Case Study – Electropolishing

A puzzling surface defect on a 304 stainless steel round bar, resulted in a Sassda member having valid concerns over the material quality but a subsequent the investigation by Sassda’s Technical Team revealed a different story. The culprit wasn’t in the alloy, but in the process. Here’s how we traced the problem and what it means for quality assurance in stainless steel finishing…

The case that started it all

A batch of 304 stainless steel round bar emerged from electropolishing with an unexpected matte finish and dull patches breaking up what should have been a bright, reflective surface. There was an initial finding of high carbon content, but independent laboratory tests quickly ruled that out. The carbon level was comfortably below 0.08%, well within the 304 specification limits.

Closer examination showed that the dullness was confined to the outer surface and disappeared completely after light mechanical polishing. When Sassda’s technical team reviewed the mill certificate, photographs, and test data, they found no metallurgical concerns or inclusions, carbide precipitation or surface defects.

All signs pointed to a single conclusion: the fault didn’t lie in the steel itself, but in the electropolishing process.

Why do we electropolish?

Stainless steel’s reputation rests on its corrosion resistance, durability, and cleanability. The standard 2B finish from the mill is more than adequate for most hygienic and industrial uses. But in sectors where every microscopic detail matters, from food and pharmaceuticals to semiconductors and surgical equipment, electropolishing provides the next level of finish.

The process smooths and brightens the metal surface, enhances corrosion resistance by removing embedded iron, and creates an ultra-clean, passive chromium-rich layer. It also improves cleanability, eliminates micro-burrs, and delivers the lustrous appearance often associated with premium stainless steel.

A short history of the process

Electropolishing isn’t new. The process was first observed in 1907 when a researcher Buetel, noticed a satin-like finish forming on gold immersed in an acid bath.

By the 1930s, researchers such as Shpitalskiy in Russia and Jacquet in France had refined it into a controllable industrial process.

The real boom came in the 1950s and 60s as stainless steel became the material of choice for hygienic and high-performance industries. With improved rectifiers, temperature controls, and acid management, electropolishing moved from laboratory curiosity to standard practice.

The science behind the shine

In essence, electropolishing is the reverse of electroplating. Instead of depositing metal ions, the process removes them selectively dissolving high points on the surface and leaving a smooth, bright, and passive finish.

A 304 stainless component is connected as the anode in an acid electrolyte (commonly a mix of phosphoric and sulfuric acids) while a direct current passes through the bath. Material removal is controlled by current density, temperature, and exposure time. Even slight changes in any of these parameters can make the difference between a mirror-bright surface and a dull, uneven one.

Material or method?

Not all dull finishes stem from poor processing sometimes the material itself is at fault. Common metallurgical factors that can affect electropolishing include:

• Non-metallic inclusions (often sulphides in freemachining grades like 303) that dissolve unevenly.
• Carbide precipitation caused by improper annealing, producing patchy lustre or “orange peel.”
• Surface decarburization during heat treatment, altering the surface chemistry.
• Residual cold work from machining or rolling, introducing stress zones that resist uniform polishing.

But in this case, none of these were present. Metallurgical analysis confirmed the batch met all specifications. That left one conclusion: the cause was process-related.

Where electropolishing goes wrong

Electropolishing success depends on the delicate interplay of chemistry, current, and time.

The most common process issues include:

• Electrolyte degradation: Over time, acid strength drops and dissolved metal ions build up, altering current flow and causing cloudy or etched finishes.
• Incorrect current density: Too low produces dullness; too high causes pitting or “burning.” The sweet spot for 304 stainless is 10–25 A/dm².
• Contaminated surfaces: Oils, oxides, or residues from machining interfere with uniform dissolution. Thorough pre-cleaning degreasing, ultrasonic cleaning, and acid pickling is essential.
• Over-polishing: Leaving parts in the bath too long can attack grain boundaries and create a hazy film.
• Poor rinsing or drying: Hard water or contaminated rinse tanks can deposit residues that dull the finish.

Troubleshooting the finish

Once the material has been verified as compliant, attention must turn to the process line. A systematic approach helps pinpoint the issue:

1. Check the electrolyte: Analyse acid ratios, contamination, and metal iron buildup. Replace or refresh as needed.
2. Verify current and temperature control: Calibrate the rectifier and ensure stable current density within the optimal range.
3. Audit pre-cleaning steps: Confirm consistent use of degreasers and acid pickling to remove contaminants.
4. Control cycle times: Limit exposure to 30 seconds–2 minutes depending on part geometry.
5. Inspect rinsing and drying: Use only deionized water and dry in a clean, controlled environment.

If the problem persists, surface microscopy can help reveal etching patterns or signs of contamination, providing vital clues to the root cause. For long-term reliability, every batch should have traceable records electrolyte condition, current settings, operator logs, and inspection results. Preventive control beats post-process correction every time.

Setting the standard

Several international standards govern electropolishing practices and surface quality requirements. The most relevant include:

• ISO 15730: Electropolishing for smoothing and passivating stainless steel.
• ASTM B912: Standard electropolishing specification for stainless steels.
• ASTM A967: Chemical passivation treatments for stainless steel parts.
• ASTM F86: Surface preparation for metallic surgical implants.
• ASME BPE: Bioprocessing equipment surface finish and cleanability criteria.
• SEMI F19: Surface condition specifications for wetted stainless steel components.

These standards outline key process parameters, inspection methods, and surface finish targets. Adherence to them ensures both consistency and compliance in critical applications.

Lessons learned

This investigation confirmed what experienced metallurgists already know: when electropolishing results fall short, the material is rarely to blame.

The 304 stainless steel in question fully met specification. The dull finish was the product of process drift, a reminder that even a small lapse in control can tarnish an otherwise flawless material.

The takeaway for stainless steel finishers and users is simple:

• Maintain chemistry and current balance.
• Keep pre-cleaning thorough and consistent.
• Use pure rinse water.
• Record and monitor every batch.

Electropolishing rewards precision and punishes complacency.

The final polish

In the end, the 304 round bar was cleared of any metallurgical blame. The root cause was identified within the process a powerful reminder that achieving a flawless shine in stainless steel is as much about discipline as it is about design.

When stainless steel loses its lustre, the solution isn’t to question the alloy it’s to refine the process.

To find out more about Sassda’s Technical Consulting Service or log a request for assistance e-mail:  michel@sassda.co.za