Corrosion & Fixings – What You Really Need to Know
Choosing the right material and finish for a fixing isn’t optional — it governs how long the fixing lasts and whether you’ll see staining, structural failure or costly callbacks. Corrosion is predictable; it’s your job to anticipate it, not react to it.
This page explains corrosion mechanisms, how materials behave, compatibility concerns, and gives clear, practical rules for specifying fixings that last.
Types of Corrosion
Atmospheric Corrosion - This is oxidation driven by humidity and oxygen. It’s the reason carbon steel rusts — and why stainless steel and aluminium naturally form protective oxide films.
Pitting & Crevice Corrosion - These are localised forms of corrosion, especially critical in chloride environments (coastal areas, de-icing salts). Stainless steels can still pit if incorrect grades are specified.
Galvanic Corrosion (Dissimilar Metals) - When two different metals are in electrical contact and bridged by an electrolyte (rainwater, damp), the less noble metal corrodes faster. Effectively a small cell is set up rather like a very inefficient battery. The metal which is less noble on the galvanic series will corrode faster than it otherwise would have done, while the other is protected. This effect is one reason why zinc plating is used to protect steel. When the zinc plating is scratched or removed over a discrete area, the zinc, which is less noble than steel, corrodes faster while corrosion of the steel is slowed and thus it is protected (when plating is removed over a large area normal atmospheric corrosion takes place). The greater the potential difference between the two metals, the faster the corrosion. The phenomenon is also area related so careful choice of metals can minimise the effect – if the more noble metal has a relatively large area, the less noble will corrode more quickly. Galvanic corrosion only happens when moisture is present.
Relative nobility series of key metals:
| ANODIC (Least Noble) | |
|---|---|
| Magnesium | |
| Zinc | |
| Aluminium | |
| Carbon Steel or cast iron | |
| Copper Alloys (brass, bronze | |
| Lead | |
| Stainless Steel | |
| Nickel Alloys | |
| Titanium | |
| Graphite | |
| CATHODIC (Most Noble) | |
Fastener Material Type: Carbon Steels (Zinc-Plated)
Zinc-plated carbon steel has a thin sacrificial coating that delays corrosion — but only in non-corrosive conditions. In practice:
- Zinc plate is typically 5–10 µm thick and can disappear quickly outdoors.
- In a typical external urban environment, most zinc plating is consumed in under two years, and in coastal conditions it can fail in months.
- When to use it: Dry internal environments with no long-term moisture.
- When not to use it: External exposure, damp environments or applications where staining or fastener loss is critical..
Fastener Material Type: Stainless Steels
Stainless steel does not “never rust.” It forms a passive chromium oxide film that resists corrosion far better than zinc-based coatings or bare steel. Grades related to fasteners include:
- A2 (302/304) – suitable for general external use in rural and urban conditions with moderate humidity and low chloride exposure. Not ideal near salt spray or industrial pollutants.
- A4 (316/316L) – contains molybdenum, significantly improving resistance to chlorides and pitting. This is the correct grade for coastal, marine and aggressive industrial environments.
- Martensitic Stainless (410) – is hardenable through heat treatment, for drilling and tapping into steel. It has less corrosion resistance than A2/A4 and should not be used with aluminium without isolation. Screws usually have an organic ceramic thin film coating to achieve better corrosion protection.
