1. What Electropolishing Can and Cannot Do in Chemical Service
Corrosion in chemical plants takes two broad forms. Uniform (general) corrosion removes metal evenly across the wetted surface at a rate set by alloy, chemical, concentration and temperature. Localised corrosion (pitting, crevice corrosion and stress corrosion cracking) starts at specific sites: inclusions, scratches, embedded iron, deposits, crevices and stressed areas.
Electropolishing acts on the second form. It dissolves surface inclusions and embedded particles, removes the folded and work-hardened layer left by grinding or mechanical polishing, and leaves a chromium-enriched passive film. Fewer initiation sites and a stronger film raise the threshold for pitting and delay crevice attack under deposits. Electropolishing does nothing for uniform corrosion: if the alloy dissolves at 0.5 mm per year in a given acid, a smoother surface dissolves at essentially the same rate. Alloy selection comes first; electropolishing is the finishing step that gets the most from the right alloy. Our specialty chemicals electropolished pipes page lists where the finish is typically justified.
2. Pitting Resistance: PREN as a First Screen
The pitting resistance equivalent number, PREN = %Cr + 3.3 × %Mo + 16 × %N, ranks alloys for chloride pitting. It is a screening tool, not a design value, but it shows clearly why grade matters more than finish.
| Alloy | UNS | Approximate PREN | Typical role in chemical plants |
|---|---|---|---|
| 304L | S30403 | ~19 | Nitric acid service, mild organics, utilities |
| 316L | S31603 | ~24 | General process piping, dilute acids, many organics |
| 2205 duplex | S32205 | ~35 | Chlorides, brines, CSCC-prone service |
| 904L | N08904 | ~34 | Sulphuric and phosphoric acids, seawater-cooled duty |
| 254 SMO (6Mo) | S31254 | ~43 | High-chloride and acid chloride service |
| 2507 super duplex | S32750 | ~42 | Seawater, hot brines, high pressure at thinner wall |
Values vary with the actual heat chemistry; use the certified analysis on the mill certificate when PREN is a contractual requirement.
3. Sulphuric Acid
Sulphuric acid is reducing at low and intermediate concentrations and becomes oxidising only when highly concentrated. Austenitic stainless steels rely on an oxidising environment to keep their passive film, so 316L performs acceptably only in very dilute acid at low temperature and in highly concentrated acid near ambient temperature. In the intermediate range, and as temperature rises, corrosion rates climb steeply.
Alloys designed for this service extend the window. 904L seamless pipe, with about 25% nickel, 4.5% molybdenum and 1.5% copper, was developed for sulphuric acid; copper is the key addition because it improves resistance in reducing acid. Alloy 20 was designed for the same duty. For hot intermediate concentrations, nickel-molybdenum and nickel-chromium-molybdenum alloys such as Hastelloy C-276 are used. Always select from isocorrosion diagrams (commonly the 0.1 mm/year line) for your concentration and temperature, and check the effect of impurities: chlorides and oxidising ions such as ferric or cupric ions change the picture considerably.
4. Phosphoric Acid
Pure phosphoric acid is far less aggressive to stainless steel than sulphuric, and 316L handles it across a wide concentration range at moderate temperatures. Industrial wet-process phosphoric acid is different: it carries fluorides, chlorides and residual sulphuric acid from phosphate rock digestion, and those impurities drive both general and localised attack. 904L and other high-alloy austenitics are the common choice for wet-process acid, particularly as temperature and impurity levels rise.
Electropolishing is valuable in food-grade and pharmaceutical-grade phosphoric acid handling, where cleanliness and low metal pick-up matter as much as corrosion rate.
5. Organic Acids
- Acetic acid: 304L is adequate in cold, dilute acid; 316L is the standard for hot and concentrated acetic acid. Contamination with formic acid or chlorides makes hot acetic service considerably more aggressive.
- Formic acid: one of the more aggressive organic acids, especially hot; 316L is limited and higher alloys or duplex grades are often needed.
- Citric, lactic and tartaric acids: generally handled well by 316L in food and pharmaceutical processing; electropolishing is commonly specified here for cleanability rather than corrosion.
Selecting pipe for an acid or chloride duty?
Send the chemical, concentration, temperature and impurities. We will propose the alloy and finish, and quote pipe, fittings and flanges together.
6. Chloride Stress Corrosion Cracking (CSCC)
CSCC needs three things at once: tensile stress (applied or residual, including weld residual stress), chloride (or other halides such as bromide), and temperature. Austenitic 304L and 316L become susceptible above roughly 60 °C, and failures are common under insulation, where chloride-bearing water evaporates and concentrates on hot pipe, and in marine and coastal plants. Cracks are branched and transgranular and can perforate a pipe with little visible general corrosion.
Electropolishing does not make an austenitic steel immune to CSCC. It can delay initiation by removing surface defects and the stressed, cold-worked layer from grinding, but the decisive variables are alloy, stress and temperature. The effective remedies are:
- Change the alloy. Duplex grades, with roughly half ferrite in their structure, are far more resistant: duplex 2205 seamless pipe is the standard upgrade, and super duplex 2507 pipe and 6Mo austenitics such as 254 SMO pipe extend resistance further.
- Control chloride at the surface: coat stainless steel under insulation and use low-chloride insulation.
- Reduce stress: avoid cold-formed bends without stress relief in hot chloride service, and support lines to avoid bending loads.
7. Flanged Joints and Gaskets in Corrosive Service
Flanged joints combine two corrosion risks, the crevice between gasket and flange face and galvanic or crevice effects at bolting, with a sealing requirement. ASME B16.5 raised-face flanges are supplied with a serrated finish of about 3.2–6.3 µm (125–250 µin) Ra, because spiral-wound and graphite-faced gaskets need that texture to grip and seal. Do not electropolish gasket seating faces smooth unless the gasket manufacturer approves it for that finish.
Practical rules for high-integrity flanged joints in acid and chloride service:
- Use weld neck flanges in the same alloy as the pipe, so the weld and bore are continuous and the joint is not the weak point.
- Match gasket material to the chemical: PTFE-based gaskets for most acids; spiral-wound gaskets with a corrosion-resistant winding and PTFE or graphite filler where pressure and temperature demand.
- Use nickel-alloy flanges, such as Hastelloy C-276 flanges, where the line itself is nickel alloy; never let a lower alloy flange become the sacrificial point of a high-alloy line.
- Torque bolts to a controlled procedure; under-loaded gaskets leave crevices, over-loaded gaskets crush and extrude.
8. When to Specify Electropolishing in a Chemical Plant
| Service | Electropolishing worthwhile? | Reason |
|---|---|---|
| High-purity or pharmaceutical-grade chemicals | Yes | Low metal pick-up, cleanability, particle control |
| Chloride-bearing media near the pitting threshold | Often | Fewer initiation sites, stronger passive film |
| Fouling or crystallising media | Often | Less adhesion, easier cleaning |
| Uniform-corrosion service (e.g. intermediate sulphuric acid) | No | Alloy decides the corrosion rate; finish adds little |
| Bulk utilities and non-critical transfer | No | No measurable benefit for the cost |
Procurement: Corrosion-Resistant Pipe, Fittings and Flanges
Sakshi Forge supplies stainless, duplex, super duplex and super austenitic pipe, matching fittings and flanges, with electropolishing where the duty justifies it. Every lot carries an EN 10204 3.1 certificate with heat analysis, so PREN and composition can be verified, plus PMI records. Send the process conditions, sizes and pressure class; we will recommend the alloy and confirm dispatch and transit in writing with the quotation.