Aluminium is widely used in the fields of construction, transportation,
electronics and packaging due to its light weight, high strength and
easy processing. However, many users have a misunderstanding of
the corrosion resistance of aluminium: some people think that
aluminium ‘never rusts’, while others worry that it will corrode and
fail like steel in long-term use. In fact, the oxidation and corrosion
mechanisms of aluminium are very different from those of conventional
metals. In this paper, in-depth analysis of the corrosion principle of
aluminium, environmental factors and protective technology, to
provide scientific reference for the industry.
First, the aluminium ‘rust’ truth: the protective
role of the oxide film
1. The nature of the oxidation reaction of aluminium
Steel rust is iron and oxygen, moisture chemical reaction to generate loose
Fe₂O₃ (red rust), its structure is porous and easy to peel off, resulting in
corrosion continues to deepen. Aluminium, on the other hand, when
exposed to air, rapidly combines with oxygen to form a dense Al₂O₃
oxide film, the characteristics of which include:
Spontaneity: It can be formed at room temperature and has a thickness
of about 2 to 5 nanometres (natural oxide layer).
Densification: Aluminium oxide molecules are larger than aluminium
atoms, forming a continuous non-porous barrier.
Self-repairing: after a surface scratch, the exposed new aluminium will
immediately re-oxidise to repair the film layer.
This oxide layer gives aluminium excellent corrosion resistance in most
environments, but it is not ‘never corrodes’ - extreme conditions or
specific media can still destroy its protective mechanisms.
2. Two forms of aluminium corrosion
Chemical corrosion: aluminium reacts directly with strong acids
(pH < 4) and bases (pH > 9).
Electrochemical corrosion: When aluminium is in contact with other
metals (e.g. copper, steel), a microcell is formed in the electrolyte
(rainwater, salt spray) and the aluminium is accelerated to corrode
as the anode. For example, in the marine environment, the rate of
galvanic corrosion can reach 0.1mm/year.
Second, the five major causes of aluminium
corrosion and typical cases
1. Environmental media influence
Chloride ions (Cl-): salt spray in coastal areas (Cl- concentration > 5mg/m³)
triggered pitting corrosion, the formation of holes with a diameter of 0.1 ~ 1mm.
Acidic pollution: Acid rain (pH 4.05.6) in industrial areas leads to dissolution
of the oxide film and corrosion of up to 25μm per year.
Alkaline cleaners: cleaning solutions with pH>10 cause soluble sodium
meta-aluminate (NaAlO₂) to be generated on the aluminium surface,
accelerating material loss.
2. Alloy element differences
Pure aluminium (1xxx system): best corrosion resistance, but low strength,
only used for chemical tank lining.
Aluminium alloy:
5xxx series (Al-Mg): seawater corrosion resistance, ship deck life of
more than 20 years;
2xxx-series (Al-Cu): intergranular corrosion risk increases significantly
with copper content >3%.
3. Processing and installation defects
Welding residues: Chlorides in weld slag initiate stress corrosion
cracking (SCC).
Dissimilar metal contact: When aluminium-steel is directly connected,
the galvanic coupling corrosion rate is enhanced by more than 10 times.
The four core technologies of aluminium
corrosion prevention
1. Anodic oxidation treatment
Generate 10~25μm aluminium oxide layer on the surface of aluminium
by electrolysis, and close the micropores:
Hard anodising: film thickness 50~100μm, hardness up to HV400,
suitable for gears, hydraulic cylinders.
Dye oxidation: add organic dyes, both anti-corrosion and decorative
function, annual fading rate of architectural curtain wall <5%.
2. Coating protection
Powder coating: epoxy-polyester hybrid coating (thickness 60~120μm),
salt spray resistance >1000 hours.
Fluorocarbon spraying (PVDF): UV aging resistance of more than 25 years,
used for ultra-high-rise building facades.
3. Alloy optimisation design
Manganese (Mn) is added: grain refinement, enhancement of corrosion
resistance, 3003 aluminium alloy (containing 1.2% Mn) roof panels life
of more than 30 years.
Rare earth element modification: Cerium (Ce) makes the self-repairing
speed of the oxide film increase by 3 times.
4. Electrochemical protection
Sacrificial anode method: Zinc or magnesium blocks are installed on
aluminium components to preferentially corrode to protect the main
body, with a ship protection efficiency of >90%.
Applied current method: apply cathodic current to aluminium to
inhibit anodic dissolution, applicable to submarine pipelines.
Fourth, the corrosion resistance performance
of aluminium in different environments
1. Daily environment
Indoor dry environment: stable oxide film, annual corrosion <0.1μm,
no need for additional protection.
Urban atmosphere: when SO₂ pollution is slight, the service life of
aluminium window frame can reach 50 years.
2. Severe environment
Marine environment:
5052 aluminium alloy hull with coating, salt spray resistance life > 15 years;
Unprotected aluminium corrodes up to 0.5mm per year in the tidal zone.
Chemical environment:
Concentration of 30% of nitric acid, pure aluminium corrosion rate <0.02mm/year;
But 40% sodium hydroxide solution can dissolve aluminium up to 10mm/year.
3. High temperature environment
<100℃: the oxide film is stable, aviation aluminium (7075-T6) can be
used for a long time;
> 400 ℃: oxide film rupture, grain boundary corrosion accelerated,
need to use aluminium composite materials.
V. Aluminium use and maintenance guidelines
1. Suggestions for material selection
Building doors and windows: 6063-T5 aluminium alloy + electrophoretic
coating, the best cost performance;
Car body: 6016 aluminium alloy + self-repair coating, 30% weight reduction
and gravel impact resistance;
Electronic radiator: 1050 pure aluminium + anodic oxidation, thermal
conductivity 229W/(m-K).
2. Installation considerations
Isolation design: Use nylon spacer or coated with insulating adhesive
when aluminium is in contact with steel;
Drainage design: avoid waterlogged areas to reduce the risk of
crevice corrosion.
3. Daily Maintenance
Cleaning method: neutral detergent (pH 6~8) + soft cloth to wipe,
disable steel wire ball;
Damage repair: local scratches can be sanded and sprayed with
acrylic varnish;
Regular inspection: test the integrity of the coating every 2 years
in coastal areas and repair the damaged areas.
Conclusion
Although aluminium will not ‘rust’ like steel, its corrosion resistance is
highly dependent on environmental conditions and protective measures.
Through scientific alloy design, surface treatment and installation
maintenance, aluminium products can achieve long service life in most
scenarios. With the breakthrough of new technologies such as
micro-arc oxidation and graphene coating, the corrosion-resistant
boundary of aluminium is expanding, providing more reliable
lightweight solutions for green buildings, new energy vehicles
and other industries.