Electropolishing vs. Passivation

Electropolishing Example: Raw, Passivated & Electropolished

After passivating your parts, do you still have issues with corrosion, microcracks, microburrs, contamination, or cleanability?

Do you want to treat metals beyond stainless steel, like Nitinol or titanium?

While passivation is useful for removing free iron and contaminants from stainless steel and is more well known, here’s what you should know about electropolishing:

  • Electropolishing is 30x more effective than passivation at eliminating corrosion and surface defects
  • Metal parts are passivated as a result of the electropolishing process
  • Electropolishing can be used on far more metal alloys than passivatoin

Electropolishing Overview

Electropolishing removes a microscopically precise layer of surface material, to tolerances as low as +/-.0001″, depending on the alloy. This degree of precision and consistency, along with the ability to remove a variety of surface imperfections in one process, is why electropolishing is so often specified for critical parts used in medical implants and surgical instruments, aerospace components and fragile or critical parts used in other industries. Electropolishing is a one-stop process for a micro-smooth, ultraclean, corrosion and pathogen-resistant surface finish – with 30 times more corrosion resistance than passivation.

Electropolishing also eliminates the need for additional metal finishing processes like hand deburring, tumbling, pickling, and cleaning.

Electropolishing Compared to Passivating Stainless Steel

Electropolished part after 144 hours of salt spray

Electropolished part after 144 hours of salt spray

Passivated part after 144 hours of salt spray

Passivated part after 144 hours of salt spray

Fig. 1a - After 2560 hours of salt spray-Passivated (left), Electropolished (right).

Fig. 1a – After 2560 hours of salt spray-Passivated (left), Electropolished (right).

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 homogeneous 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.

04-24hrs

Fig. 2a – After 24 hours ASTM B-117 salt spray testing.

After 888 hours ASTM B-117 salt spray testing.

Fig. 2b – After 888 hours ASTM B-117 salt spray testing.

The Stainless Steel Passivation Process

Passivation of metal, similar to electropolishing, is a metal finishing operation that many industries specify for their parts. Passivation is a chemical process designed to increase the corrosion resistance of stainless steel parts by removing free iron and other foreign matter on the surface of the metal.

To passivate stainless steel parts, they are placed in a basket and submerged in a nitric or citric acid bath. The acid concentration, temperature and time the part spends in the bath depends on the stainless steel alloy of the part itself. The process of passivating stainless steel effectively removes free iron and foreign materials from the part’s surface, leaving the part clean and more corrosion resistant.

Passivated stainless steel, however, doesn’t benefit from removed 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.

Attaining a contaminant free part with passivation may also require a pre-cleaning operation, e.g. vapor degreasing or alkaline cleaning. In the past, stainless steel 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.

Which Metal Alloys Can Be Passivated?

Many grades of stainless steel can be treated with passivation to remove free iron and contaminants from the surface. However, stainless steel passivation is not recommended for some grades of stainless steel due to inadequate levels of chromimum and nickel. Passivation of stainless steel is also not recommended for parts that have been welded or brazed.

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 – .0010″ of the “skin” metal, leaving an ultra-smooth, fully passivated and brightened surface that will resist corrosion for many years.

Fig. 3a - Before 888 hours ASTM B-117 salt spray testing.

Fig. 3a

Fig. 3b - After 888 hours ASTM B-117 salt spray testing.

Fig. 3b

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 866-465-8427 to schedule your FREE sample.

Case Study 2: Electropolished Vs. Passivated Stainless Steel

Fig. 4a - Auger Raw

Fig. 4a – Auger Raw

Fig. 4a -Auger Raw

Fig. 4b -Auger Passivated

Fig. 4c - Auger Electropolished

Fig. 4c – Auger Electropolished

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.

Fig. 5a - Wire Weldment Raw

Fig. 5a – Wire Weldment Raw

Fig. 5b - Wire Weldment Passivated

Fig. 5b – Wire Weldment Passivated

Fig. 5c - Wire Weldment Electropolished

Fig. 5c – Wire Weldment Electropolished

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 with stainless steel 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 Passivated Stainless Steel 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.

Contact us to learn more about electropolishing vs. passivation

Medical Device Manufacturing

Our tightly controlled quality process and ISO 9001 and ISO 13485 certifications make us the preferred partner for many medical device manufacturers across the globe. From implantable devices to surgical instruments, electropolishing is a single-process treatment that results in biocompatible, safe and ultra clean parts.

Learn More  

Aerospace

We offer aerospace vendors and original equipment manufacturers our signature metal finishing services to increase fatigue-resistance for their metal parts, rendering them capable of withstanding the stress and corrosion that comes with long-term use. Surface abnormalities lead to part failure, but with electropolishing, parts become more reliable and long lasting.

Learn More  

Pharmaceutical

Our strict adherence to industry standards ensures that pharmaceutical components are electropolished in accordance with ASTM B912 and ASME BPE specifications. These products are clean and smooth, corrosion resistant and contaminant free.

Learn More  

Automotive

Corrosion resistance and microfinish improvement are essential for critical automotive parts, making electropolishing an increasingly beneficial option for automakers. Electropolishing both interior and exterior auto parts ensures they will resist premature failure and function at an optimal level.

Learn More  

Consumer Appliance

Notable examples of appliance parts we electropolish are 400-series stainless steel alloys used in many consumer appliances. Our metal finishing services help increase corrosion resistance for these parts, and by removing the outermost layer of metal withelectropolishing the durability and longevityof these appliance components is increased.

Learn More  

Food & Beverage

We have a long history of working with the food and beverage industry. Electropolishing materials for this industry eliminates the buildup of bacterial biofilms, as well as keeping the equipment free of contaminants like Salmonella. When it comes to food and beverage processing equipment, safety and cleanliness are of the upmost importance and electropolishing delivers the level of sanitation required by a number of regulatory agencies.

Learn More  

Hydraulics & Pneumatics

From robotics to heavy mobile equipment, the uses of electropolishing in the hydraulics and pneumatics industry are many. By helping prevent premature part failure, electropolishing reduces the amount of downtime and increases the life cycle of these components. Proper surface finish and cleanliness is key in this industry, and electropolishing attains the finishour customers require.

Learn More  

Electronics

Improving conductivity and performance is just one benefit electropolishing offers for electronics parts. The surface finish is improved by as much as 50 percent, removing imperfections from the material without removing a lot of material removal from the component.

Learn More