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 removing it.
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.
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.
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.
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 methods:
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:
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.