Why have my galvanised screws gone rusty?
The four ways a zinc coating stops protecting — thickness, damage, treated timber and contact with another metal — and what to specify next time.
- Difficulty
- beginner
- Time
- 12 min
- Read
- 3 min
Short answer
Either the coating was electroplated zinc rather than hot-dip galvanising, or it was cut, scratched or driven through, or the timber's preservative attacked it, or the fixing was in contact with a different metal in wet conditions. Only the last is unavoidable — the other three are specification errors.
Galvanised is a word that covers coatings differing by a factor of ten in thickness and life. That, more than anything else, is why two fixings that both say galvanised on the box behave completely differently on the same fence. Working out which of the four causes applies tells you what to buy instead.
Step by step
- Establish which coating you actually had.Bright zinc plated — shiny, silvery, smooth — is a few microns thick and is an indoor coating whatever the packet implies. Hot-dip galvanised is dull grey, visibly thicker and often slightly rough. Sherardised is a matt grey diffusion coating, thicker than plating and good on fixings. Organic-coated screws have a coloured resin layer over a thin zinc base.
- Look at where the rust started.Rust at the point, at the head recess and along the thread crests means the coating was mechanically damaged during driving — very common on hot-dip fixings whose coating is thick but soft, and on any screw driven hard through dense timber.
- Check whether the timber is preservative-treated.Modern copper-based preservatives replaced the old CCA formulations and are considerably more aggressive to zinc. Ordinary galvanised fixings in treated timber can lose their coating in a couple of years, especially where the timber stays wet.
- Check for a second metal in the joint.Galvanised fixings in contact with stainless steel, copper, brass or aluminium in a wet joint corrode preferentially, and fast. A galvanised screw through a copper or aluminium flashing is a classic example.
- Check what has been running over it.Rainwater off a copper or lead surface, or off a cedar shingle, carries dissolved metal and tannin onto the fixings below and eats them from the outside in.
- Specify for the exposure, not the label.Sheltered exterior joinery: a good organic-coated exterior screw. Wet exterior structure, treated timber, decking, ground contact: A2 or A4 stainless, or a fixing explicitly rated by its maker for treated timber. Coastal or pool: A4.
- Do not mix metals in a wet joint.Keep the fixing and everything it touches consistent, or isolate them with a nylon or neoprene washer and a sleeve so there is no metal-to-metal path.
Tips
- Cutting a galvanised bolt or nail leaves a bare cut end, which is where the rust always starts. Prime the cut or use a shorter fixing.
- Rust weeping down brickwork or render from a fixing is a stain that needs chemical treatment, not scrubbing, and it will come back until the fixing is changed.
- Where a galvanised fixing has failed but the item is fine, replace all of them rather than the visibly bad ones. They have all had the same exposure.
Common mistakes
- Treating "zinc plated" as an outdoor coating — It is a few microns of zinc intended to stop shelf corrosion indoors. Outdoors it can start rusting in a single wet season.
- Using galvanised fixings against aluminium — In wet conditions the pair sets up a galvanic cell. Which one corrodes depends on the metals and the relative areas, but something does, and it is usually the smaller component — the fixing.
Questions people ask
Is stainless always the safe answer?
For corrosion, usually. But stainless is weaker than a high-tensile steel bolt of the same size and it accelerates corrosion of aluminium and galvanised steel it touches when wet, so it is not automatically right.
Can I paint over rusting fixings?
Only after removing the loose rust and applying a rust-inhibiting primer, and it is a delaying tactic. If the fixing has lost section it needs replacing, not painting.