Passivation is a chemical process that is used to help restore contaminated stainless steel to original corrosion specifications. Parts are placed in baskets and submersed into a diluted nitric acid or citric acid solution for a specified length of time and temperature. This process is designed to remove free iron or other foreign matter from the surface of the metal. Passivation, however, is not effective for removing heat tint or oxide scale as a result of heat treating or welding. Other than giving the part a clean finish, the process does not brighten or change the part appearance.
To attain a contaminant free part, it may be necessary to add a pre-cleaning operation, e.g. vapor degreasing or alkaline cleaning. In the past, passivation was typically preceded by a pre-cleaning process known as “pickling”. The pickling solution, made from a hydrofluoric acid mixture, was used for dissolving embedded steel flecks and removing the heat tint and oxide scale. Due to EPA regulations, most companies have eliminated pickling. Without this aggressive pre-cleaning operation, in many cases, passivation does little to improve the corrosion resistance of stainless steels.
The Electropolishing Process
Electropolishing is essentially a “reverse plating” technique that can be performed at Able Electropolishing on nearly any metal alloy. By using a combination of chemicals and electrical current, electropolishing carefully removes surface imperfections and contaminants from metal parts (and can be used for removing heat tint and oxide scale). Precise monitoring removes as little as .0001” up to .0015” (per surface) to produce the desired result.
The electropolishing process is often used as a more aggressive alternative to passivation (per ASTM B912 and ASTM A967) and does not require a separate treatment for passivation. Since the process removes the outer skin of metal, embedded contaminants which can cause sub-surface corrosion are eliminated. The electropolished part is left in a homogenous and passive condition which offers superior corrosion resistance to chemical passivation.
*Able Electropolishing provides passivation per ASTM A967, ASTM A380 and AMS2700. In addition, we passivate titanium per ASTM F86.
Case Study 1: Electropolished Stainless Steel
The samples shown in Fig. 2 were submitted to the ASTM B-117 salt spray test by an independent laboratory. These photos show how the electropolished stainless steel samples have a superior corrosion protection compared to the raw parts. Stainless steel applications that require high resistance to corrosion for food, semiconductor and biopharmaceutical equipment often specify electropolishing. The electropolished stainless steel parts shown above have no visible signs of rust even after 888 hours of exposure to salt spray. [Fig. 2b]
As the two actual scan electron micrographs [Fig. 3a and 3b] clearly show, the un-polished stainless steel has many flakes of die steel embedded, which will eventually be the source of sub-surface corrosion. The Able Electropolishing technique typically dissolves .0005 – .001″ of the “skin” metal, leaving an ultra-smooth, fully passivated and brightened surface that will resist corrosion for many years.
Able is America’s largest electropolishing specialist, employing over 150 people on three shifts. The company operates a state of the art facility in Chicago, IL. Literally thousands of companies in every industry worldwide rely on Able technology for their metal parts.
The best way to determine if electropolishing is the best solution for YOUR part is to send us a sample. We will electropolish it for FREE and send it back to you in a few days. Call us at 888-868-2900 to schedule your FREE sample.
Case Study 2: Electropolishing Vs. Passivation
To portray the benefits of electropolishing vs. passivation, we subjected two sets of parts to the ASTM B-117 salt spray test at an independent testing facility. The test parts included a formed auger made from 430 stainless steel and a wire weldment fabricated from 304 stainless steel. The testing was performed on each part in the raw [Fig. 4a], passivated [Fig. 4b] and electropolished state [Fig. 4c].
The pictures show the auger after 144 hours of exposure in the salt spray cabinet. As you can see, after 144 hours of exposure, the raw [Fig. 4a] and passivated [Fig. 4b] parts show advanced red corrosion on the extremities and in the hole. The electropolished [Fig. 4c] part exhibits light red corrosion at the extremities and shows the superior corrosion resistance gained with the use of Able’s electropolishing process.
After 72 hours of exposure to the salt spray, the raw and passivated wire weldment parts [Fig. 5a, 5b] developed red corrosion in the resistance welded areas. The electropolished [Fig. 5c] part showed no visible corrosion after 144 hours of testing.
Electropolishing is effective in treating the carbide precipitation condition that occurs in the heat affected zone during welding. It also improves the chrome to iron ratio on the surface which greatly improves the corrosion resistance.
One of the most common applications for electropolishing is to enhance corrosion resistance on a wide variety of metal alloys, specifically stainless steel. Electropolishing is quickly becoming a replacement process for a long established treatment: Passivation. Passivation is a chemical process that has been used for years to help restore contaminated stainless steel to original corrosion specifications.
Applicable to ALL grades of Stainless Steel
Since electropolishing removes surface metal, all surface impurities are removed as well. Heavily contaminated surfaces such as machined parts, welded or brazed assemblies or other components that typically respond poorly to passivation alone are good candidates for electropolishing. Unlike passivation, 300, 400 series and precipitating grade stainless steels can be electropolished without distortion, flash attack or hydrogen embrittlement.
Anti-corrosion Solution for Many Alloys
Just as electropolishing is used to enhance corrosion resistance on stainless steel, it offers corrosion resistance on other alloys as well. Many companies use the process to delay or retard the corrosion properties of copper, brass, aluminum and carbon steels. On these and other alloys, the removal of surface skin and impurities enhances the corrosion resistant properties of the component. For components needing light corrosion resistance that cannot afford an expensive plating operation, electropolishing may be a solution.
Works Where Passivation Does Not
Several popular grades of stainless steel cannot be passivated due to low levels of chromium and nickel. When these stainless steels are passivated by standard methods, the resultant flash attack actually weakens the base material and does irreparable damage to the part.
By dissolving surface metal, we remove deeply imbedded contamination, reduce surface area and remove the false or amorphous layer that is produced by grinding, machining, stamping or lapping metal. Less imbedded contamination and less retained surface moisture mean less chance for corrosion to begin.
Our electropolishing process is performed in accordance with ASTM B912 and many other company specifications.