QUESTION:
My 316 stainless-steel valve is rusty and attracts a magnet. Did I get the wrong alloy?
ANSWER:
Let's answer the second part of the question first. Forged 300-series stainless steels should be non-magnetic. However, cast
versions of the 300-series stainless steels-such as CF8 (304), CF3 (304L), CF8M (316), CF3M (316L), CG8M (317), CG3M
(317L), CF8C (347), etc.-are all formulated to contain some ferrite. The presence of ferrite makes the alloy less prone to
cracking as the hot casting cools in the mold. Weld filler materials are also formulated to contain some ferrite for the same
reason. The ferrite makes the material attract a magnet. The exact amount of ferrite, which influences the strength of
magnetic attraction, is dependent on the exact chemical composition and the thermal history of the casting. In any event,
CF3, CF8, CF3M, CF8M, CG3M, CG8M, and CF8C should all attract a magnet to some degree. In fact, if you have a casting in
one of these alloys that doesn't attract a magnet, you should wonder whether something is wrong.
Now let's discuss the rusting portion of the question. While the term stainless steel implies that such an alloy is "stainless"
or will not rust, the reality is that stainless-steel castings can exhibit a rusty appearance if not processed correctly. The rusty
appearance is only superficial surface rust, i.e. iron oxide, and for most applications it is neither a problem nor an indication
as to how the equipment will perform in service. However, there are some services where this surface rust is objectionable
and special processing is necessary to make sure the stainless-steel castings are truly "stainless." Some of the industries
where any rust is undesirable are food and beverage, pharmaceutical, and electronics.
Shot blasting is routinely used by foundries and forge shops to remove burned-in sand, core material, and scale that are
formed during casting, forging, and/or heat-treating processes. Steel shot is normally used because it is the quickest and
most economical cleaning abrasive. A slight amount of iron contamination remains on the surface of the stainless steel, and
may later cause surface discoloration or rusting.
Sand blasting or grit blasting with various abrasives may also be used to clean castings and forgings. If the sand or abrasive
was previously used on steel, it may contaminate the stainless-steel surface, which can result in rusting. Therefore, if sand
or grit blasting is utilized, the sand or grit should either be new or only used previously for cleaning stainless-steel items.
Pickling, or the more technically correct term acid cleaning, is used to remove iron oxide that formed during the heat
treatment of stainless steel or to remove free iron contamination that may have occurred from shot blasting, grit blasting,
and/or grinding of stainless- steel castings or forgings. Normally, acid cleaning is only necessary for alloys that do not have
sufficient chromium content to prevent oxidation of the surface during heat treatment, such as the 300-series stainless
steels. The higher-chromium-content duplex stainless steels do not require acid cleaning unless they are to be used in one
of the rust-sensitive industries mentioned above. Pickling is normally done as a last step before machining.
Passivation is the process by which a stainless steel will form a thin, invisible, chemically inactive surface when exposed to
air or other oxygen-containing environments. At one time it was considered necessary to apply an oxidizing treatment to
stainless steels in order to establish this protective oxide film. It is now accepted that this oxide film will form spontaneously
in any oxygen-containing environment provided the surface has been thoroughly acid cleaned, i.e. pickled or descaled. This
oxide film is the mechanism by which corrosion-resistant alloys achieve their corrosion resistance. Although a stainless steel
is basically self passivating in any oxygen-containing environment, even air, some users of stainless-steel equipment still feel
it is necessary to require a separate passivation treatment. If passivation is performed, it is generally done after finish
machining.
Acid cleaning and passivation are covered by ASTM specifications A380 and A967. These specifications provide complete
details on the acid cleaning and passivation of stainless steels as well as a variety of tests to measure the effectiveness of
these processes, including simple wet-dry immersion tests to chemical testing for the detection of free iron. As with any
special order requirement, there should be discussions between the manufacturer and customer up front so that everyone is
in complete agreement on the processes to be used and how the results will be measured.