Corrosion & Fixings

Most metals occur naturally as oxides and oxidation (which occurs in the presence of oxygen and water) and atmospheric corrosion is just the natural tendency to revert to that condition. In iron and steel “Rusting” is an aggressive phenomenon producing prodigious growth which, in the case of unprotected components contained within a structure, can exert forces sufficient to crack certain building materials. The oxidation of aluminium occurs immediately it is exposed to the atmosphere producing a protective layer hence the dull patina and apparent corrosion resistance of this material. Stainless steel benefits from a similar protection mechanism in the development of a chromium based passive protection layer. The corrosion of zinc, when exposed to the atmosphere, results in zinc carbonate (white rust) which develops at a rate of about one tenth that of red rust.

Depending on the durability required, and the degree of pollution, for most atmospheric exposure situations stainless steel will be the answer in grade A2 (304) for long term rural and urban exposure with low chloride concentrations or Grade A4 (316) for urban locations with higher chloride concentrations and industrial or coastal exposure.

Pitting corrosion is the local breakdown of the passive layer on passively protected materials, such as stainless steel and aluminium, and results in pitting which can affect appearance, may cause some staining and, depending on section thickness, can eventually lead to complete perforation. It can be initiated by chemical contamination including seawater and other chlorides or even by steel fragments from non-stainless tools.

Often also referred to as “bi-metallic corrosion”, this occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte e.g. rainwater. If the metals are dry though, galvanic (bi-metallic) corrosion cannot occur.

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.

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)

 Guidelines for selection of fasteners based on Galvanic Corrosion

KEY

A. The corrosion of the base metal is not increased by the fastener.
B. The corrosion of the base metal is marginally increased by the fastener.
C. The corrosion of the base metal may be markedly increased by the fastener material.
D. The plating on the fastener is rapidly consumed, leaving the bare fastener metal.
E. The corrosion of the fastener is increased by the base metal.

NOTE: Surface treatment and environment can change activity

Crevice corrosion occurs in chloride containing solutions where narrow gaps or crevices restrict the access of oxygen while allowing access for the solution and can even affect stainless alloys with good resistance to atmospheric corrosion. The crevices which allow this can be those that exist between washers and nuts (or flange heads) and fixtures. Deposits on the fixings e.g. mortar, sand, iron or accumulated dirt can eventually lead to crevice corrosion. The use of isolating washers does not prevent crevice corrosion.

Internal Hydrogen Embrittlement (IHE) is a brittle failure mechanism caused by the absorption and diffusion of hydrogen atoms into the metal, weakening internal bonds and leading to cracking. It primarily affects high-strength carbon and low-alloy steel fasteners, usually those heat-treated to high hardness or tensile strength. Hydrogen can be introduced during manufacturing processes such as acid pickling, electroplating, or phosphating. It can also be introduced during service through corrosion reactions in damp or aggressive environments. ISO 4042 recognises this susceptibility and restricts the use of electroplated coatings on very high-strength fasteners unless hydrogen-relief heat treatment is carried out immediately after plating.

In service, hydrogen may also be generated by corrosion, particularly in chloride-rich or industrial atmospheres and in confined oxygen-starved locations such as recesses beneath washers or under gaskets. When high tensile stress is present from tightening service loads or residual forming stresses, hydrogen-assisted cracking can occur without obvious surface corrosion and can lead to sudden brittle fracture. By contrast, austenitic stainless steels are generally not susceptible to hydrogen embrittlement in atmospheric service, whereas martensitic stainless steels and some precipitation-hardened stainless steels can be affected under certain conditions.