Complete Solution for Surface Treatment of Welded Stainless Steel Structures

Introduction: Welding Is Only the Beginning – Surface Treatment Is the Key to Quality

Stainless steel is widely used in chemical, food, pharmaceutical, construction, and marine industries because of its excellent corrosion resistance. However, the welding process inevitably destroys the original passive film on the stainless steel surface. During the welding thermal cycle, a layer of oxide film of varying thickness and colour forms on the surface of the weld and the heat‑affected zone – commonly referred to as “heat tint” or “temper colour” .

Welding heat tint forms at temperatures higher than those for heat‑treatment discolouration – the lightest colour begins to form above 430°C. As the colour deepens (from light straw → golden → blue → dark blue → black), the surface oxide film thickens, and chromium beneath the surface is drawn to the surface to form a chromium‑rich oxide layer, leaving a chromium‑depleted zone immediately below the surface. This chromium‑depleted zone is the root cause of the decline in corrosion resistance.

Research has shown that welding heat tint significantly reduces corrosion resistance, and the darker the colour, the more pronounced the drop in pitting potential – even the lightest straw colour fails to meet the corrosion resistance requirements for fluid‑containing vessels. Therefore, post‑weld surface treatment is not an “option” – it is a “must‑do” to restore the corrosion resistance of stainless steel.

This article provides a systematic introduction to complete solutions for surface treatment of welded stainless steel structures, covering the principles, operational points, and application scenarios of mainstream processes.

Part 1: Pre‑Weld Preparation – The First Line of Defence in Surface Treatment

Surface treatment work begins before welding. Before welding, all surface contaminants – including grease, paint, and rust – must be removed from the groove and at least 20 mm on both sides. Grease, as a source of hydrogen, can cause porosity in the weld if not removed; while low‑melting‑point metal contamination (such as zinc‑rich paint) can cause weld cracking.

Recommended practice: Use acetone wiping to clean oil and grease, and use abrasive cloth or a stainless‑steel‑specific wire brush to remove paint and rust, then wipe again with acetone. Studies show that acetone cleaning gives the best overall results for stainless steel preparation.

If there is a thick oxide layer on the welding area, use a dedicated stainless steel wire brush (never mixed with carbon steel tools) to grind down to a metallic sheen.

Part 2: Causes and Hazards of Welding Heat Tint

2.1 What Is Heat Tint?

Heat tint is caused by the thickening of the oxide film that naturally forms on a metal surface at high temperatures. Its colour arises from the “light interference effect” – a difference in optical path between light reflected from the oxide film surface and light reflected from the oxide/metal interface, producing a series of colours that depend on the oxide film thickness.

The oxide film thicknesses and formation temperatures corresponding to different colours are as follows:

2.2 Why Must Heat Tint Be Removed?

The harm of welding heat tint to stainless steel is mainly in three aspects:

First, chromium depletion: During the formation of heat tint, chromium beneath the metal surface is drawn to the surface to form a chromium‑rich oxide layer, reducing the chromium content immediately below the surface layer. Chromium is the core element for stainless steel corrosion resistance; its reduction directly weakens the material’s “self‑passivation” capability.

Second, reduced pitting resistance: Studies confirm that the pitting potential decreases significantly as heat‑tint colour darkens. Even the lightest straw colour cannot meet the corrosion resistance requirements for fluid‑containing vessels.

Third, crevice corrosion risk: The heat‑tint layer itself is a non‑uniform surface that can act as a site for corrosive media to accumulate, leading to crevice corrosion during service.

Key principle: The UK Drinking Water Inspectorate explicitly states: “To achieve optimum corrosion resistance of stainless steel welded joints, all crevice features, contaminants, and weld heat tint darker than light straw must be removed by mechanical grinding supplemented by pickling.”

Part 3: Mainstream Solutions for Surface Treatment

Surface treatment of welded stainless steel structures typically requires a combination of multiple techniques, as a single method alone is often insufficient. The four main categories are described below.

3.1 Mechanical Methods

(1) Grinding

Grinding is the most basic mechanical method, using abrasive paper, grinding wheels, or grinders to remove oxide layers and contaminants from the stainless steel surface by friction. The ground surface is usually smooth, suitable for applications with high weld appearance requirements.

Key operational points:

• Use a grinder to remove 80% of the weld discolouration first, then treat residual oxides

• Never use grinding tools previously used on carbon steel – prevent iron contamination

• For thin‑wall stainless steel, prefer light mechanical cleaning combined with chemical treatment

(2) Blasting (Sandblasting / Shot Blasting)

Use glass beads, ceramic particles, or specialised abrasives to impact‑clean the weld surface. Shot blasting can convert residual tensile stress on the welded component surface into compressive stress, thereby preventing stress corrosion cracking.

Application scenarios: Large‑area welds, cases where residual stress relief is required.

3.2 Chemical Methods

(1) Pickling

Pickling uses acid to remove high‑temperature oxides, weld discolouration, and the chromium‑depleted layer from the stainless steel surface. The purpose of pickling is to remove the scale from the weld and the heat‑affected zone.

Common pickling methods:

• Immersion: Immerse the workpiece in a pickling tank – suitable for small to medium‑sized parts

• Brushing: Apply pickling solution to the weld surface with a brush

• Paste method: Apply pickling‑passivation paste to the stainless steel surface at a thickness of 1–2 mm, leave for 5–30 minutes, then rinse thoroughly with clean water after scale and weld discolouration are completely removed

Common pickling formulation: 20% nitric acid + 5% hydrofluoric acid mixture.

(2) Passivation

Passivation is carried out after pickling. Its purpose is to re‑form a colourless, dense oxide film on the pickled surface for corrosion protection. Passivation is less aggressive than pickling and therefore cannot remove scale or the chromium‑depleted layer – it is mainly used to remove light iron contamination and form a passive protective film.

Passivation effect: Restores the self‑repairing capability of the passive film in air. The treated surface should exhibit a uniform silver‑white appearance.

(3) Integrated Pickling and Passivation Process

Typical process flow:

Dust removal (cold water rinse) → Degreasing → Rinse → Pickling → Rinse → Remove residual oxides and weld spatter → Inspection → Passivation → Rinse → Drying

3.3 Electrochemical Methods

(1) Electropolishing

Electropolishing is a finishing process that removes microscopic surface irregularities through electrochemical action, producing a smooth and glossy surface. When higher surface smoothness is required – whether for aesthetic reasons or to further optimise corrosion resistance – a final electropolishing step can be applied.

Advantages: Not only restores the material‘s corrosion resistance but also significantly enhances the overall appearance and texture of the finished product.

(2) Electrochemical Cleaning (Anodic Polarisation)

A recent patent from TISCO (CN119243304A) provides an innovative rapid cleaning method for stainless steel welds – using constant‑current anodic polarisation in sodium sulphate solution for welded stainless steel parts. This method can quickly and completely remove surface discolouration after welding. It is simple to operate and the solution is safe and non‑polluting; the corrosion resistance of the treated weld surface is significantly improved.

3.4 Laser Cleaning

Laser cleaning is a high‑precision surface treatment technology that has developed rapidly in recent years. Nanosecond pulsed lasers can effectively remove scale from stainless steel joint surfaces and help improve corrosion resistance.

Latest research findings:

• Gaussian beam can effectively remove scale and form a passive zone

• Two‑step method (Gaussian beam first, then flat‑top beam) effectively removes scale and further improves corrosion resistance

• After treatment, a smooth, chromium‑rich new oxide film forms, significantly enhancing corrosion resistance

Advantages: Non‑contact, high precision, no chemical waste – ideal for post‑weld treatment of precision and thin‑wall stainless steel structures.

Part 4: Method Comparison and Selection Recommendations

Selection principles:

• General industrial structures: Prefer the combined solution of mechanical grinding + pickling & passivation

• High appearance requirements: Add electropolishing after pickling & passivation

• Thin‑wall / precision parts: Prefer laser cleaning or light mechanical cleaning + chemical treatment

• Environmentally restricted sites: Choose electrochemical cleaning (anodic polarisation) or laser cleaning

• High‑chromium stainless steels: Heat tint forms at higher temperatures, making pickling more difficult – allow extended treatment time accordingly

Part 5: Complete Process Flow and Quality Control

5.1 Standard Process Flow

Pre‑weld preparation:

1. Remove grease, paint, and rust from the groove and at least 20 mm on both sides

2. Wipe with acetone to clean oil and grease

3. Use stainless‑steel‑dedicated tools to remove paint and rust

Post‑weld treatment:

1. Mechanical pre‑treatment: Grind and clean the weld using stainless‑steel‑dedicated tools – remove slag and spatter

2. Pickling: Immersion / brushing / paste method to remove scale and chromium‑depleted layer

3. Rinsing: Thorough rinse with clean water

4. Passivation: Re‑form a dense oxide film

5. Final rinse and drying: Rinse with warm water and dry

Quality inspection:

• Visual inspection: Treated surface should show a uniform silver‑white colour

• Adhesion test: Cross‑cut or pull‑off test when required

• Salt spray test: Verify corrosion resistance per ASTM B117

5.2 Common Problems and Prevention

Conclusion: Surface Treatment Is the “Final Gateway” for Welded Structure Quality

Surface treatment of welded stainless steel structures is not “icing on the cake” – it is a fundamental requirement for restoring corrosion resistance and ensuring the service life of the structure.

Key takeaways:

1. Welding heat tint must be removed – heat tint darker than light straw significantly reduces corrosion resistance

2. Pickling and passivation is the most reliable mainstream solution – mechanical grinding + pickling is the golden combination for restoring corrosion resistance

3. Upgrade processes for high‑requirement applications – electropolishing and laser cleaning offer superior options for high‑end uses

4. Full‑process control is essential – from pre‑weld cleaning to post‑weld inspection, every step affects final quality

Engineering practice recommendations:

• For general industrial stainless steel welded structures: mechanical grinding + pickling & passivation is sufficient

• For food, pharmaceutical, drinking water, and other applications with strict corrosion resistance requirements: mechanical grinding + pickling & passivation + electropolishing

• For thin‑wall, precision, or environmentally restricted applications: laser cleaning or electrochemical cleaning are ideal choices

Remember: Stainless steel is not inherently “stainless” – its corrosion resistance depends on an intact passive film. Welding destroys it, and surface treatment is the only way to restore it.