Weld defects and corrosion damage on stainless steel: causes and remedies

By Robert Lahnsteiner, International Welding Engineer (IWE) · Updated: November 2026

Two kinds of damage

There are two groups of damage at stainless steel welds: defects that occur during welding, and corrosion that only shows up in service. Many cases of corrosion damage, however, already originate in fabrication.

Typical examples are temper colours that were not removed, iron abrasion from the wrong tools, or a crevice in a weld without full penetration. The tables give cause and remedy and refer to the articles in this series.

Defects during welding

The most common weld defects in stainless steels and what helps against them.

Defect	Cause	Remedy
Hot cracks (solidification cracks) in the weld centre	weld metal solidifies without ferrite, e.g. with fully austenitic grades; too much heat	filler metal with some ferrite, low heat input, stringer beads; Microstructure
Crater cracks	the weld pool at the end of the weld solidifies last and cracks	fill craters; with fully austenitic grades, grind tack welds and weld ends
Cracks in dissimilar joints	too much dilution with carbon steel, martensite in the weld metal	over-alloyed filler metal such as 309L; Dissimilar joints
Cracks at tack welds on duplex	rapid cooling, highly ferritic weld metal	tack with filler metal, preheat if necessary; Tack welding
Lack of fusion	viscous weld pool, groove angle too narrow, weld pool running ahead of the arc	groove angle of 70 to 80° for duplex, adjust torch guidance; Joint preparation
Pores	disturbed gas shield, damp hoses, too much nitrogen, hydrogen in the arc	check gas shield, limit nitrogen; Shielding gases
Root concavity, blown-out root	forming gas escapes only through the gap, overpressure too high	separate gas outlet, only slight overpressure; Root protection
Excessive root penetration	too much heat or too wide a root gap	avoid because of the notch effect; grind out the root before a backing run
Arc strikes next to the weld	arc struck on the plate	strike only in the groove; arc strikes can initiate cracks and rust
Oxidised, “burnt” root	no or poor root protection	purge, measure residual oxygen; Root protection
Heavy temper colours	too much heat, poor gas shield, moisture	Avoiding and removing temper colours
Distortion	high thermal expansion, poor thermal conductivity	low heat input, close tack welds, fixtures; Heat control

Based on DVS 0931, DVS 0937, DVS 0946 and ISER information sheet 823. Which imperfections are permissible and to what extent is governed by ISO 5817 with its quality levels.

Corrosion damage at welds

Stainless steel rusts when the passive layer is locally destroyed and cannot renew itself. At welds, the causes are usually known and avoidable.

Type of corrosion	Appearance	Cause	Remedy
Pitting corrosion	small, deep pits	chlorides, especially at elevated temperature; deposits such as extraneous rust, slag, temper colours	molybdenum-bearing grade or higher PREN; clean post-weld treatment
Crevice corrosion	attack in narrow crevices, under washers, in stagnant media	there is no oxygen in crevices, so the passive layer cannot renew itself	avoid crevices by design, use full-penetration welds; molybdenum helps
Intergranular corrosion	grain boundary attack next to the weld in the heat-affected zone	chromium carbides at the grain boundaries; steels above 0.03 % C with thicker walls, carbon from CO₂-rich gas	L grades or stabilised steels; MAG gas with little CO₂
Stress corrosion cracking	fine, branched cracks running through the grains	chlorides, elevated temperature and tensile stress together; austenitic grades with 8 to 12 % Ni are the most susceptible	duplex or a grade with higher nickel content; avoid grinding stresses, pickle
Extraneous rust	rust spots on the surface, often after a few days	iron particles from tools or flying sparks from carbon steel work	separate tools; remove rust and pickle
Galvanic corrosion	attack on the less noble partner of a joint	contact with carbon steel or aluminium in an electrolyte	insulate, consider the area ratio; Dissimilar joints

Based on ISER information sheet 823 (2019) and DVS 0937. Advice on selecting the material according to the corrosive load is given in What makes stainless steel stainless.

Rust at the weld: the usual suspects

If rust appears on or next to a stainless steel weld shortly after fabrication, the material is rarely to blame. Usually it is one of the following causes.

Temper colours not removed: The attack starts in the chromium-depleted zone beneath them, often as a reddish-brown fringe along the weld.
Extraneous rust: iron abrasion from brushes, discs or clamping tools that were also used on carbon steel, or grinding sparks from nearby work.
Slag and spatter not removed, especially after manual metal arc and flux-cored arc welding.
Wrong filler metal or too much dilution, for example carbon steel wire on stainless steel or 308L on a molybdenum-bearing steel.
Crevice, for example a root without full penetration or a single-sided fillet weld, in which moisture collects.

Testing and assessment

Visual testing is always the first step. For surface cracks on austenitic grades, penetrant testing is suitable, but magnetic particle testing is not, because austenite cannot be magnetised.

The test methods at a glance: Testing, health and safety.

Frequently asked questions

Why is my 304 weld rusting?

Usually because of temper colours, extraneous rust or slag that were not removed. In a chloride-bearing environment, 1.4301 itself can also reach its limit; a molybdenum-bearing grade such as 1.4404 is then the better choice.

How do I recognise hot cracks?

Usually as a longitudinal crack in the weld centre or in the end crater, often visible to the naked eye or in penetrant testing. They are typical of fully austenitic weld metal.

Where do pores in stainless steel come from?

Usually from a disturbed gas shield or moisture in the gas. With duplex and super duplex, also from too much nitrogen in the weld metal.