By: Eaton Filtration

Every so often we get a call from someone who has just purchased one of our stainless steel strainers and they want to know, “Is it really stainless?” Why? Because the strainer shows rust spots or is somewhat magnetic.

This is a problem which has plagued producers of stainless steel castings for years. Can something which is supposed to be type 316 stainless really be 316 and yet show rust and/or magnetism? The answer is “yes.”

Let us take rust first. It can (and frequently does} occur on stainless alloys of controlled composition and heat treatment as the result of surface contamination. Among the many sources of contamination, the following are the leading offenders:

1. An iron film left on the surface as a result of a machining or other manufacturing operation will tend to rust in the presence of moisture.
2. Microscopic scale particles left on the surface after pickling may become visible as rust under suitable conditions.
3. Pickling solution oozing from minute pores in the metal may stain the surface and oxidize to a brown rust color due to the iron which it contains.
4. The accumulation of the natural corrosion products of the alloy in corrosive service on a rough surface may cause a brown stain due to oxidation.
5. Discoloration may be caused by the accumulation of any extraneous processing material which is of such a nature as to cause a “rusty” appearance on a rough surface.

A smooth or polished surface will always stay cleaner and brighter under mildly corrosive conditions than a rough surface. Although it is true that stainless steel is at its best when highly polished, (it should be remembered that, under strongly corrosive conditions, this polish is soon removed. It is the inherent resistances of the alloy that counts and “rust” conditions such as those described are relatively harmless. They are the results of surface contamination and in no way reflect the composition of the alloy.

Now for magnetism. Users of stainless steel are accustomed to finding the wrought types of 304 and 316 practically non-magnetic. It Rust & Magnetism in S.S. Strainers comes as a surprise to many that castings of somewhat similar composition are often found to have considerable magnetism. The result is that such compositions are suspected of being improperly made, or outside specified limits, and lacking in proper corrosion resistance. However, such is not the case.

Stainless alloys, wrought or cast, are composed of elements like carbon, nickel and manganese, which tend to promote a non-magnetic austenitic structure, and elements such as chromium, silicon, and Molybdenum, which promote. a magnetic ferrite. The amount of non-magnetic austenite and magnetic ferrite varies with composition and can co-exist.

A stainless steel composition that is to be produced in the wrought form must be one that has satisfactory rolling or forging properties, while a somewhat similar cast composition is designed
to give the foundryrnan good “castability.” Hence, the wrought composition will be made with higher nickel and manganese than the comparable cast type.
This is particularly true of type 316, where to overcome the ferritizing effect of molybdenum, considerably more nickel is used, resulting in a composition that can withstand the rigorous rolling or forging operations. Such a composition is virtually non-magnetic.

The corresponding cast form is lower in nickel, because the hotworking difficulties caused by molybdenum need not be counterbalanced. On the other hand, the foundryrnan, to gain “castability”, will increase silicon. This change in composition tends to promote the formation of ferrite with an increase of magnetism in the product.

For process industry use, our interest is not in how much magnetism an alloy has, but how corrosion-resistant it is. Corrosion rates are related to the amount of each element present in composition. Chromium imparts oxidation resistance with increasing amounts of the alloy present. Nickel and manganese, within normal ranges, have little effect on corrosion rates but carbon and silicon tend to decrease corrosion resistance. Studies made of comparable types of wrought and cast alloys show that corrosion resistance is approximately the same, even though the wrought type is non-magnetic and the cast type magnetic.

There are other phenomena which affect magnetic characteristics. Heavy sections tend to be more magnetic than thin ones. The cause is “mass effect”, which means that different sections have cooled in the mold at different rates, promoting the segregation of ferrite. Since these ferrite areas are not changed by subsequent heat treatment, they remain in the finished casting.

It must not be assumed that this ferrite segregation is harmful to corrosion resistance. In some cases, it can be helpful because ferritic material- is less •susceptible to intergranular attack. It will trap chromium carbides that ordinarily would be precipitated• along- the- grain boundaries of a completely austenitic (and non-magnetic) composition.


For more information please visit Eaton Filtration
Article Copyright Eaton -2011


About Eaton's Filtration Business
Eaton’s Filtration business is a global leader in manufacturing filtration products that include automatic self-cleaning and fabricated pipeline strainers, mechanically cleaned filters and strainers, bag and cartridge filtration systems, and gas liquid separators for industrial customers worldwide.

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