Iron Residue / Contamination on Stainless Steel Surfaces
Iron residue and contamination on stainless steel surfaces (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.
Sources of Free Iron
No list can possibly include all the potential sources of iron residue 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 residue 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 iron residue on the surface.
One of the most difficult sources of iron residue 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 residue 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 methods:
- 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.
- 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.
- 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 Iron Residue on Stainless Steel Surfaces
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:
- 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 re-contaminated. 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.
- 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 passive film.)
- 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.
- Electropolishing – Like machining and grinding, this process removes the surface of the part, including any embedded iron.
Contamination on 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.