Thursday, 24 September 2009

Corrosion in Climbing Carabiners - Corrosion in 7000 series Aluminum Alloys

Each year DMM receive back a few carabiners where the surface of the carabiner has corroded and exfoliated. Almost invariably these carabiners have been to Thailand (although DMM have seen similar corrosion from carabiners used in Hawaii). They are often asked for the reasons why this corrosion occurs.

Aluminum alloys are widely used in the manufacture of climbing products because these alloys can offer a high strength-to-weight ratio combined with good ductility and toughness.

There are currently 8 families of aluminum alloys and each family group is identifiable by the first number of its name. The families differentiate themselves from each other by their principal alloying elements:

Major Alloying Element

Series Identifier

None (99%+ Aluminum)

1XXX

Copper

2XXX

Manganese

3XXX

Silicon

4XXX

Magnesium

5XXX

Magnesium + Silicon

6XXX

Zinc (Magnesium + Copper)

7XXX

Lithium

8XXX

Unused

9XXX0

These main alloying elements impart specific properties to the alloy. These properties can be further modified both by the addition of extra alloying elements and processing treatments that occur during the production of the finished alloy product.

The vast majority of carabiners used in recreational climbing are made from 7000 series aluminum alloys with aluminum alloy 7075-T6 being the most widely used. 7075 describes the grade and T6 describes the generic heat treatment process used on the metal.

7000 series alloys have 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. There are stronger/tougher alloys on the market, but cost and productivity issues often work against them in a commercial environment.

Chemical

Composition Limits (Wt. %)

Si

0.4

Fe

0.5

Cu

1.2-2.0

Mn

0.3

Mg

2.1-2.9

Cr

0.18-0.28

Zn

5.1-6.1

Ti

0.2

Aluminium 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), aluminium reacts very quickly to form aluminium oxide. This aluminium oxide layer is chemically bound to the surface and it seals the core aluminium 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. Aluminium’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. It 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.

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.

However certain other factors often need to be in place to de-stabilise the oxide layer and accelerate the corrosion process, these include:

  • The oxide is not stable in acidic (pH <> 9) environments
  • Aggressive ions (chlorides, fluorides) may attack the oxide locally i.e. sea water
  • Certain elements (Ga, Tl, In, Sn, Pb) may become incorporated in the oxide and destabilise it
  • Contact with dissimilar metals including pressure treated wood (which contains copper) carbon steel and mild steel.
  • Heat

  • Sea salt (mostly sodium chloride) can help destabilise the normally protective oxide film, leading the localized attack (pitting). This happens because it acts as a facilitator for galvanic metal corrosion.

When the two dissimilar metals come into contact 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 electrical circuit causes the anode to lose ions and the cathode to gain ions i.e. the more active metal dissolves. The copper is relatively inactive and the other metals in the 7075 alloy, including the aluminium base metal dissolve/change form. These adverse effects are magnified if the copper is out of solution at grain boundaries.

The most common form of corrosion on carabiners is pitting – small localized corrosion on the surface of 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 delamination of the surface of the aluminum with white corrosion products forming between the layers.

Anodising can reduce corrosion in aluminum alloys by increasing the thickness and hardness of the 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.

Anodising does help a lot in reducing corrosion, but it is not an alternative to looking after your kit – if the surface of the carabiner is nicked or scratched, such as happens when a carabiner is loaded on a steel bolt hanger, then this will 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 anodize on the carabiner.

Keep you carabiners dry, cool, away from salts and away from acids. If you do climb by the sea wash your gear in clean, cool water and dry it in a warm environment naturally and lubricate when dry if necessary.

There is more information on a corroded quickdraw set on my web site - see Strength of Old Carabiners

Click on the tab below the main , central text.



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