Stainless Steel Cable Cleats – Preventing Galvanic Corrosion Of Cable Fixings
Published 20 Mar 2018
Uploaded By Chris Dodds – Thorne & Derrick Sales & Marketing Manager
In the following article we provide an evidence based specification case for stainless steel cable cleats where levels of atmospheric corrosion preclude use of other cable fixing materials.
One of the most important issues to consider when specifying cable cleats is the risk of material corrosion – not just as a result of the installation environment, but also from other metals which the cable cleat is in contact with.
Galvanic corrosion occurs when dissimilar metals are placed in contact with each other in the presence of an electrolyte. There are two factors that affect the rate of galvanic corrosion, the first is the distance between the two metals in the galvanic series.
The further apart the two metals are in the galvanic series, the greater the risk of galvanic corrosion – with the metal higher up the list (more anodic) being the one whose rate of corrosion is accelerated.
The second factor to consider is the relative surface areas of the different metals.
If the more anodic (higher up the list) metal has a smaller surface area than the metal it is in contact with, the difference in surface area causes the rate of corrosion of the anodic metal to increase.
Conversely, if the more anodic metal has a much larger surface area than the cathodic metal, it may be sufficient for the effects of galvanic corrosion to be discounted. In terms of cable cleat selection, the surface area of the cleat is generally significantly smaller than the structure it is mounted on.
Therefore, if it is made from a metal that is more anodic than its support structure it will be susceptible to galvanic corrosion.
Conversely, if the cable cleat is more cathodic than its support structure, there is little risk of galvanic corrosion.
Using this criteria, if galvanised ladder is the support structure, and there are no other significant factors, it is safe to use stainless steel or aluminium cleats. However, if the support structure is stainless steel, separation should be provided if aluminium or galvanised cleats are used.
Galvanic corrosion is not easily predictable and can be influenced by the type of electrolytes present such as salt water or fresh water containing impurities. In general terms when guarding against galvanic corrosion, the safest course of action is to separate dissimilar metals with polymer separation washers.
This separation should be carried out between the cable cleat and its mounting surface and the cleat’s mounting fixing.
All Ellis cable cleat products constructed from dissimilar metal are designed in a way that completely avoids bimetallic contact. As a result of this you can be confident that cable cleats will have a service life measured in decades.
All Ellis stainless steel cable cleats are produced from 316L austenitic stainless steel.
In general, cable cleats are manufactured from austenitic stainless steel due to its non-magnetic and corrosion resistant properties – the former ensuring the cable cleat will not induce eddy currents or localised heating of the LV-HV cable.
Austenitic stainless steel does become a little magnetic as a result of work hardening when processed. This magnetism can barely be detected with a magnet and so is not significant from an eddy current perspective.
304 austenitic stainless steel, often referred to as A2, is one of the most commonly used stainless steels. It has excellent corrosion resistant properties in most circumstances, although is susceptible in atmospheres where chlorides are present, making it unsuitable for use in coastal or marine environments.
316 austenitic stainless steel, often referred to as A4, contains Molybdenum, which provides resistance against chlorides. 316 is often referred to as marine grade stainless steel due to its suitability for use in coastal and offshore applications.
If unsure a simple chemical test can determine whether Molybdenum is present and so differentiate between 304 and 316 types of stainless steel.
There are many different types of stainless steel, but there are two principal variants when it comes to cable cleats. 304 and 316 stainless steel are available in low carbon variants, namely 304L and 316L. These variants are immune to sensitisation (grain boundary carbide precipitation).
Any cable cleat which is manufactured from stainless steel and includes welding in the manufacturing process should be made in a low carbon (L) variant.
Should you require any further assistance in the selection or specification of stainless steel cable cleats for LV Low Voltage, MV Medium Voltage or HV High Voltage cables please do not hesitate to contact us.
Cable Cleat Coatings
The corrosion resistance properties of stainless steel are a result of chromium, which reacts with oxygen and forms a self-healing impervious layer of chromium oxide on the surface of the steel.
In most circumstances the chromium oxide layer is extremely durable and helps in resisting galvanic corrosion. However, in certain installation locations, such as railway tunnels, the oxide layer can be continuously penetrated.
This occurs due to trains frequently applying their brakes, which releases mild steel dust into the atmosphere that then settles on the stainless steel. If moisture is present, then corrosion occurs at an exaggerated rate. In such circumstances, if regular washing is not feasible, use of aluminium as an alternative to stainless steel products and/or coating processes are strongly recommended.
Ellis Patents offers special coatings to suit specific cable installation environments – e.g. our London Underground Approved electrostatic plastic coatings.
Cable Cleat Fixings
Closure fixings on cable cleats are fundamental to the loop strength of the cable cleat and its short-circuit withstand capability.
All Ellis Patents 316L stainless steel cleats use 316 fixings, which are manufactured to a precise and specific tensile strength. Fixings are sourced directly from approved manufacturers and any fixing on any cleat is directly traceable back to the batch quality records at that manufacturer.
Contracts often require a guarantee regarding the life expectancy of a cleat. If the installation is designed correctly and all other corrosion issues have been considered this is a relatively simple exercise for stainless steel products. With galvanized steel, life expectancy is determined by the thickness of the zinc coating.
The resistance of galvanizing to atmospheric corrosion depends on a protective film that forms on the surface of the zinc. When the newly coated steel is withdrawn from the galvanizing bath, the zinc has a clean, bright, shiny surface. With time a corrosion process occurs which produces a dull grey patina as the surface reacts with oxygen, water and carbon dioxide in the atmosphere.
This leads to the formation of a tough, stable, protective layer, which is tightly adherent to the zinc. As the corrosion process is continuous, the thickness of the zinc layer reduces over time and it is the speed of this reduction that is used to accurately predict the life span of the cable cleat.
Corrosion Rates for the UK
Permission to use the information relating to galvanising was granted by the Galvanizers Association for galvanised steel. If a galvanised steel cable cleat is specified for use in a zone 3 area then the corrosion rate is 1.5 microns (µm) per year.
If the contract for this specification states a required life expectancy of 40 years, then the initial galvanising thickness will need to be a minimum of 60 µm in order to meet the required longevity.
Zinc corrosion rates are represented by 5 categories indicated by the colour codes below:
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Thorne & Derrick International are specialist distributors of LV, MV & HV Cable Installation, Jointing, Duct Sealing, Substation & Electrical Equipment – servicing UK and global businesses involved in cable installations, cable jointing, substation, overhead line and electrical construction at LV, 11kV, 33kV and EHV.
THORNE & DERRICK Product Categories: Duct Seals | Cable Cleats | Cable Glands | Electrical Safety | Arc Flash Protection | Cable Jointing Tools | Cable Pulling | Earthing | Feeder Pillars | Cable Joints LV | Joints & Terminations MV HV
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