A Comprehensive Guide to Protective Coatings for Fixings

Blindbolt UK | 20th December 2023

Heavy Duty Bolt

In common with many other manufacturers, BlindBolt fixings can be specified in different materials and with alternative coatings to suit different exposure conditions. This article offers advice on the suitability of BlindBolt materials and protective coatings for different corrosion conditions.  BlindBolt supplies fixings which have been hot dip galvanised, fixings manufactured from A4 stainless steel and fixings with a zinc flake coating.

Corrosion Categories

Corrosion categories are tabulated in BS EN ISO 9223, ranging from C1 (very low corrosivity) to C5 (very high corrosivity). There is one even more onerous classification, CX, described as “extreme”.

Examples of typical environments falling into each corrosivity category are given in Annex C of BS EN ISO 9223, for both indoor and outdoor environments. Selected environments are given below.

C1 Heated spaces – offices, schools, museums Dry or cold zone – deserts or polar regions
C2 Unheated spaces, low frequency of condensation – storage, sports halls Temperate zones, low pollution – rural areas, small towns
C3 Moderate frequency of condensation, moderate pollution from production processes – food-processing plants, laundries Temperate zone, medium pollution – urban areas, coastal areas
C4 High frequency of condensation, high pollution from production processes – industrial plants, swimming pools Temperate zone, high pollution – industrial areas, coastal areas, exposure to de-icing salts
C5 Very high frequency of condensation, high pollution – mines, unventilated sheds in tropical zones Temperate and sub-tropical zones, very high pollution – industrial areas, coastal areas

BS EN ISO 9223 also tabulates the classification category with respect to the corrosion rates for different metals that might be expected to occur in the first year. After this period, the corrosion rate generally slows.

Considerable uncertainty is associated with an estimation of atmospheric corrosivity. As an example, the variability in corrosion rates given for zinc is given in Table A.1 of BS EN ISO 9223 as -33% to +50%.

The Galvanisers Association provide a corrosion map for the UK and Ireland, which may be searched to indicate the atmospheric corrosion rate for hot dip galvanising within a 10 km grid.

Hot Dip Galvanised Fixings

A galvanised fixing would be expected to have a mean coating thickness of 85µm.  At a typical corrosion rate in corrosivity category C3 of 1.4µm in the first year, even if this rate continued, the coating might be expected to be effective for 60 years. The Galvanisers Association suggest the average corrosion rate in the UK and Ireland is less than 1µm per year, indicating a maintenance-free life of over 85 years.

Stainless Steel Fixings

Stainless steel is highly corrosion-resistant. Depending on the environment, a maintenance-free lifespan of more than 60 years can be expected. Common grades of stainless steel for fixings are A2 and A4. A4 stainless is more resistant to corrosion.

Galvanitic corrosion occurs when two dissimilar metals are in contact and also bridged by an electrolyte (an electrically conducting liquid). Seawater is a strong electrolyte, although rain or humidity can trigger a galvanic reaction.

When a stainless steel fixing is used to connect a carbon steel element, the risk of attack on the carbon steel member is negligible. Conversely, using carbon steel bolts in stainless steel members should be avoided.

Zinc flake coating

A zinc flake coating system is produced by applying a zinc flake dispersion to the surface of a steel fastener, usually with the addition of aluminium flakes, in a suitable medium. As the coating cures, bonds form between the flakes and the substrate, forming an inorganic surface sufficiently electrically conducting to ensure cathodic protection. The sacrificial cathodic coating corrodes preferentially to protect the fastener.

The resistance of a zinc flake coating is often given as a number of hours of salt spray protection (SSP). The standardised test is undertaken in an enclosed chamber with an aggressive salt environment. The test is primarily used to detect defects in the coating and to ensure a consistent coating process. There is no correlation between the duration in the test and the service life of the coating. Experience suggests that in service, a coating reported as having 100 hours SSP might indicate a service life of six weeks in a humid environment, to many years in a dry environment.

Which material or coating should be specified?

From the above, it should be clear that zinc flake coating is only appropriate for dry, protected environments. In less benign environments and external environments, a galvanised fixing should provide excellent resistance to corrosion. In aggressive environments such as marine installations, or in swimming pool environments, stainless steel is the appropriate material.

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