Iron residue on the surfaces of Stainless steel parts (either cast or wrought)
has been a recurring problem for many years – probably since Stainless steels
were first developed. Here we will discuss the possible sources of the residue,
the consequences of it being present, methods for detecting it, and methods for
First, a word about what iron residue is and what it is not. The residue is free
(unAlloyed) iron on the surface. Free iron should not be confused with Alloyed
iron which is a major component of the Stainless steel, or with ferrite which is
a specific type of crystalline structure and a normal component of Stainless
steel, especially cast Stainless steel.
Sources of Free Iron
No list can possibly include all the potential sources of free iron
contamination. Please consider the following as examples of sources which may or
may not play a part in a particular situation.
Any steel or iron item which comes in contact with the Stainless steel is a
potential source of contamination. This includes chains, slings, metal shipping
containers, work benches, tools (hammers, wrenches, pliers, etc.), machine tools
(lathes, mills, machining centers, jaws of chucks, etc.), fork lift trucks, and
steel shot or grit used to remove scale, etc.
Iron may also be transferred to Stainless steel surfaces from materials which
were previously used on steel or iron parts. This includes blasting, grinding,
and polishing abrasives; the iron parts they were previously used on may be
the containers or the systems used to handle the abrasives, such as blasting
cabinets. Of course, iron or steel shot or grit used to remove scale or other
materials from the Stainless steel items would leave residue on the surface.
One of the most difficult sources of iron to avoid is the atmosphere itself.
Industrial areas have a surprising amount of iron in the air. This iron “falls
out” onto exposed items, including previously cleaned Stainless steel parts.
Also, water which is used to “clean” the surfaces may itself contain iron which
will be deposited onto the surfaces thought to be clean. Note that water may
also contain other chemicals which may leave rust-colored deposits which may be
mistaken for indications of the presence of free iron.
As mentioned above, there are so many possible sources of iron contamination
that no list of potential sources of iron residue can be complete. Those listed
here should be considered examples of the types of sources which should be
considered when trying to avoid the contamination.
Consequences of Free Iron Contamination
Again, no one can list all the possible consequences of iron contamination.
However, there are some broad categories:
Appearance – Free, unAlloyed iron on the surface of any item will oxidize (rust)
given the appropriate conditions (warmth, moisture, and oxygen). The reddish
brown deposits are easily recognized. People around the world see rust as a
deterioration of metal items and work to avoid it where possible. Thus, even the
appearance of rust is taken as objectionable.
Material Identification – Because rust is associated with iron or steel, items
which appear rusty are often assumed to be iron or steel. Thus when Stainless
steel parts (or brass parts or nickel parts or ...) are covered with rust, it is
often assumed that they are not Stainless steel (or brass or nickel or ...).
Since the buyer of the parts paid for and was expecting Stainless steel (or
brass or nickel or ...), his/her first reaction is usually “I’ve been cheated!”
Process Contamination – Stainless steels are often used to handle pure
substances such as chemicals, cosmetics, and pharmaceuticals. Even small amounts
of iron in these materials can change their color or behavior or both and
possibly render them unfit for use.
Some things that free iron does not do is to cause galvanic corrosion or pitting
corrosion, etc. It MAY be possible that the iron could accelerate some forms of
corrosion if there is enough present. For example, iron accelerates the general
corrosion rate in nitric acid as demonstrated in the Huey test.
Detecting Free Iron
Free iron cannot be seen on the surfaces of contaminated parts. Therefor, it
must be converted into a visible form. There are at least the following three
1. Moisten the surface, either by spraying with clean water or by immersing the
part in clean water. The water accelerates the reaction of the iron with oxygen
to form iron oxide (rust) which is readily visible.
Sometimes, just allowing the part to sit undisturbed for a period of time,
especially in a humid environment will result in the conversion of iron to iron
oxide. However, this can take several days or weeks.
It is important that the water is clean. If it contains iron (from iron
plumbing) or certain other chemicals, it will give a false indication of iron on
the part surface.
2. Spray the surface with a solution of copper sulfate in water. If free iron is
present, a copper film will form. In this test, the chemical reaction is:
Fe + CuSO4 = FeSO4 + Cu
The copper film is immediately visible.
3. Use the “ferroxyl test.” Spray the surface with a solution of potassium
ferricyanide. If free iron is present, a blue color will appear. This test is
extremely sensitive and often gives false positive results, that is, it gives an
indication of iron being present when it really is not. The ferroxyl solution
must be made fresh each day.
Both the copper sulfate and the ferroxyl tests are described in ASTM A380.
Removing Free Iron Contamination from Stainless Steel
One of the first points which should be made regarding the removal of iron
residue is that mechanical methods such as abrasive blasting have not been
successful. The abrasive merely moves the iron around on the surface; it does
not remove it. The only mechanical methods which are successful are those which
remove the surface, such as machining or grinding.
The only known methods for removing iron from the surfaces which are not
machined are chemical and electro-chemical methods. And not all chemical methods
are successful – nitric acid alone does not do the job. The known useable
chemical methods include:
1. Oxidation – This is most readily accomplished by heating the part in air to
normal heat treating temperatures. The iron is converted to iron oxide which can
then be removed by abrasive blasting. This method is acceptable for unmachined
sand castings since the scaling which occurs is not detrimental. The sand
blasting abrasive must be free of iron contamination or the part will be
However, heat treating in air is not suitable for parts with machined surfaces
and often not for investment castings. Heat treating in vacuum or in protective
atmospheres is also not suitable since the iron is not oxidized.
2. Pickling – This is probably the most commonly used method. Use a solution of
nitric and hydrofluoric acids in water. We use ASTM A380, solution D. The
formula is specified in A380 as 6 - 25% HNO3 and ½ - 8% HF in water at 70 -140 F
(21 - 60 C) for about 30 minutes. This is a strong cleaning solution and may
etch highly finished surfaces.
Pickling should not be confused with passivating. Stainless steel self-passivates
on exposure to air – no special passivation treatment is required. (However, it
may be that using a “passivating” treatment such as nitric acid, also described
in ASTM A380, may accelerate the formation of the passive film or form a thicker
3. Chemical Cleaning – Some citrus-based cleaners have been shown to remove free
iron contamination. However, there are some concerns about the stability of
these cleaners since they may be subject to bacteria growth.
4. Electropolishing – Like machining and grinding, this process removes the
surface of the part, including any embedded iron.
Contamination of Stainless steel surfaces with free iron is common. It can be
avoided only with very careful handling. The presence of free iron on the
surfaces of interest can be detected by a variety of tests, including the copper
sulfate and ferroxyl tests. Iron contamination can be removed by certain
chemical or electro!chemical methods; abrasive blasting alone is not effective.