Weld Tint vs Weld Sensitisation: Two Different Problems Too Often Confused

In stainless steel fabrication, few topics create more confusion than the relationship between weld tint and sensitisation.

They are often spoken about as though they are the same issue — but they are not.

Understanding the Difference

When stainless steel is welded, the Heat Affected Zone (HAZ) can experience several metallurgical changes depending on temperature and exposure time.

Weld Sensitisation

Sensitisation occurs when a relatively small portion of the HAZ remains within the critical temperature range (typically around 450–850°C) long enough for chromium carbides to form at grain boundaries.

This matters because:

 Chromium combines with carbon

 Local chromium depletion occurs near grain boundaries

 There may no longer be enough free chromium to maintain the protective chromium oxide (Cr₂O₃) passive layer

 Intergranular corrosion risk increases

This is a genuine metallurgical issue — but importantly, it is usually confined to a much narrower region than many people assume.

What Weld Tint Actually Is

Weld tint is primarily a surface oxide scale formed when the stainless surface is exposed to elevated temperatures in the presence of oxygen.

It can extend well beyond the sensitised zone because surface oxidation can occur over a broader thermal footprint.

Weld tint may indicate:

 Poor shielding or purge practice

 Surface oxidation

 Reduced local corrosion resistance at the surface due to oxide thickness

But weld tint itself is not proof of sensitisation.

The Common Misconception

A widespread belief is:

“If I remove the weld tint, I’ve solved the problem.”

Not necessarily.

Mechanical or chemical removal of weld tint (pickling, passivation, brushing, etc.) can restore surface condition and help rebuild the chromium oxide layer on the surface.

However:

If chromium depletion has already occurred within the grain boundaries due to sensitisation, removing surface tint does not reverse that metallurgical change.

In simple terms:

Removing weld tint improves the surface. It does not undo sensitisation.

Why This Matters

Confusing weld tint with sensitisation can lead to:

 False confidence in corrosion performance

 Overreliance on cosmetic cleaning

 Poor material selection

 Inadequate welding procedure qualification

The Bigger Picture

Preventing sensitisation is more about:

 Correct grade selection (e.g. low carbon “L” grades or stabilised grades)

 Heat input control

 Interpass temperature management

 Proper welding procedure

While controlling weld tint is more about:

 Shielding gas quality

 Purging

 Oxygen control

 Post-weld cleaning

Final Thought

Immediately post welding, weld tint is visible, whereas sensitisation is not.

That visibility can cause people to focus on what they can see, rather than what may actually compromise long-term corrosion resistance.

Surface oxide and subsurface metallurgical depletion are different phenomena — and treating them as the same can be a costly mistake.

In metallurgy, appearance matters… but microstructure matters more.

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