APPLICATION NOTE # D1
Different forms of aluminum corrosion in natural waters
The aluminum’s resistance to corrosion depends on the stability of aluminum oxide formed on the
surface of the metal. Corrosion of aluminum in natural waters can occur in two main ways : pitting
corrosion, which is a localized form of corrosion, characterized by the formation of irregularly
shaped cavities on the surface of the metal (their diameter and depth depend on several
parameters related to the metal), and galvanic corrosion, which is a form of corrosion that occur
when two dissimilar metals are in direct contact in a conducting liquid.
Furthermore there are many factors that can influence corrosion on aluminum, the most
important are:
 Influence of pH
 Influence of ions solved in fresh water on aluminum
 Influence of temperature
 The influence of water movement and flow speed
The effect of these factors on aluminum should always be considered in order to make the correct
choice in terms of product design.
Pitting corrosion
Pitting corrosion is a form of localized degradation of metal that results from electrical potential
differentials in a single body of metal. Pitting corrosion can initiate in locations where
compositional heterogeneity, emerging dislocations or slip steps, inclusions or contaminants, or
externally induced breaks in passive or protective films occur, and different areas of the metal
take on different electrical potentials.
Like all passive metals, aluminum is prone to pitting corrosion in aqueous media close to
neutrality. Under these conditions, pitting corrosion depends more on the quantity of anions, such
as chlorides, than on variations in the pH value of the aqueous medium.
Pitting corrosion of aluminum in waters develops at preferential sites where the natural oxide film
is less resistant because of heterogeneous features such as Al 3Fe intermetallics or defects related
to very localized thinning or rupture of the natural oxide film. These sites are anodic with respect
to their vicinity, and corrosion pits can develop according to the electrochemical mechanism.
Pitting is initiated by anions that penetrate into the defects of the natural oxide layer.
Chlorides have the highest propensity to penetrate into the natural oxide layer, since they are
small and very mobile . They may replace oxygen atoms in the network of the natural oxide film.
This leads to a decrease in resistance of the film, which facilitates the release of aluminum atoms
that diffuse into the water. The aggression of a given water to metal in general, and to aluminum
in particular, largely depends on its chloride content.
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APPLICATION NOTE # D1
The deepening rate of pitting on alloy 1100 follows the equation:
d = kt1/3
where d is the pitting depth, t the time and k is a constant that depends on the alloy and on the
service conditions: nature of the alloy, temperature, flowing speed, etc.
Experience shows that in certain cases, pitting corrosion in contact with water may develop during
the first weeks of service. In freshwater tanks, significant pitting corrosion can be observed after a
few months of service as large pits, several millimeters in diameter and 1–2 mm in depth, covered
by white plaques of alumina gel.
Several deep pits may develop during the first weeks of service, and then nothing more for 25
years.
Galvanic corrosion
When two dissimilar metals are in direct contact in a conducting liquid, experience shows that one
of the two may corrode. This phenomenon is called galvanic corrosion.
There are three conditions that must exist for galvanic corrosion to occur:
 There must be two electrochemically dissimilar metals present
 There must be an electrically conductive path between the two metals
 There must be a conductive path for the metal ions to move from the more anodic metal
to more cathode metal
If any one of the above conditions does not exist, galvanic corrosion will not happen.
Aluminum and aluminum alloys are more electronegative than most common metals (with the
exception of zinc, cadmium and magnesium). As a consequence, aluminum may develop galvanic
corrosion when in contact with a lot of other metals when permanently immersed in water. For
this reason it has to be absolutely avoided the use of aluminum and copper within the same
cooling circuit.
An increase in temperature may modify the dissolution potentials and accelerate galvanic
corrosion.
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APPLICATION NOTE # D1
Blackening of aluminum
Blackening is not a form of corrosion: it is only an alteration of the visual properties of the outmost
oxide layer. It does not alter the corrosion resistance of aluminum in water. This is an unavoidable
phenomenon. Blackening is not irreversible and can disappear in acidic media.
Blackening of aluminum is due to the structure of the uppermost layer of the natural oxide film.
No aluminum alloy that resists blackening is known yet.
Blackening occurs if the water contains bicarbonates HCO3 2 3 and if its pH is between 8 and 9.
Blackening is said to be caused by the adsorption of bicarbonate ions at the porous natural oxide
film.
Blackening may also appear when aluminum is coupled with a less electronegative metal in water:
silver, stainless steel, or copper. Blackening can be eliminated with acidic solutions containing 10%
phosphoric acid or 1% tartaric acid and 1% sodium fluoride at 60 °C .
Figure D.3. Clear oxide layer formed on 3003 by
immersion in boiling distilled water (50000x).
Figure D.4. Grey-blak oxide formed on 3003 by immersion
in boiling tap water (50000x).
Influence of pH
When an aluminum medium is exposed to air or water a superficial film forms on the surface of
the body. This film is made of alumina or aluminum oxide and protects the material from
corrosion.
The natural oxide film is composed of two layers (Figure D.6):
– an internal layer, normally amorphous, in contact with the metal;
– an external layer, whose structure changes in contact with water as a function of temperature.
Up to 70 °C, the reaction of the oxide film with water leads to the formation of bayerite
(Al2O3·H2O). Above 70 °C, boehmite (Al2O3·H2O) is formed ; its thickness may reach a micrometer.
This layer is protective.
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APPLICATION NOTE # D1
Figure D.6. Layers and adsorption phenomena on oxide film.
The stability of natural oxide film, which governs the corrosion resistance of aluminum, depends
on pH (Figure D1). Most natural, untreated and unpolluted surface waters have a pH between 6.5
and 7.5. Since the solubility of alumina is minute and is practically constant in this pH range (Figure
D.2), pH is not an important factor for the corrosivity of natural waters.
Figure D.2. Solubility of alumina in water as a function of pH.
The dissolution rate of alumina depends on the pH value, as shown in Figure D.1. It is higher at
acidic and alkaline pH values, which reflects the amphoteric properties of aluminum oxide.
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APPLICATION NOTE # D1
However, the pH value is not the sole parameter to be considered when predicting the stability of
the natural oxide film in aqueous media, and therefore, of aluminum itself: at acidic or alkaline pH,
the dissolution rate of aluminum also depends on the nature of the acid or of the base dissolved in
water, as shown in Figure D.2.
Figure D1. Dissolution rate of alumina in aqueous media
as a function of pH.
Figure D.2. Influence of the nature acids and bases on the
corrosion of 1100 H14.
Natural waters, both surface waters (river water, spring water) and seawater, generally have a pH
close to neutral. Municipal waters mostly have a pH between 6.5 and 7.5. Distilled water is slightly
more acidic, with a pH ranging from 6 to 6.5.
The corrosion resistance of aluminum depends on the stability of the natural oxide film that covers
the metal. This film is most stable in the pH range of 6.5–7.5 (Figure D.1), which is why aluminum
resists natural aquatic environments well, including weathering. If this film is damaged or
destroyed, for example in contact with certain organic acids at their boiling point, the underlying
aluminum may be attacked.
On the other hand, in this pH range, at room temperature and up to about 80 °C, aluminum is
prone to pitting corrosion.
This is the common form of corrosion in contact with water
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APPLICATION NOTE # D1
Influence of ions solved in fresh water on aluminum
Among all anions, chloride ions have the highest power of penetration into natural oxide film,
because they are small and very mobile. Chlorides (Cl  ), as well as fluorides (F  ), bromides (Br  )
and iodides (I  ) belong to the anions that activate corrosion of aluminum in water, while sulfates
(SO 24 ), nitrates (NO 3 ) and phosphates (PO 34 ) hardly activate it or do not activate it at all.
Chlorides may substitute oxygen atoms in the alumina network. This leads to a decrease in the
film’s resistivity, which facilitates the release of aluminum atoms that diffuse into the water .
Influence of :




sulfate concentration. In water, sulfates are associated with calcium, magnesium and
sodium. Being bulkier and less mobile than chlorides, they penetrate less easily into the
oxide film.
calcium concentration. Calcium, as carbonate or bicarbonate, has no influence on the
corrosion resistance of aluminum in water, even at concentrations as high as 500 mg·l 1 of
calcium carbonate (CaCO3), or even higher.
carbonate concentration. The so-called hard waters are not more aggressive towards
aluminum and aluminum alloys than waters having a low concentration of carbonates or
bicarbonates.
Influence of freshwater treatment. Chlorination is the most common method for biological
purification of water’s biological contamination. This addition of chloride is generally very
low compared to chlorides that are naturally contained in water. Aluminum resists water
and the chlorine environment of swimming pools very well. When unprotected, it will
develop the usual water staining, and when anodised at 20 mm, neither pitting corrosion
nor water staining will develop for several years
Other metals can be dissolved in water; they can be of natural origin, such as in the case of ironbearing water, or originate from the corrosion of other metals present in the circuit or from
discharge of industrial wastewater.
Among common metals that can be found in this way, a distinction can be made between:
– those that attack aluminum by a reduction according to the process described above:



copper,
mercury,
lead;
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APPLICATION NOTE # D1
– those that have no effect:


manganese,
cobalt;
– those that form a film on aluminum without attack:



iron,
chromium,
zinc.
Influence of temperature
Several temperature domains corresponding to different forms of corrosion need to be
distinguished. Their limits are not defined strictly, but may depend on the composition and the
nature of the water.
The corrosion resistance of aluminum in water depends on the reaction of the natural oxide film
with water.
The limits of the temperature ranges may vary significantly, by several tens of degrees, depending
on the composition of aluminum and water.
In freshwater up to 60/70 °C, the dominating corrosion tendency is pitting. At higher
temperatures, the pitting depth sharply decreases, while the density may increase in certain cases
(Figure D.7). Above 70 °C and up to 150 °C, the propensity of pitting corrosion progressively
disappears.
The thickness of these oxide layers increases with temperature and with time.
Table D.1
Temperature domain
<100 °C
100–150 °C
150–250 °C
>250 °C
Forms of corrosion
Pitting corrosion (in freshwater,the tendency to pitting corrosion decreases above 60–70 °C)
General corrosion
General corrosion and intercrystalline corrosion
Intercrystalline corrosion with destruction of the metal
Globally, aluminum’s corrosion resistance in freshwater is better at a temperature between 70
and 100 °C than at room temperature. Intercrystalline corrosion is accompanied by a modification
of the barrier layer structure that tends to crystallize.
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APPLICATION NOTE # D1
Figure D.7. Influence of the water temperature on corrosion.
In distilled water, freshwater or seawater, above 60–70 °C, the formation of more or less colored
layers with a structure similar to that of boehmite is observed.
The thickness of these oxide layers increases with temperature and with time.
They can reach several micrometers at 100 °C in distilled water. These layers protect aluminum
against corrosion in water, including seawater.
The increase in temperature has several effects:
– the solubility of dissolved gases decreases,
– the solubility of carbonates and sulfates (with calcium and magnesium) decreases.
Globally, aluminum’s corrosion resistance in freshwater is better at a temperature between 70
and 100 °C than at room temperature (Figure D.8).
Pitting corrosion is also observed. Above 250 °C, a sharp increase in corrosion is found: the kinetics
changes to a linear law (Figure D.9). A rapid destruction of the metal is observed.
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APPLICATION NOTE # D1
Figure D.8. Corrosion rate in water at 165°C.
Figure D.9. Corrosion rate in water at 363°C.
The influence of water movement and flow speed
Based on experience, it is known that the corrosion resistance of aluminum in moving water is
always better than in stagnant water, if all the other parameters are kept constant. Tests over one
week in freshwater at 20 °C have shown that the density and depth of pitting decreases with
increasing flow speed (Table D.2) . Water movement regularly eliminates corrosion products and
makes uniform the cathodic and anodic zones by removing a possible local excess of H+ and OH 
ions.
In an open circuit, moving water is aerated, and oxygen uptake contributes to repairing the oxide
layer.
In a closed circuit, the movement of the liquid prevents the formation of deposits under which
corrosion can easily develop. Aluminum can withstand a water flow speed up to 2.5–3 m·s 1
without any risk of erosion.
Pits are often disseminated, of large diameter (1–5 mm), covered with voluminous white plaques
of alumina gel, and sometimes with a deposit of hard, light yellow scale made up of carbonates.
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APPLICATION NOTE # D1
Table D.2. Influence of the flow speed of water on pitting
244
Maximum
220
Depth (mm)
Minimum
100
Average
148
145
26
58
25
15
50
0
0
150
100
140
80
60
60
0
0
80
50
60
40
20
10
0
0
107
79
90
50
35
29
0
0
Speed (m·min-1)
Mean density
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
3
Figure D.10. Aspect of pitting in a tank with stagnant water.
Considering the parameters affecting the corrosion of aluminium, the firm has chosen the best
solution available so as to produce devices guarantee for a life of tenths of years under the
following operating conditions:




Usually there is some corrosion inhibitor (eg. antifrogen)
Temperature range [40°-90°C]
Water speed higher 0.6 m/s
Chemical properties under control (no salt dissolved)
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than which it is disclosed. Complete or partial reproductions are forbidden without written permission from the owner
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APPLICATION NOTE # D1
REFERENCES
[1] Christian Vargel, CORROSION OF ALUMINIUM, Consulting Engineer, Member of the Commission of Experts within
the International Chamber of Commerce, Paris, France http://www.corrosion-aluminium.com , 2004 Elsevier.
[2] Seligman R., Williams P., The action of aluminium of hard industrial waters, Journal of the Institute of Metals, vol.
23, 1920, p. 159–184.
[3] Britton S.C., Evans U.R., Journal of the Chemical Society, 1930, p. 1773.
[4] Heine M.A., Keir D.S., Pryor M.J., The specific effects of chloride and sulfate ions on oxide covered aluminium,
Journal of the Electrochemical Society, vol. 112, 1965, p. 24–32.
[5] Godard H.P., The corrosion behavior of aluminium in natural waters, The Canadian Journal of Chemical
Engineering, vol. 38, 1960, p. 167–173. June 1956.
[6] Sawyer D.W., Brown R.H., Resistance of aluminium to fresh water, Corrosion, vol. 3, 1947, p. 443–457.
[7] Bourbon R., Adenis D., Moriceau J., Relation entre le noircissement a` l’eau des alliages d’aluminium et la structure
des couches d’alumine, rapport Pechiney CRV, avril 1966.
[8] Britton S.C., Evans U.R., Journal of the Chemical Society, 1930, p. 1773.
[9] Dacres C.M., An investigation of the influence of various environmental factors upon the aqueous corrosion of
aluminum alloys, American University, Washington DC, PhD, 1977.
[10] Bell W.A., Effect of calcium carbonate on corrosion of aluminium in waters containing chloride and copper,
Journal of Applied Chemistry, vol. 12, 1962, p. 53–55.
[11] Arnold G.G., The effect of swimming pool atmospheres on aluminium, Anti-Corrosion Materials & Methods, vol.
19, 1972, p. 5–9.
[12] Davies D., Pitting of aluminium in synthetic waters, Journal of Applied Chemistry, vol. 9, 1959, p. 651–659.
[13] Bryan J.M., Action of boiling distilled water on aluminium, Journal of Society Chemical Industry, vol. 69, 1950, p.
169–173.
[14] Herenguel J., Lelong P., Les me´canismes d’attaque de l’aluminium de haute purete´ par l’eau a` tempe´rature
e´leve´e, Revue de l’Aluminium, vol. 35, 1958, p. 991–998.
[15] Dillon R.L., Observations on the mechanisms and kinetics of aqueous aluminium corrosion. Part II. Kinetics of
aqueous aluminium corrosion, Corrosion, vol. 13, 1957, p. 13t–16t.
[16] Wright T.E., Godard H.P., Laboratory studies on the pitting of aluminium in aggressive waters, Corrosion, vol. 10,
1954, p. 195–198.
[17] Jakson E.W., Aluminium vs corrosion by water, Chemical & Process Engineering, vol. 38, 1957, p. 391–393.
[18] Binger W.W., Marstiller C.M., Aluminium alloys for handling high purity water, Corrosion, vol. 13, 1957, p. 591t–
596t.
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than which it is disclosed. Complete or partial reproductions are forbidden without written permission from the owner
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