Corrosion Resistance of Continuously Anodised Aluminium

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PRODUCT CHARACTERISATION Part I 1.611/2005

Corrosion Resistance of Continuously Anodised Aluminium

Abstract

An untreated aluminium surface will spontaneously react with water or moist air to form

an aluminium oxide layer. This oxide layer acts as a protective barrier, preventing furtheroxidation. However, in unfavourable environments, this natural oxide layer may quicklydeteriorate and different forms of local corrosion attacks are then possible.

Anodising replaces the thin natural oxide layer by a structured, highly resistant anodiclayer, which provides an excellent protection against these local corrosion attacks.

Through accelerated corrosion tests (Salt Spray and Sulphur Dioxide Atmosphere) theperformance of continuously anodised aluminium in corrosive environments is assessed.

The test results show excellent resistance against corrosion of continuously anodisedaluminium in both types of aggressive atmospheres. There were no visible signs ofpreferential corrosion on the bent, deformed or machined parts. None of the exposedsamples show filiform corrosion.

It is extremely difficult to correlate results of accelerated tests in simulated environmentswith the actual performance in real life situations.Therefore, an independent company was commissioned to carry out a detailed field studyof results in situ on buildings constructed over the past 25 years.

The results of this study confirm the excellent resistance against corrosion of class 15 mcontinuously anodised aluminium in real life, as was already indicated by theaccelerated laboratory tests. In line with this, none of the inspected cassette systemsshowed any preferential corrosion forming on the edges.

Topics Covered

A Basic Understanding of Corrosion

Summary of Accelerated Corrosion Test Results

Details of Accelerated Corrosion Test Procedures


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1. A Basic Understanding of Corrosion

Corrosion is the transformation of a metal through a chemical or electrochemical reaction,starting at its surface.

Although all metals have a tendency to be oxidized, some will oxidize more easily than others.

Metals can be ranked in their relative tendency to oxidation. A metals relative position isdefined in the so-called galvanic series

When in contact with an electrolyte like water or moist air, an untreated aluminium surfacewill react spontaneously to form aluminium oxide. This natural, stable oxide layer is an

integral part of the aluminium surface, thus protecting it from further oxidation.

However, in acidic or alkaline environments, this natural oxide layer may deteriorate andinitiate local corrosion attacks.

For aluminium used in exterior architectural applications, aggressive elements such aschloride ions (Cl-), sulphates (SO4--) or others, may be potential causes of corrosiondepending on the local environment. For this reason, an effective lifetime protective surfacetreatment is essential for architectural applications.

There are specific types of corrosion possible:

Uniform corrosion

Pitting corrosion

Galvanic corrosion

Crevice corrosion Intergranular corrosion

Stress corrosion Filiform corrosion

Uniform corrosion: Here,the entire surface is uniformly attacked and the corrosion

progresses at a uniform rate. Aluminiums natural oxide layer will protect it from this form ofcorrosion, unless exposed to strongly acidic or alkaline environments.

Anodising will replace the thin natural, non-structural, and imperfect, oxide layer by a perfectlystructured, uniformly thicker anodic layer.

Pitting corrosion: This localized form of corrosion, which may occur following chemical ormechanical damage to the protective oxide film, produces tiny cavities (pits) in the metal.

Pitting corrosion is an electrochemical attack of the metal, creating an increasingly acidic localenvironment. This will result in the formation of a white powder (Aluminium Hydroxide) whichadheres to the surface of the cavity. Under most conditions, the speed of propagation of thecavity will decrease rapidly and finally stop.


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Both the chemical and the mechanical resistance of a thoroughy sealed anodic protection

layer will substantially retard this local corrosion form.The degree of protection is directly influenced by the layer thickness (cfr. Acceleratedcorrosion test results later in this text).

Galvanic corrosion: When two different metals or alloys are in direct contact or directly

connected with a conductive material in a humid or conductive environment, one metal maycorrode whilst the other remains unaffected.

It is important to avoid the creation of any galvanic coupling. For example, couplings between

aluminium and less electronegative metals (e.g. steel, copper, stainless steel, bronze,titanium, etc.) should be avoided, since they may cause corrosion of the aluminium. Similarly,couplings with more electronegative metals (e.g. zinc, magnesium, etc.), may cause corrosionof these metals.

This corrosion form is mainly caused by design errors, combining different metals withoutsufficient electrical insulation. Anodising as such is not an adequate protection againstgalvanic corrosion. In any case, other protective means are to be applied to avoid galvaniccorrosion, such as insulating spacers when combining aluminium to another metal, usage ofaluminium bolts when combining aluminium with aluminium, etc.

Crevice corrosion: This corrosion occurs in restricted areas such as pockets, crevices, etc.

where there is insufficient aeration of the aluminium surface.

Generally, crevice corrosion does not propagate due to the formation of aluminium oxidewhich fills the affected areas. However, this corrosion will create an undesirable visualappearance. This can be avoided during the design stage of components to eliminatecorrosion traps.

Crevice corrosion is due to inappropriate geometry, insufficient draining and poor ventilation.This can be avoided by good design of the structures.

Intergranular corrosion:

This structural corrosion is directly linked to the metal structure. Intergranular corrosionproceeds along the metal grain boundaries because there is a potential difference betweenthe grain boundaries and the grain core. Susceptibility to intergranular corrosion is mainly

dependent upon alloy composition and heat-treatment. It concerns practically only aluminiumalloys of harder structure like the 2000 and 7000 series.

The aluminium alloys, generally used for continuous anodising, are of a quality and

composition which is not vulnerable at this type of problem.

Stress corrosion:

Simultaneous occurrence of static tension and corrosion is called stress corrosion.


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This corrosion form is limited to alloys with high mechanical characteristics, such as 2000 and

7000-series or 5000 series with more than 4% Mg.

The aluminium alloys, generally used for continuous anodising, are not sensitive to stresscorrosion.

Filiform corrosion:Filiform corrosion is a specific corrosion form of painted metals.Filiform corrosion occurs under the paint layer and has the appearance of a thin threadlikeattack progressing along the surface of the metal. Filiform corrosion always starts from

defects in the coating, like scratches, holes, raw etches, etc.Filiform corrosion has a mainly aesthetic effect but can cause delamination of the paint layer.

Filiform corrosion does not exist for anodised material. The anodic layer is an integral part ofthe treated metal and thus cannot be delaminated. Even when the layer is damaged throughto the blank substrate by piercing, folding, carving, etc., filiform corrosion will never takeplace.

2. Summary of Accelerated Corrosion Test Results

There are two commonly used simulated tests of accelerated corrosion , which are approvedwith ISO specifications and used by our inspection centres. It should be remembered thatthese accelerated tests do not necessarily reflect real on-site performance.

i. Neutral Salt Spray Accelerated Test, designed to evaluate the corrosion

resistance of a metal in a NaCl (salty) environment. Chloride ions are thecorrosive ions of most practical importance, notably in marine atmospheres.These tests were according to ISO 9227 1000 hours and carried out by CRM(Centre de Recherches Mtallurgiques), in Lige, Belgium.

ii. Kesternich Sulphur Dioxide (SO2) Atmosphere Accelerated Test, designed

to evaluate the corrosion resistance of a metal in a sulphur dioxide (SO2)environment, which can be in certain cases representative of an industrialenvironment. These tests were according to ISO 6988 and carried out by CRM(Centre de Recherches Mtallurgiques), in Lige, Belgium.


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A detailed description of the tests is given below in paragraphs 4.i (Details of Accelerated

Corrosion Test Procedures - Neutral Salt Spray test) and 4.ii (Details of AcceleratedCorrosion Test Procedures - SO2 atmosphere - Kesternich test).

In summary, the results of these tests are as follows:

Neutral Salt Spray - 1000 Hours Accelerated Test EN ISO 9227

Flat Surface

(ISO 8993)(Chart

Method)

Flat Surface

(ISO 8994)(Grid

Method)

Substrateattack

Flat Surface 10 - 15m

Natural

No pitting No pitting None

Flat Surface 10 - 15mElectrolytic Colours

No pitting No pitting None

Flat SurfaceOrganic Colours No pitting No pitting None

PittingCorrosion

PreferentialCorrosion

FiliformCorrosion

Perforated All None None None

Hemispherical Draw All None None None

Conical Bend All None None None90 Bend All None None None

Kesternisch - 1000 Hours Accelerated Test- EN ISO 6988

Flat Surface(RA - ISO 10289)

SubstrateAttack

Flat Surface10 15 m

Natural

Index 9meaning

0% < area of defects = 0,25%None

Flat Surface 10 15 m

ElectrolyticColours

Index 9None

Flat SurfaceOrganic Colours

10 - 15 mIndex 9 None

In brief, the above tests indicate that a flat aluminium surface can be anodised

with 10-15 m in natural, electrolytic and organic colours and will demonstrate nosubstrate attacks on the metal after 1000 hours of Neutral Salt Spray or Kesternichtest.


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iii. Pre-anodised Building Faades Independent Inspection by Straditec Sprl ,The entire published study results can be found on our websitehttp://www/coil.be, or can be obtained from [email protected].

Laboratory tests provide useful indicators to the performance of a metal in corrosive

environments. It has, however, been proven extremely difficult to directly correlate the resultsof accelerated tests in simulated environments with the actual performance in real lifesituations. In real life the effects of the environment are accumulated over a long period oftime. For this reason, an independent company was commissioned to perform a detailed fieldstudy, pursuing the accumulation of results in situ on buildings constructed over the past 25

years. The observed overall tendencies and conclusions from these building inspections aresummarized below:

Durability of the anodic layer:None of the building faade claddings investigated showed any trace of erosion of the anodiclayer. Individual measurements of the anodic layer proved that the layer thicknessesremained unchanged when compared to their initial thicknesses, regardless of the number ofyears the cladding had been in place.

In all the buildings inspected, it was established there had been no deterioration of the anodiclayer due to any adhesion failures, blooming, peeling, blistering, cracking, etc.

The overall state of the faade:The pre-anodised aluminium clad faades aged exceptionally well, regardless of the longyears of service.

None of the clear (natural) anodised or bronze coloured surfaces had been subject to colourfading, colour shifting or yellowing.

Corrosion:

The inspected faades showed an excellent resistance against corrosion, for flat surfaces aswell as for bent edges.It is also interesting to note that none of the applied cassette systems showed any preferentialcorrosion forming on the edges.

The few isolated traces of corrosion occurred under the following unfavourable conditions:

Poorly ventilated construction elements in specific bays. (See 2. A Basic

Understanding of Corrosion: Crevice Corrosion) Design or construction faults like insufficient protection against galvanic corrosion or a

poor sealing of specific construction elements. (See 2. A Basic Understanding ofCorrosion: Galvanic Corrosion)

Specific micro-environments containing unusually high concentrations of aggressive,

polluting elements. (See 2. A Basic Understanding of Corrosion: Crevice Corrosion)


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3. Details of Accelerated Corrosion Test Procedures

i. Neutral Salt Spray test:

The Neutral Salt Spray test (NSS) evaluates a metals corrosion resistance in a NaCl (salty)

environment. Chloride ions are the corrosive ions of most practical importance, notably inmarine atmospheres.

For this test, continuously anodised samples of different metal gauges with varying anodiclayer thicknesses, colours and textures are taken from our COIL SA/NV production lines.

These samples are cut and deformed after anodising to simulate typical industrial postprocessing applications.

The applied deformations are: a perforation of 18 mm a hemispherical punch stretching (Erichsen) a conical bend with radius variable from (0,78 + T/2) to (9,8 + T/2), with T being the

metal gauge a right angle bend with curvature radius of 1 mm

In accordance with international standard ISO 9227 standard, which describes theaccelerated corrosion tests in detail, the prepared samples are subjected to the NSS for 1000hours.

Any resulting pitting corrosion is made visible by immersing the exposed samples into a 30%nitric acid solution for 5 to 10 minutes. After that, the samples are examined using twodifferent standardized rating systems for the evaluation of pitting corrosion on anodised

aluminium and aluminium alloys:

A. The Chart method (ISO 8993) compares the pitting density and sizes on the flatsurface with standard figures. The performance rating indicates the surfacepercentage impacted by pitting corrosion, and is expressed as a letter code (A = nopitting corrosion; B1-6 = 0,02%; H1 -6 > 0,5%).


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B. The Grid method (ISO 8994) applies a grid of 5 mm squares to the surface and

counts the number of squares which include at least one pit. This evaluation isperformed separately on the flat surface, the conical bend, the right angle band andon the hemispherical punch stretching.

Observations:

No pitting corrosion attack on any clear (Natural) and/or organic colour anodisedsamples was observed. (image A in accordance with ISO 8993 after 1000 hours saltspray test).

No preferential corrosion was observed on the bent, deformed or machined parts of

the samples.

None of the samples showed any traces of filiform corrosion.

Electrolytically coloured surfaces thinner than 15m were slightly attacked after 1000hours of the salt spray test (as the presence of tin metal in the electrolyticallycoloured surfaces makes them slightly less corrosion resistant). Also here, there areno more corrosion attacks in high deformation areas than in flat areas.

ii. SO2 atmosphere - Kesternich test:

The SO2 atmosphere - Kesternich test investigates a metals corrosion resistance in a sulphur

dioxide (SO2) environment, which can be in certain cases representative of an industrialenvironment. This rather tough accelerated corrosion test is described in EN ISO 6988.

For this test, continuously anodised samples of different metal gauges and with varyinganodic layer thicknesses, colours and textures are taken from our COIL SA/NV production

lines.


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The flat A4-size samples are positioned under an angle of 15 (vs. vertical) in a climate

chamber.

During this cyclic wetting and drying test, the samples are exposed for 8 hours to an SO2-atmosphere (100% RH at 40C) and for 16 hours to an ambient atmosphere (70% RH at23C). Thus, one complete test cycle lasts 24 hours.

The samples are tested during 42 cycles (~1000 hours).

After that, the corrosion behaviour is evaluated following the applicable ISO 10289 standard.This standard provides a rating method for the condition of metallic and other inorganiccoatings on metal subjected to corrosion tests.

In rating these coatings for the effects of corrosion, two evaluations are made:

A. A protection rating (RP) relating to the corrosion of the base metal, thus evaluating theability of the coating to protect the substrate from corrosion.It provides a numerical rating, based on the area of the substrate that showscorrosion defects (10 = no defects; 9 = 0% < area of defects = 0,25%; 0 =50% < area of defects).

B. An appearance rating (RA) relating to the deterioration of the coating, thus evaluatingthe ability of the coating to retain its integrity. The appearance rating, expressed as asequence of letters and numbers, is assessed in terms of the type of defect (letter

code) and the area affected by a particular defect (scale 10 to 0).

Observations:

Below 20 cycles, the majority of the samples didnt show any signs of pittingcorrosion.

Although anodic layer thicknesses are tested from 5m up to 15m, very littledifference is observed in corrosion effects.


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iii. Conclusions:

The accelerated corrosion tests show that continuously anodised aluminium providesexcellent corrosion protection. Salt spray tests show that even aggressive chloride ions (Cl-)contained in the salt spray have little impact on the anodic oxide.

Furthermore, there are no visible signs of preferential corrosion on the bent, deformed ormachined parts. None of the exposed samples shows filiform corrosion.

From comparisons of exposed samples with different anodic layer thickness (e.g. StainlessSteel look-alike 5m, 10m and 15m), the corrosion resistance increases with the increasinglayer thickness. Samples with thinner layers score marginally lower.

The presence of tin metal in the electrolytically coloured anodic layer explains why bronzes(with an anodic layer thickness less than 15m) tend to be slightly more vulnerable tocorrosion. However, there are no more corrosion attacks in high deformation areas than in flatareas and the anodic layer was never completely pierced. Therefore, the substrate metal itselfwas never affected.

All ofthe above results indicate that a well sealed anodic layer protects the aluminium verywell against corrosive attacks, even when the pre-anodised material is post-formed and/ormachined.

In the event that you require additional explanations or clarification of the information

contained in this paper, we shall be pleased to help you in any way possible. Please contactus at [email protected] .

Corrosion Resistance of Continuously Anodised Aluminium

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