8.4 Corrosion, Wear & Protective Layers

8.4 Corrosion, Wear & Protective Layers

Quick Answer: A weld can pass dimensional and RT checks yet fail in service by corrosion or wear. Inspectors must recognise how pitting, crevice corrosion, and stress corrosion cracking (SCC) exploit welds and HAZs; understand that hardfacing is a wear system with its own defect modes; and verify that cladding / protective layers remain continuous, correctly alloyed, and undamaged by fabrication. Modules WT2.12 and WT2.20–2.21 connect materials knowledge to service damage modes.

Why Welds Are Corrosion Hot Spots

Even when filler “matches” the base metal, the welded region differs:

• Microstructure changes in weld metal and HAZ (grain size, precipitates, ferrite/austenite balance)
• Residual tensile stress remains after cooling
• Surface films (heat tint, slag residues, iron contamination) disrupt passivity
• Geometry creates crevices at weld toes, backing bars, and incomplete penetration
• Composition gradients from dilution or segregation act as local anodes/cathodes

Therefore corrosion engineering and welding quality are inseparable. The IWI-S role is to enforce the surface, material, and procedure requirements that the corrosion design assumes.

Uniform vs Localised Corrosion (Inspector Framing)

Uniform corrosion thins metal relatively evenly. It is often predictable and allowed for by corrosion allowance. Welds may still corrode slightly differently, but the dramatic shop and field failures taught for stainless and alloy systems are usually localised:

Pitting corrosion

Pitting is deep, local attack that can pierce walls while most of the surface looks intact. Chloride-bearing waters and films that break the passive layer are classic drivers for stainless steels. Aggravating factors the inspector influences:

• Embedded iron particles and grind debris
• Heavy heat tint not removed to the specified standard
• Rough, slag-filled, or porous weld surfaces that trap chlorides
• Wrong grade or under-alloyed weld metal (e.g. insufficient Mo in environments that need it)

Pitting resistance is often discussed with indices such as PREN in materials engineering; the inspector’s practical lever is correct alloy + clean passive surface + qualified weld quality.

Crevice corrosion

Crevice corrosion occurs in shielded stagnant zones: under lap joints, deposit piles, gaskets, incomplete penetration roots, and tight gaps at attachments. Oxygen depletion inside the crevice changes local chemistry aggressively. Design should avoid crevices; fabrication should not create new ones through poor fit-up, excessive reinforcement traps, or leaving backing strips against process fluid when forbidden.

Inspector actions: flag unwelded laps in corrosive service when drawings require full sealing welds; ensure specified seal welds exist; report sludge traps created by weld spatter and overlap.

Stress Corrosion Cracking (SCC) — Introduction

SCC is cracking from the combined action of:

1. A susceptible material (certain stainless, brass, high-strength steels, etc., each with specific environments)
2. Tensile stress (applied + residual—welds almost always contribute residual tension)
3. A specific corrosive environment (e.g. chlorides with austenitic stainless under tensile stress at elevated temperature in classic teaching cases; other systems have other media)

Key teaching points for inspectors:

• SCC cracks can be branched and hard to see until leak or brittle failure
• Grinding smooth without addressing stress and environment does not “cure” SCC risk
• Controls include material selection (e.g. duplex or higher alloys in some chloride duties), stress relief where permitted, avoiding chloride concentration under insulation (CUI-related issues), and correct PWHT on susceptible ferritic systems when specified
• Do not confuse SCC with hydrogen cold cracking: both need stress, but mechanisms, timing, and material/environment pairs differ (Chapter 6 vs this corrosion frame)

Exam answers should name the three concurrent requirements and note that weld residual stress often supplies the stress leg.

Other Corrosion Modes Linked to Welds (Awareness)

• Intergranular corrosion after sensitization (Section 8.1) along HAZ grain boundaries
• Galvanic corrosion when dissimilar metals connect in electrolyte (Section 8.3)
• Erosion-corrosion where high-velocity fluid strips protective films at weld ripples and misalignment steps
• Microbiologically influenced corrosion (MIC) in water systems—biofilms under deposits at rough welds

You need recognition vocabulary; detailed corrosion engineering remains a specialist field.

Wear and Hardfacing — Awareness for Inspectors

Wear removes metal by abrasion, adhesion, impact, or fretting. Instead of upsizing the entire structure in exotic alloy, industry deposits hardfacing or wear-resistant overlays on local surfaces (mining, agriculture, valves, screws).

What differs from structural welding

• Deposit hardness may be very high; cracking of the overlay can be acceptable or forbidden depending on specification
• Dilution with soft base metal lowers surface hardness—procedure may require buffer layers
• Multiple layers, specific patterns, and interpass limits are common
• Consumables range from chromium carbides to complex proprietary wires—identity must match WPS

Inspector focus:

• Correct product and layer sequence
• Surface preparation (remove old cracked hardface if required)
• Cracking, spalling, and bond defects to the acceptance standard
• Avoid structural load paths that assume hardface is a tough, crack-free weld when it is not

Hardfacing is not a substitute for fixing an under-designed structural weld.

Protective Layers and Cladding

Clad plate and weld overlay cladding

Cladding applies a corrosion-resistant alloy layer on a cheaper structural substrate (e.g. stainless or nickel alloy on carbon steel). Methods include roll bonding, explosion cladding, and weld overlay.

Inspection implications:

• Thickness of clad layer must meet drawing minimums after fabrication and any grinding
• Bond integrity — disbond is a critical defect; UT bond testing may be specified
• Edge preparation of clad plate: cladding is often stripped back and restored with qualified weld overlay so the process side remains alloy-continuous
• Iron dilution at the first overlay passes can drop chromium/nickel below corrosion needs—qualified low-dilution procedures and sometimes barrier layers are mandatory
• Stampings and marks on the carbon-steel side must not be confused with the process side alloy identity—PMI (positive material identification) may be required

Coatings, linings, and passive films

Organic coatings, rubber linings, galvanising, and shop primers interact with welding:

• Weld zones usually require coating removal to bare metal for a defined distance
• Reinstatement of coating is a hold point in many ITPs
• Welding over zinc or thick paint without removal produces porosity, fumes, and lack of fusion
• Passivation/pickling of stainless is a chemical restoration of the passive film after fabrication—when specified, it is not optional cosmetics

Linking Corrosion Control to Everyday Inspection

Service Failure Mindset for Oral and Written Exams

When a scenario describes “leak next to the weld after months in chloride service,” structure the answer:

1. Localised corrosion or SCC is more likely than uniform thinning
2. Check material grade, filler, heat tint, residual stress, and environment concentration (insulation, deposits)
3. Name inspection controls that should have been verified (purge, cleaning, PMI, cladding thickness)

When a scenario describes “wear plate life too short,” discuss dilution, wrong hardfacing product, and missing buffer layers—not only operator travel speed.

Integration with Earlier Chapters

• Stainless sensitization and heat tint (8.1) feed intergranular and pitting risks
• Aluminium (8.2) has its own oxide and corrosion behaviours; do not apply stainless passivation myths blindly
• Dissimilar joints (8.3) create galvanic couples and carbon migration issues that appear as corrosion or interface damage later
• NDT chapters later choose PT on non-magnetics for surface-breaking corrosion cracks; MT only where ferromagnetic

Closing for Chapter 8

Advanced materials success for the welding inspector is identity + procedure + surface condition. Stainless, aluminium, nickel, cast iron, dissimilar transitions, hardfacing, and cladding all fail in characteristic ways when those three are neglected. Master the failure language—sensitization, oxide film, hydrogen porosity, dilution, buttering, pitting, crevice, SCC, overlay bond—and you can both pass WT materials items and protect real fabrications in service.