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(888) 868-2900 | Electropolishing, passivation and pickling are all common, chemically based methods of metal finishing — but they don’t deliver identical results or benefits. Here’s how each works, and how they compare.
Surface imperfections can threaten the fit, function and fatigue life of metal parts.
From microcracks to burrs to oxide scale, contaminants and more, surface imperfections left behind by machining and welding can compromise metal parts — in addition to creating an unclean, unfinished or unattractive appearance.
For some industries and applications, the criteria comes with particularly high stakes, as surface imperfections can become initiation sites for corrosion and pathogens.
While all three processes are chemically based, they differ in their efficacy, metal compatibility, precision, impact on roughness average (Ra), and cost.
A side-by-side overview comparing the results and effectiveness of these three metal finishing techniques.
| Point of comparison | Passivation | Pickling | Electropolishing |
|---|---|---|---|
| Removes surface contaminants | Yes | Yes | Yes |
| Typically includes pre-cleaning/degreasing | Yes | Yes | Yes |
| Effective for sizing | No | No | Yes |
| Microscopic precision | No | No | Yes |
| Consistent, uniform results | Yes | No | Yes |
| Improves fatigue life | No | No | Yes |
| Improves corrosion resistance | Yes | No | YesSee 888-hour salt spray proof ↓ |
| Deburrs metal surface | No | No | Yes |
| Removes microcracks | No | No | Yes |
| Removes heat tint | No | Yes | Yes |
| Removes oxide scale | No | Yes | Yes |
| Improves microfinish | No | No | Yes |
| Cleans welds effectively | No | Yes | Yes |
| Passivates parts | Yes | No | Yes |
Deburred, polished, cleaned and passivated in one pass. Electropolishing can eliminate the separate deburring, polishing, cleaning and passivation steps a part would otherwise need.
A cleaning process that adds corrosion resistance — but removes no surface material.
During passivation, stainless steel parts are placed into a basket and submerged into a nitric or citric acid bath. Depending on the alloy, acid concentration, temperature and time can be adjusted to effectively remove free iron and other contaminants on the part’s surface. By creating a chromium-rich, passive layer on the part surface, passivation improves corrosion resistance while also removing surface contamination. Parts are typically passivated in a bulk process.
Metals with lower levels of chromium and nickel are susceptible to flash attack during passivation, which can weaken the base material and leave the part permanently damaged.
Passivation does not remove scale and discoloration in the heat affected zone for welded or brazed components.
Passivation removes free iron and other contaminants from the surface of the part and adds corrosion resistance, but does not remove surface material.
| What materials can I passivate? | Nitric | Citric |
|---|---|---|
| 300 series stainless steel (except 303) | Yes | Yes |
| 303 stainless steel | No | No |
| 416 stainless steel | No | No |
| 440 stainless steel | No | Yes |
| Carbon steel | No | No |
| 410, 420, 430, 455, 465 stainless steel | Yes | Yes |
| Titanium | Yes | Yes |
| Nitinol | Yes | Yes |
| Non-ferrous metals | No | No |
A cruder, more dangerous process than either electropolishing or passivation.
Pickling is a pre-passivation technique that uses hydrochloric or sulfuric acid to remove surface impurities like rust, scale and other contaminants. It is sometimes used to remove weld discoloration when heavy scale is present.
Pickling requires a clean surface prior to processing — alkaline cleaning, solvents, ultrasonic cleaning and other mechanical or manual methods are required first. Because of these harsh acids, pickling etches metal in both microscopic peaks and valleys, and parts are processed in bulk.
Pickling is too corrosive for many metal alloys and the process can be difficult to control due to variable acid strength. If not controlled properly, it can create an inconsistent finish resulting in pitting and corrosion. Some alloys and high-carbon steels can also become damaged by hydrogen embrittlement. Rinsing is critical to remove residual acid.
While applicable in some industrial applications, significant drawbacks include environmentally hazardous waste. Because of these hazards, the process faces strict EPA regulations for handling and disposal — and its use has declined.
Microscopically precise surface material removal — the reverse of electroplating.
Electropolishing combines a chemical bath with an electric current that precisely removes a uniform layer of surface material from the part. It strips away contaminants and microscopic imperfections such as cracks, burrs, jagged edges and more — improving roughness average (Ra) by up to 50%, depending on the alloy.
Electropolishing can eliminate the need for additional metal finishing processes like hand deburring, tumbling, sanding and other mechanical processes.
Precision is a key advantage: electropolishing removes a microscopically precise layer of surface material, controlled to +/- .0001″ – .0005″ depending on the alloy and the application.
The ability to establish precise, customizable material removal specifications makes electropolishing effective at improving both the fit and function of a treated part. It acts like lightning, removing the high points first — resulting in an evenly smooth surface.
One limitation: because electropolishing removes surface material, it may not be suitable for parts with extremely tight tolerances that cannot accommodate at least .0002″ of material removal.
.0005″ – .0015″ removal per surface, or as required per application
.0001″ – .0005″ removal per surface, or as required per application
Stock removal to be determined for each application
.00025″ – .0015″ removal per surface, or as required per application
Varies to accepted standard sample
Stock removal and finish vary from bright to dull; standards based on sample approval
Foreign material introduced during manufacturing can create initiation sites for corrosion.
Corrosion resistance is essential in virtually every industry. Sources of corrosion that can be eliminated by electropolishing include:
In ASTM B-117 salt spray testing after 24 and 888 hours, electropolished samples of 304, 410, 420 and Trinamet stainless steel consistently outperformed raw and passivated-only samples.
By leaving parts in a passive state, electropolishing eliminates the need for additional passivation, while also significantly improving the corrosion resistance of stainless steel and other alloys. And with racking customized for each part, electropolishing delivers consistent, thorough results for small and fragile parts — and parts with interior diameters as small as ¼″.
Electropolished parts have 30 times the corrosion resistance of parts that are passivated only. The independent salt spray results below show why.
Sample panels of 304, 410, 420 and Trinamet stainless steel — raw, passivated only, and electropolished — were subjected to ASTM B-117 salt spray exposure. The electropolished samples resisted corrosion at both checkpoints, while raw samples corroded heavily.
Photography from independent ASTM B-117 salt spray testing at an accredited outside laboratory.
Passivation, pickling and electropolishing are all chemical processes for improving the surface finish of metal parts — but only electropolishing can consistently and precisely remove surface imperfections and improve Ra by up to 50% for a wide variety of metal alloys, including stainless steel, aluminum, titanium, nitinol and many other metal alloys. Don’t trust your critical parts to anything less.
The best way to determine if electropolishing is the best solution for your part is to send us a sample. We’ll electropolish it for free and send it back to you in a few days.
From implantable devices to surgical instruments, electropolishing is a single-process treatment that results in biocompatible, safe and ultra clean parts.
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.
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.
Corrosion resistance and microfinish improvement are essential for critical automotive parts, making electropolishing an increasingly beneficial option for automakers.
Our metal finishing services help increase corrosion resistance for these parts, and by removing the outermost layer of metal withelectropolishing the durability and longevity of these appliance components is increased.
Electropolishing materials for this industry eliminates the buildup of bacterial biofilms, as well as keeping the equipment free of contaminants like Salmonella. Electropolishing delivers the level of sanitation required by a number of regulatory agencies.
From robotics to mobile equipment, the uses of electropolishing in the hydraulics and pneumatics industry are many. By helping prevent premature part failure, electropolishing reduces downtime and increases the life cycle of these components.
Improving conductivity and performance is just one benefit electropolishing offers for electronics parts. The surface finish is improved by as much as 50%, removing imperfections from the material without material removal.
The extreme heat resistance and resilience characteristic of refractory metals like niobium, molybdenum, tungsten, tantalum and TZM make them useful in applications like aerospace components, small modular reactor components and other parts that must withstand extreme environments.
A wide array of modern assemblies for everything from aerospace parts to medical devices depend on high-quality springs that must perform to incredibly precise specifications, often in very tiny spaces.
Small modular reactors are entering component fabrication under tight schedule pressure. Across every coolant technology, surface integrity is becoming a qualification gate — not an afterthought.
Electropolishing is a precise and controllable process for eliminating defects from the surface of metal parts. But it’s not the only finishing process available to manufacturers; there are mechanical, chemical and plating processes that can remove defects and improve the…