Anodising

27 August, 2011

Anodising is one of the key processes used in the manufacture of DMM's aluminium based hardware. DMM anodise products made from aluminium alloys because it:

  • Increases corrosion resistance by creating a thicker protective layer on the surface of the alloy.
  • Allows the surface hardness to be raised thus increasing abrasion resistance.
  • Reduces friction and hence lowers drag.
  • Allows products to be coloured.

Anodising reduces corrosion in aluminum alloys by increasing the thickness and hardness of the natural protective oxide layer. The standard anodising process used on carabiners uses chromic acid or sulphuric acid; the anodising process causes a chemical transformation of the aluminum material itself to create a thicker than normal aluminum oxide surface layer. This layer is often dyed and then it is sealed by a secondary chemical treatment, normally immersion in boiling water. This gives maximum corrosion protection by filling the micro-columns in the layer and preventing access to the substrate.

Thus anodising not only offers the benefit of helping increase the life-span of climbing equipment, but in products such as nuts and cams it allows them to be quickly and easily identified on your harness. If you need any convincing of the corrosion benefits of anodising have a look at the above video around the 6:11 and 7:32 minute mark.

Anodising for easier kit identification
Anodising allows for quick and easy identification of kit such as cams and nuts with colour coded sizing. © Ray Wood
Anodising for easier kit identification
Anodising allows for quick and easy identification of kit such as cams and nuts with colour coded sizing. © Ray Wood

Anodising is the only step in the process of carabiner manufacture that is carried out away from our Llanberis base, as it requires specialist waste management. DMM use Colour Anodising in Manchester to give our hardware this protective finish as they have a modern plant specialising in all types of anodising. Using a state of the art ion exchange recycling system they reuse over 50% of the water from the anodising process, ensure that any liquid effluent is totally clean and scrub all the air leaving the building. Colour Anodising are a Quality Management System ISO 9001 company, approved by the Enviromental Agency and hold the Environmental Management System ISO 14001:2004.

In the above video Colour Anodising Director, Andrew Buckley, explains the electrochemical process of anodising and takes the viewer on a short but insightful tour of their plant, highlighting the steps taken to reduce any potential environmental impact along the way. DMM's Chief Design Engineer, Fred Hall, points out the benefits of anodising, whilst emphasising the need for equipment to be well maintained especially after sea cliff climbing in warm climates.

We are seeing increasing amounts of exfoliation corrosion on carabiners as climbers explore more remote, exotic destinations and it is worth pointing out that although anodizing does a fantastic job of helping equipment last longer it is not a complete solution.

This is largely because the alloy used to make most climbing carabiners - 7000 series aluminum alloy - is quite susceptible to galvanic and exfoliation corrosion: this alloy series has zinc as the primary alloying element together with magnesium. The further addition of copper to the aluminum-zinc-magnesium system together with small, but very important amounts of chromium and manganese gives high strength aluminum alloys that, although initially developed in 1943, are still the benchmark for use in climbing carabiners.

Climbing on the Phra Nang Peninsula
Libby Peter bridged out on the classic Thai F6c, Kratoy. The Phra Nang Peninsula being by the sea with its high humidity and temperatures is renowned for its corrosive effect on climbing gear. © Ray Wood
Climbing on the Phra Nang Peninsula
Libby Peter bridged out on the classic Thai F6c, Kratoy. The Phra Nang Peninsula being by the sea with its high humidity and temperatures is renowned for its corrosive effect on climbing gear. © Ray Wood

Aluminum metal is a very active metal that oxidises very quickly. While this would be a weakness for most metals, this quality is actually the key to its ability to resist corrosion. When oxygen is present in the atmosphere or in the environment (in the air, soil, or water), aluminum reacts very quickly to form aluminum oxide. This aluminum oxide layer is chemically bound to the surface and it seals the core aluminum body from any further reaction.

This is different from oxidation (corrosion) in steel, where the metal oxide (rust) exfoliates, and constantly exposes new metal to corrosion. Aluminum’s natural oxide film is tenacious, hard, and instantly self-renewing. It is this combination of fast oxidation and a protective air-formed oxide film that enables most aluminum alloys to offer good resistance to most corrosive environments.

However the very alloying elements that make the 7000 series perfect for building carabiners make the metal more susceptible to corrosion. In particular it has been established that the addition of copper (2000 series alloys) and zinc/copper/magnesium (7000 series alloys) to the aluminum solid decreases the corrosion resistance of the alloy. This is due in part to the fact that the aluminum oxide film is now not consistent over the surface of the metal and contains oxides of copper, magnesium and zinc which can decrease its protective qualities.

When the two dissimilar metals come into contact in a humid or conductive environment an electrical loop is closed, and the natural voltage differential between them causes electrons to flow. One metal becomes the anode (negative) and one will become the cathode (positive). This galvanic coupling causes the anode to lose metal ions and the cathode to gain ions i.e. the more active (anodic) metal dissolves. Fishermen may use zinc bars in their steel crab pots as sacrificial anodes to stop them corroding.

The alloying components of the 7000 series alloys – copper, zinc, magnesium – have different galvanic reactivities (copper is less reactive or noble and zinc/magnesium are more reactive or anodic) to the aluminum base metal and thus a galvanic reaction and hence corrosion can occur. These adverse effects are magnified if the alloying components (copper especially) are out of solution at grain boundaries. In addition any metal ions passing into solution as a result of corrosion can then be deposited on the surface of the alloy and set up galvanic cells on the aluminum surface. This can dramatically increase the rate of corrosion.

Anodising reduces the potential effect of galvanic action by providing electrical insulation. However, certain factors can de-stabilise the oxide layer and accelerate the corrosion process, these include:

  • Exposure to acidic (< pH 5) or very alkaline environments.
  • Aggressive corrosive ions (chlorides, fluorides) such as found in sea water may attack the oxide locally. Sea salt (mostly sodium chloride) acts as a facilitator for galvanic metal corrosion.
  • Heat
Comparison of anodised versus unanodised on a sea cliff
The protection afforded by anodising can clearly be seen in the comparison of these two carabiners recently removed from a lower-off, dating back to the late-80's, in Pigeons Cave at Pen Trwyn. © Ray Wood
Comparison of anodised versus unanodised on a sea cliff
The protection afforded by anodising can clearly be seen in the comparison of these two carabiners recently removed from a lower-off, dating back to the late-80's, in Pigeons Cave at Pen Trwyn. © Ray Wood

The most common form of corrosion on carabiners is pitting – small localized corrosion on the surface of the metal, which may occur following chemical or mechanical damage to the protective oxide film, producing tiny pits in the metal. Exfoliation corrosion takes longer to happen and is often more serious – it is a severe form of inter-granular corrosion that occurs along aluminum grain boundaries and causes lamination of the surface of the aluminum with white corrosion products forming between the layers.

Galvanic corrosion due to contact between dissimilar metals can happen if your damp climbing gear is left in a sack after a trip to a seaside crag and your nut key or pegs are touching a carabiner with a damaged anodized layer (i.e. the surface of the carabiner is nicked or scratched). It can also happen when a carabiner is loaded on a steel bolt hanger, the harder steel of the hanger can scratch through the anodizing layer and this will then allow corrosion to potentially begin.

In this situation there is a galvanic cell between the steel hanger (even if it is stainless) and the carabiner. The situation is worsened by the large cathodic surface of the hanger concentrating the current flow through the small anodic area of the scratched anodizing on the carabiner.

Anodising does help a lot in reducing corrosion, but it is not an alternative to looking after your kit. Keep you carabiners dry, cool, and away from salts and acids. If you do climb by the sea wash your gear in clean, cool water and dry it naturally in a warm environment and lubricate it when dry. Thanks to Silvia Fitzpatrick for help in writing this article that was largely taken from her site – rockclimbingcompany.co.uk