Should I use A2 or A4 stainless steel fixings?

Stainless is not one material. The two grades sold for domestic fixings behave identically in a dry garden and very differently anywhere chloride is present, and the failure when you get it wrong is not gradual — it is pitting corrosion, which drills small deep holes into a fixing and can leave it looking almost fine while it has lost most of its section.

Step by step

1. Know what the numbers mean.A2 is austenitic stainless roughly equivalent to 304 grade. A4 is roughly equivalent to 316 and contains 2 to 3 per cent molybdenum, which is the addition that resists chloride pitting. A suffix like A2-70 or A4-80 gives the strength class as well.
2. Use A2 for general exterior work inland.Fencing, garden structures, sheds, general joinery, anything wet with rain. A2 is entirely adequate and considerably cheaper.
3. Use A4 near the coast.Salt-laden air carries chloride inland on the wind. Within a few miles of an exposed coast, A2 will pit and streak, and A4 is the sensible default for anything you do not want to redo.
4. Use A4 around pools, hot tubs and salt-treated surfaces.Chlorinated water, bromine and the vapour above them attack A2. So does road salt, so exposed metalwork near a gritted road or a driveway falls into the same category.
5. Use A4 with some treated timbers and with oak.Modern copper-based preservatives and the tannins in oak, cedar, chestnut and western red cedar are aggressive. A2 is generally acceptable in oak; A4 is safer where the timber will be persistently wet.
6. Do not assume stainless means strong.A2-70 is roughly comparable to a mid-grade steel bolt and considerably weaker than a high-tensile 8.8. Where strength is the requirement rather than corrosion resistance, stainless can be the wrong choice.
7. Keep stainless away from mild steel and from aluminium in wet conditions.In a wet joint, stainless in contact with a less noble metal accelerates the corrosion of the other metal. Use isolating washers or keep the metals consistent.

Tips

• Stainless is comparatively soft and galls — two stainless threads can seize solid when run together dry. Use an anti-seize compound or a smear of grease on stainless nuts and bolts.
• Brown speckling on stainless outdoors is usually surface contamination from steel dust rather than the stainless itself corroding. It cleans off; pitting does not.
• A magnet is not a reliable test. Cold-formed A2 and A4 fixings are often slightly magnetic, and being magnetic does not mean a fixing is not stainless.

Common mistakes

• Using A2 on a coastal balcony, balustrade or handrail — Chloride pitting attacks locally and deeply, so a fixing can look sound and have lost most of its cross-section. On anything that a person leans on, this matters.
• Assuming stainless never needs maintenance — Stainless relies on a passive oxide film that needs oxygen. Under a permanently wet, oxygen-starved deposit — trapped dirt, a lap joint, an algae film — it can corrode.

Questions people ask