Atmospheric Corrosion
 

Aluminium and Atmospheric Corrosion

Aluminium enjoys the advantage of a hard, inert oxide film that forms instantaneously when the metal is cut or abraded. This oxide film inhibits corrosion and as a result, in some uses such as industrial and agricultural roofing and cladding, aluminium is routinely supplied in mill finish. Even windows, doors and casements can be supplied in mill finish. The demands of corrosion resistance, surface protection and aesthetics have however relegated mill finish to those situations where corrosion is slow and the product does not have to be finished in a specific brand colour or where dulling and roughening of the surface is not important.
 

The Concept of Corrosivity

The corrosivity of an atmosphere can be defined as the ability of the atmosphere to cause corrosion in a given metal or alloy. ISO 9223 specifies the key factors in the atmospheric corrosion of metals and alloys and defines categories of corrosivity on the basis of the constituents of the atmosphere, and in particular the levels of chloride and sulphur dioxide present, in combination with the time-of-wetness of metal surfaces.

ISO 9223 also allows the classification of corrosivity to be done using corrosion rate measurements of standard specimens of common metals and provides a table relating the corrosivity category to these corrosion rates for the first year of exposure.

While the category can be determined from a one-year exposure of a suitable coupon, the corrosivity category thus derived can in turn be used to estimate the long-term effects on metals: given the category, the average corrosion rate over the first 10 years of exposure can be found in ISO 9224 [4]. This can be useful in estimating, for example, whether an unpainted galvanized coating over steel is a viable protection, or whether mill-finish aluminium will achieve a particular service lifetime.

Conversely, if 10-year or equivalent data are available from long-term exposure and analysis of metal samples, then these may be used to determine the associated corrosivity categories.
 
 

NOTE:

  • Metal corrosion depends on time of exposure to moisture and relative humidity and temperature.
  • Rate of corrosion reduces rapidly the greater the distance from the sea.
  • For aluminium, aggravating conditions can be a strong sulphurous atmosphere, the occurrance of chloride salts and stagnant water - but the right aluminium alloy still
    outperforms copper, zinc and carbon steel! Regular cleaning and surface coatings reduce the affect on aluminium.
  • Pitting corrosion in aluminium may be unsightly, but it is not structurally significant.
 
Corrosivity areas derived for Southern Africa with associated ISO corrosivity ratings
 
Code
Geographic Area
Distance from Ocean
Description
ISO
Category
Comments
A
Namibia and NW Cape
shore-line
To 5 km
Desert shore-line and
coastal fog zone
Above C5
N. of Olifants River
B
W. Cape Atlantic
shore-line
To 3 km
Arid shore-line with fog or
strong winds
Above C5
False Bay to Olifants River
C
W. Cape coastal
To 5-15 km
Coastal area
C4
To range of fall-out of
salt aerosols
D
W. Cape urban
To 25 km
Coastal urban/industrial
C5
Cape Town and
surrounds
E
S. and E. Cape
shore-line
To 1 km
Temperate shore-line
C5
Distance from ocean varies
with terrain
F
S. and E. Cape and
Natal south coastal
To 5-10 km
Temperate coastal
C4
Distance from ocean varies
with terrain
G
KZN shore-line
To 4 km
Subtropical shore-line
Above C5
KZN to Maputo
H
KZN coastal
To 15-25 km
Sub-tropical coastal
C4
I
Durban urban
To 10 km
Urban and industrial, inland
of shore-line
Upper C5
Amanzimtoti to Durban
North
J
Richards Bay
To 15 km
Urban and industrial, inland
of shore-line
Lower C5
K
Coastal cities
To 10-15 km
Industrial and heavy
traffic areas
Lower C5
Areas of Port Elizabeth,
East London, Pinetown
L
Highveld general area
-
Rural and suburban areas
C3
M
Highveld urban and
industrial
-
High-traffic urban, or close
to heavy
industry
C4
East Rand, areas of
Pretoria, Witbank
 
Remainder of the region: C2 to lower C3 depending on climate.
 

Corrosion Resistance

Many decades of experience with applications in a variety of markets including construction, land-based installations and shipbuilding have shown that aluminium and its alloys in the

1000, 3000, 5000, 6000 and 8000 series have excellent resistance to atmosheric corrosion and to marine, urban and industrial environments.

Combined with its light weight, aluminium’s excellent corrosion resistance accounts for the growth of aluminium applications and offers users a number of advantages:

  • aluminium equipment can have a very long service life. It is not uncommon to find roofing, wall cladding panels, marina installations and boats with decades of service behind them. This is also true in the field of transport and many other applications,
  • aluminium maintenance is minimal even when no extra protection (painting, anodising) is provided. When aluminium is painted, repairs are fewer and less urgent because the parent metal generally resists corrosion very well. Aluminium alloy products do not entirely eliminate the need for maintenance, especially in buildings. It is well known that surfaces that are not cleaned are more sensitive than others, and that a surface build-up can aggravate corrosion (this is true of all metals and alloys).
  • aluminium’s corrosion resistance maintains the appearance of the equipment made from it. This is a useful sales argument, especially in industries where users want to keep their products looking good for less cost. Commercial vehicles, outdoor municipal amenities and traffic signs (indicator boards, gantries) are good examples of this.

Finally, the process of anodising to a depth of a few microns can help create and preserve visual properties (high-quality reflectance for retro-reflectors) or decorative features (luxury packaging for cosmetics, decorative paneling for buildings). Aluminium’s resistance to corrosion comes from the oxide surface which is impermeable and integral with the base metal, staying stable for pH values between 4 and 9. The most frequent types of corrosion are galvanic, pitting and crevice corrosion. Galvanic corrosion can be easily prevented by inserting an insulating material between the two different metals or by cathodic protection. Pitting can be prevented by surface coating and crevice corrosion by judicious design.