainless steel in 
accordance with the invention will be of use in such environments.

DETAILED DESCRIPTION 
An embodiment of the invention will now be described by way of example 
only: 
In apparatus substantially in accordance with U.K. Patent No. 2004454B, 
sintered stainless steel molds were used. The stainless steel had the 
designation 316L and the analysis from the British Steel Corporation was 
as follows: 
______________________________________ 
Max. % Min. % 
______________________________________ 
C 0.07 0.00 
Si 1.00 0.20 
Mn 2.00 0.50 
Ni 13.00 10.00 
Cr 18.50 16.50 
Mo 3.00 2.25 
Ti 0.00 0.00 
S 0.03 0.00 
P 0.045 0.000 
Fe approx. 65% 
______________________________________ 
In the preparation of sausages an aqueous solution of acetic acid of pH 
2.25 was passed through the mold walls. After termination of use of the 
apparatus a dark colored deposit formed gradually. After one run with new 
molds, the amount of deposit eventually formed was not great. After two 
days, however, the amount of deposit was such as to severely restrict the 
flow of acid. The deposit did not form immediately when use of the 
apparatus was terminated, but appeared gradually. The deposit could be 
cleared to a certain extent by flushing through with more acetic acid, but 
it reappeared after the flow was terminated and the porosity of the 
sintered stainless steel was reduced. 
An analysis of acid which had passed through a fresh untreated mold, showed 
the following metal contents: 
______________________________________ 
Fe 0.7 ppm 
Cr less than 0.2 ppm 
Ni less than 0.1 ppm 
______________________________________ 
After a mold had been left standing for two weeks--after use with the 
acid--it was found that a new run with acetic acid resulted in the acid 
having the following metal contents: 
______________________________________ 
Fe 190 ppm 
Cr 23 ppm 
Ni 42 ppm 
______________________________________ 
This is an indication of the extent of corrosion when the mold is left in 
an acidic environment without a continuous flow. 
Analysis of the solid deposit showed it to have the following metal 
contents: 
______________________________________ 
Fe 21.8% w/w 
Ni 2.59% w/w 
______________________________________ 
In accordance with the invention, after initial clearing of the deposit 
with acetic acid, the molds were flushed through with an aqueous solution 
of trisodium orthophosphate having a pH of about 11.7 for a period of say, 
5 to 10 minutes. Following that, air was pumped through the molds. It was 
found that no deposits formed, and that even after further use of the 
molds with the acetic acid solution, no such deposits appeared. From this 
it was deduced that a protective layer of e.g. ferric phosphate had been 
formed at least in those regions where acid attack would normally take 
place. 
It having been discovered that sintered stainless steel filters were 
discoloring whisky, an analysis was sought as regards the acid content of 
such whisky. It was found that proprietary whisky has an acid content of 
about 15-23 grams per 110 l of which about 80%-90% is acetic. The pH of a 
proprietary blended whisky was found to be about 3.6. 
A sample of the sintered stainless steel used in the sausage molding 
process just described, prior to any phosphate treatment was immersed in a 
sample of proprietary whisky ("Bells"--Trade Mark), which was discolored 
within twenty minutes. A sample of the phosphate treated sintered 
stainless steel--taken from the sausage molding apparatus--was then 
immersed in a fresh sample of the same proprietary whisky. No 
discoloration was noted even after several hours. 
It will thus be appreciated that the invention permits sintered stainless 
steel to be used in corrosive conditions which before could not be 
tolerated. It has been stated previously that there is no sense in using 
sintered stainless steel in acidic solutions in non-oxidizing conditions. 
It has now been found that at least in the case of acetic acid solutions 
under the conditions described--where access of oxygen to the interior of 
the sintered stainless steel is restricted--the invention permits sintered 
stainless steel to be used. 
While the use of food grade phosphates to generate the replacement 
passivating layer on the sintered stainless steel according to the methods 
of the invention is preferred where the sintered stainless steel is to be 
used in the foodstuffs industry, it is anticipated that the present 
invention extends beyond this to include the use of materials having 
equivalent anticorrosive properties to phosphate such as chromates, 
oxalates etc to generate conversion coatings such as chromate, oxalate, or 
other inorganic coatings, as the protective layer in place of the oxide 
surface layer on sintered stainless steel. As a further possibility, the 
use of a two-stage treatment of at least partially activated sintered 
stainless steel has been considered; in such a process a treatment with a 
phosphate containing solution might be followed by treatment with an acid 
chromate solution. 
According to a yet further aspect of the invention there is therefore 
provided a method of treatment of sintered stainless steel wherein on at 
least partially activated interior surfaces thereof is generated a 
protective surface layer substantially resistant to corrosion by agents 
which attack conventionally passivated stainless steel, such as acid 
solutions in non-oxidizing conditions. Thus, the method of this invention 
is useful to enhance the corrosion resistance of sintered stainless steel 
articles. This method of treatment comprises passing such an article 
through a material which generates a conversion coating on interior 
surfaces of the article. The invention also extends to stainless steel so 
treated.