Fixings and Corrosion

The protection against atmospheric corrosion offered by the zinc plating is directly proportional to its thickness. In the case of fixings this is typically 5–10µm (microns) and can only be relied upon for long term use in dry internal conditions. Durability is affected by factors which may remove the zinc carbonate produced by normal atmospheric corrosion such as the degree of exposure and regularity of washing. Typically in external urban areas the plating would disappear in less than two years and in seawater in a matter of months.

This additional finish applied to zinc plating, which is commonly a yellow/gold colour but occasionally “Blue” (effectively clear) is only applied to prevent corrosion of the zinc and has no significant benefit in terms of atmospheric or other corrosion conditions.

As for ordinary plated components the zinc coating thickness governs durability. Thicknesses used on fixings range from 45-60µm so protection in urban areas is in the order of 10–12 years while in seawater only around 3 years can be expected. Remember that the galvanising may be damaged or removed on some fixing types during installation. If in doubt stainless steel versions should be considered.

Austenitic stainless steel derives its corrosion resistance from Chromium, Nickel and in some grades, Molybdenum.

Grade A2 (303 and 304 etc) - Contains 17–19.5% Chromium and 8–10.5% Nickel. The passive layer resists normal atmospheric corrosion in unpolluted rural areas but is susceptible to pitting and crevice corrosion in aggressive environments, such as industrial and coastal locations and may stain in polluted urban atmospheres.

Grade A4 (316) - Contains 16 – 18.5% Chromium, 10.5–14% Nickel, 2.5–3.0% Molybdenum. The molybdenum improves the resistance to pitting corrosion. This grade of stainless steel is suitable for long term use in the most aggressive conditions normally encountered i.e. industrial coastal and marine environments including total immersion in seawater. It has good resistance to pitting and crevice corrosion at normal temperatures.

Martensitic Stainless Steels (410 grade) are similar to low alloy or carbon steels. Due to the addition of carbon, they can be hardened and strengthened by heat treatment, in a similar way to carbon steels and are capable of drilling and self-tapping into steel reinforcement. They are classed as a "hard" ferro-magnetic group and are considered to have a fair relative resistance to corrosion among the common stainless steel fastener materials.

Rapierstar 400 series Stainless Steel fasteners benefit from a Magni coating that provides an additional level of corrosion protection inherent in Martensitic Stainless Steel, however, Rapierstar would not recommend the use of Martensitic Stainless Steel fasteners into aluminium.

When fixings are used externally for short or medium term applications consideration must be given to how they are to be dealt with after their use is complete. If fixings cannot be removed from the structure but will be left in place, or ground off flush with the surface, then carbon steel anchors should be avoided as they may rust and cause structural damage, in this case fixings should be specified as for long term use.

Rapierstar offer this information as best known practice and will not be held accountable for any liabilities that may result from this information.

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.

There are two usual types of hydrogen embrittlement mechanisms; the environmental type, termed EHE, when the supply of hydrogen from the environment, i.e. through corrosion leads to hydrogen assisted failure. This failure mode is sometimes referred to as Stress Corrosion Cracking (SCC). The second type is hydrogen embrittlement failure due to the processes during manufacture and can be termed hydrogen induced cracking or HIC.

Environmental Hydrogen Embrittlement (EHE) - A combination of very specific conditions leads to this problem. The chance of this type of fastener and therefore joint failure increases in applications in coastal or industrial areas due to the more corrosive environment and/or where the conditions likely to promote crevice corrosion occur, e.g. the fastener is sat in a recess, beneath a gasket or other area where moisture may accumulate. Standard fasteners when stressed, are particularly susceptible in the presence of chlorides and in temperatures in excess of 60°C. Hence swimming pool roof spaces are particularly prone to this type of corrosion. The stresses involved may be direct stresses, as in a tightened or loaded fixing, or simply the residual stresses from cold working or thermal cycles involved in fabrication. The result is not visible to the naked eye as it takes the form of fine cracks, which may lead to sudden catastrophic failure.