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Corrosion is a metal slowly returning to the oxide it was refined from. It is one of the most common reasons parts fail early, and most of it can be prevented at the design stage: by choosing the right material, protecting it with the right coating, and avoiding details that trap water or put the wrong metals together.
Why metals corrode
Corrosion is an electrochemical reaction. It needs four things: an anode (where metal dissolves), a cathode (where a reaction such as oxygen reduction takes place), an electrolyte (water, often with salt) connecting them, and an electrical path between them. Remove any one and corrosion stops. That is the logic behind every protection method: keep water off, isolate metals, or give the corrosion a sacrificial target.
Some metals protect themselves. Aluminium, stainless steel and titanium form a very thin, tightly bonded oxide film (a passive layer) that stops further attack and heals itself when scratched, as long as there is oxygen. Carbon steel’s rust is loose and porous, so the attack carries on.
Types of corrosion to design against
| Type | What happens | Typical place | Prevention |
|---|---|---|---|
| Uniform | The whole surface thins evenly | Unprotected carbon steel outdoors | Coating, corrosion allowance, better material |
| Galvanic | The more active of two metals in contact corrodes fast | Aluminium sheet with stainless steel screws | Isolate the metals, choose close metals, coat the cathode |
| Pitting | Small, deep holes in a passive film | Stainless steel in chlorides | Higher-alloy stainless (316, duplex), keep clean |
| Crevice | Attack inside tight gaps where oxygen can’t reach | Under washers, gaskets, lap joints | Seal or open up gaps, weld instead of bolting laps |
| Stress corrosion cracking | Cracks from tensile stress plus a specific environment | Austenitic stainless in warm chlorides | Lower stress, duplex grades, stress relief |
| Intergranular | Attack along grain boundaries, often near welds | Welded 304 with too much carbon | Low-carbon grades (304L, 316L) |
Galvanic corrosion
When two different metals touch and are wetted by the same water, they form a battery. The metal higher in the galvanic series (more active) becomes the anode and corrodes faster than it would alone; the more noble metal is protected. Three things decide how bad it is:
- Distance in the series. Metals close together (stainless steel and titanium) are safe; metals far apart (aluminium and copper) are not.
- Area ratio. A small anode next to a large cathode corrodes very fast. Stainless screws in an aluminium panel are usually fine (large anode, small cathode); aluminium rivets in a stainless panel fail quickly.
- Electrolyte. Salt water is far worse than rain, and dry indoor air is harmless.
The same effect is used deliberately: zinc on galvanised steel, and zinc or magnesium anodes on boats, corrode instead of the steel they protect.
Stainless steel grades and PREN
For stainless steels, resistance to pitting is compared with the Pitting Resistance Equivalent Number:
Worked example: 304 or 316?
A typical 304 has 18 % Cr, no Mo and 0.05 % N. A typical 316 has 17 % Cr, 2.1 % Mo and 0.05 % N.
As a rough guide: 304 (PREN about 18–20) for indoor and mild outdoor use; 316 (about 24–26) for coastal air, food processing and chemicals; duplex 2205 (about 35) for splash zones and chlorides; super-duplex and 6Mo grades (above 40) for continuous sea-water immersion.
Protection by coating
| Coating | Typical thickness | Notes |
|---|---|---|
| Paint system (primer and top coat) | 120–320 µm | Flexible and cheap; thickness chosen for the environment (ISO 12944) |
| Powder coating | 60–120 µm | Tough, attractive finish for steel and aluminium |
| Hot-dip galvanising | about 45–85 µm and more | Zinc protects even at scratches and cut edges; long life outdoors |
| Zinc electroplating | 5–25 µm | Thin; for fasteners and small parts, mostly indoors |
| Anodising (aluminium) | 5–25 µm | Thickens the natural oxide; can be dyed; hard anodising up to about 50–100 µm |
| Nickel or chrome plating | 5–50 µm | Wear and corrosion resistance with a bright finish |
Worked example: how long will galvanising last?
A steel frame is hot-dip galvanised with 85 µm of zinc and installed in a medium (C3) environment, such as an urban or light industrial area. In C3, zinc corrodes at roughly 0.7–2.1 µm per year (ISO 9223).
Designing parts that don’t corrode
- Let water drain. Avoid pockets, channels and upturned sections where water stands. Add drain holes to hollow sections and enclosures.
- Avoid crevices. Seal lap joints, use continuous welds instead of intermittent ones in wet areas, and avoid tight gaps under washers.
- Isolate dissimilar metals with nylon washers, insulating sleeves or sealant, or choose metals close together in the series.
- Make coatings possible. Round sharp edges (paint thins on them), leave vent and drain holes for galvanising, and avoid closed sections that can’t be coated inside.
- Specify clearly. The drawing should give the material grade, the coating, its thickness and the standard, for example “Hot-dip galvanise to ISO 1461”.
Key points
- Corrosion needs an anode, a cathode, an electrolyte and an electrical path; remove one and it stops.
- Galvanic corrosion is worst with metals far apart in the series, a small anode and salt water.
- For stainless steel, PREN compares pitting resistance: 304 for mild, 316 for coastal and chemical, duplex for harsh conditions.
- Good detail design (drainage, no crevices, isolated metals) matters as much as the material.