Protecting metal parts from corrosion and wear: coatings, design details and surface rebuilding

Most metal parts fail at the surface. Corrosion starts at the outside and works in. Wear happens where surfaces touch and slide. Fatigue cracks usually begin at surface pits and scratches. Yet surface protection is often decided last, by a default note such as “paint” or “galvanise”, without considering the environment the part will live in or how it will be maintained.

The consequences are familiar to anyone who maintains equipment: structures rusting at bolted joints a few years after installation, stainless fittings pitting at a coastal site, plated fasteners snapping because of hydrogen embrittlement, wear parts replaced every few months when a better surface would have lasted years. Each of these has a known cause and a known remedy, and most can be prevented in design.

This article explains how corrosion and wear happen, how to judge the severity of an environment, the main coatings, platings and conversion treatments and where each fits, design details that prevent corrosion, and how worn surfaces can be rebuilt by hardfacing, thermal spraying and related processes. It is general information for designers, engineers, maintenance teams and buyers. For structures and critical equipment, follow the relevant standards and coating manufacturers’ advice.

How corrosion happens

Corrosion of metals is mostly electrochemical. Where moisture is present, areas of the metal act as anodes, where metal dissolves, and cathodes, where a reduction reaction such as oxygen reduction takes place. The moisture film carries the current. Remove any part of the circuit, such as moisture, oxygen or the electrical path, and corrosion slows or stops.

The main forms are:

• Uniform corrosion: general thinning across a surface, such as rusting of unprotected steel. Predictable and usually manageable.
• Galvanic corrosion: when two different metals are in electrical contact in the presence of moisture, the less noble metal corrodes faster. Aluminium or zinc against stainless steel or copper is a common example. A small anode coupled to a large cathode is the worst case.
• Crevice corrosion: concentrated attack in narrow gaps, such as under washers, gaskets and lap joints, where the chemistry inside the gap becomes aggressive.
• Pitting: deep local attack, particularly of stainless steels and aluminium in chloride environments.
• Stress corrosion cracking: cracking under the combined action of tensile stress and a specific environment, such as some stainless steels in warm chloride conditions.
• Microbially influenced corrosion: corrosion accelerated by bacteria, for example in stagnant water.
• Erosion-corrosion: accelerated attack where fluid flow removes protective films, such as in pump casings and pipe bends.

Judging the environment

Atmospheric corrosivity is classified internationally in ISO 9223 categories from C1, very low, such as heated indoor spaces, through C2, C3 and C4 to C5, very high, and CX, extreme. Coastal sites, industrial atmospheres and areas exposed to salt spray are at the severe end. In Australia, AS 4312 describes atmospheric corrosivity zones, and AS/NZS 2312 guides the protection of structural steel using paint coatings and hot-dip galvanising.

Within a site, the microclimate matters as much as the region. A sheltered underside that collects salt and never gets washed by rain can be more corrosive than an exposed face. Splash zones, areas wetted by process fluids, places where water ponds and spaces near cooling towers all need particular attention.

Designing out corrosion

Many corrosion problems are design problems. Good details include:

• Drain water away: avoid pockets, channels facing upwards and ledges where water and dirt collect. Provide drain holes.
• Avoid crevices: seal-weld lap joints or design joints that can be fully coated, and avoid narrow gaps that trap moisture.
• Isolate dissimilar metals: use insulating washers, sleeves and gaskets, or choose compatible metals, especially where the less noble metal has a small area.
• Allow access for coating and maintenance: surfaces that cannot be reached cannot be painted or inspected.
• Round edges: coatings thin out on sharp edges, which are often the first places to rust.
• Design for the coating process: hot-dip galvanised fabrications need vent and drain holes so molten zinc and gases can flow in and out safely.
• Use closed sections sealed or fully galvanised inside, rather than partly protected hollow sections that rust from within.

Coatings, platings and treatments

The life of a zinc coating is roughly proportional to its thickness and inversely proportional to the corrosion rate in the environment. Hot-dip galvanised coatings are typically many times thicker than electroplated zinc, which is why galvanising lasts decades outdoors in moderate environments while thin electroplated zinc is best suited to indoor and mild conditions.

Hydrogen embrittlement is a particular risk when high-strength or hardened steel parts are acid cleaned and electroplated. Hydrogen absorbed during processing can cause sudden cracking under load, sometimes days after assembly. Specify baking after plating where required by the relevant fastener or plating standard, or choose coatings that do not introduce hydrogen.

How wear happens

Wear is the progressive loss of material from contacting surfaces. The mechanism decides the remedy:

• Abrasion: hard particles or rough surfaces cut and plough the surface, as in chutes, buckets, pump impellers handling slurry and agricultural tools.
• Adhesion and galling: surfaces weld together at contact points and tear, common with stainless steel on stainless steel and poorly lubricated sliding parts.
• Erosion: impact of particles or droplets in a flowing fluid.
• Impact: repeated blows that deform and crack surfaces, such as crusher parts and hammers.
• Fretting: tiny oscillating movements at clamped joints, producing oxide debris and cracks.
• Corrosive wear: wear and corrosion together removing material faster than either alone.

Choosing a surface for wear means identifying the dominant mechanism, then selecting a material or treatment that resists it. Hardness helps against abrasion; toughness is essential under impact; dissimilar materials and lubrication help against galling.

Rebuilding and protecting worn surfaces

Worn parts do not always need replacing. Surface engineering processes can rebuild them, often with a better surface than the original:

• Hardfacing by welding deposits wear-resistant alloys onto a base metal, with a metallurgical bond. Chromium carbide overlays resist abrasion; tungsten carbide in a tough matrix resists severe abrasion; cobalt-based alloys resist heat, erosion and galling; austenitic manganese overlays suit impact. Overlay plate, a mild steel plate with a hardfaced layer, is widely used for liners.
• Plasma transferred arc welding deposits overlays with low dilution and good control, suiting valve seats and precision wear surfaces.
• Laser cladding deposits overlays with a narrow heat-affected zone and low distortion.
• Thermal spraying melts or softens metal, ceramic or carbide powders or wires and sprays them onto a prepared surface. Processes include flame, electric arc, plasma and high-velocity oxy-fuel spraying. Coatings bond mainly mechanically, are thin and dense, and suit shafts, rolls, seal surfaces and corrosion protection. They are less suited to heavy impact or point loads.
• Metallising, thermal spraying of zinc or aluminium, protects steel structures from corrosion.
• Sleeves and inserts replace worn journals and bores with new surfaces.

The choice depends on wear mechanism, base metal, part size, accuracy needed after repair, heat input and cost. Weld overlays dilute with the base metal and can crack hard deposits if procedures are not followed. Thermal spray needs careful surface preparation, typically grit blasting, to bond well. Most rebuilt surfaces need finish machining or grinding.

Specifying surface protection

A coating note on a drawing or purchase order should leave nothing to guess. State the standard or system, the surface preparation (for example, abrasive blast cleaning to a stated grade), each coat’s product type and dry film thickness, any colour and gloss requirements, and the inspection required, such as film thickness readings and adhesion checks. For galvanising and plating, state the standard, the minimum coating thickness and any post-treatment such as passivation or baking. For hardfacing and thermal spray, state the alloy or coating type, deposited thickness after finishing, hardness and any crack testing. Ask for records that show the specification was met, and keep them with the asset’s maintenance history.

Maintenance and inspection

Coatings and surfaces need maintenance. Plan inspection intervals, touch-up of damaged coatings, monitoring of wear allowances and replacement of sacrificial anodes. Record coating systems and repair procedures so the same specification is used next time. The maintenance that prevents breakdowns article covers building inspection and condition monitoring into routines. Protecting surfaces well also extends product life, which has environmental as well as cost benefits, as the durability is sustainability article explains.

A worked example

This is an illustrative example. A business operates a conveyor and transfer station close to the coast. The steel structure was painted with a basic single-coat system. Within three years it shows rust at bolted joints, under ledges where salt collects and on sharp edges. The transfer chute’s mild steel liners wear through every few months from abrasive material.

Assessment. The site is assessed as a high corrosivity environment, with sheltered areas worse than exposed faces because rain does not wash them. Stainless steel fasteners have been used with galvanised brackets in places, creating galvanic couples. The chute wear is mainly sliding abrasion, with some impact at the loading point.

Changes.

• Replacement structural members are hot-dip galvanised and painted as a duplex system, with vent and drain holes, rounded edges and no upward-facing channels.
• Ledges are removed or sloped, and lap joints are seal-welded before galvanising.
• Fasteners are hot-dip galvanised to match the brackets, or stainless fasteners are isolated with insulating washers and sleeves.
• Chute liners change to chromium carbide overlay plate in the sliding zone and an impact-resistant steel at the loading point.
• Coating inspection and touch-up are added to the annual maintenance plan.

Result. The duplex system is expected to give a much longer time to first major maintenance than the original paint, according to the coating guidance for the site’s environment, and inspection shows little deterioration after the first years. Liner replacements fall from several times a year to about once a year, reducing shutdowns and hot work.

Applying this in an Australian business

• Classify the environment and microclimates before choosing protection.
• Design out water traps, crevices and sharp edges.
• Isolate or avoid dissimilar metals.
• Choose coatings for the environment and the required life, and specify thickness and preparation.
• Manage hydrogen embrittlement risk for high-strength plated parts.
• Identify the wear mechanism before choosing a wear-resistant surface.
• Consider rebuilding worn parts with hardfacing, cladding or thermal spray.
• Plan inspection and maintenance, and record specifications.

Where surface protection goes wrong

• Default coating notes without regard to the environment.
• Water traps and crevices designed into structures.
• Stainless fasteners in aluminium or galvanised steel without isolation.
• Thin electroplated coatings outdoors.
• Plating high-strength steel without embrittlement controls.
• Hard but brittle overlays under impact.
• No maintenance plan for coatings.

Questions to ask about surface protection

• What corrosivity category and microclimate will this part see?
• Where could water, salt and dirt collect?
• Are dissimilar metals in contact, and are they isolated?
• How long must the coating last before maintenance, and how will it be maintained?
• What is the dominant wear mechanism?
• Could a rebuilt or overlaid surface outlast a new standard part?

Bringing it together

Corrosion and wear are surface problems with known causes and remedies. Classify the environment, design out water traps, crevices and galvanic couples, and choose coatings, platings and treatments that suit the environment and the required life, specifying preparation and thickness. Control hydrogen embrittlement on high-strength plated parts. For wear, identify the mechanism and choose materials, overlays or coatings that resist it, and consider rebuilding worn parts with hardfacing, cladding or thermal spray. Plan inspection and maintenance and record what works. The result is equipment that lasts longer, needs less unplanned repair and costs less over its life.

Source: KEVOS editorial notes, drawing on earlier KEVOS engineering handbooks on metal surface treatment, conversion coatings and plating, and on hardfacing, thermal spraying and surface rebuilding, together with established corrosion and surface engineering practice. The worked example is illustrative. This article is general information; follow the relevant standards and coating manufacturers’ advice.