Corrosion of Aluminum

Alloys

• Aluminum is one of the most prevalent metallic elements in the solid portion of the earth’s crust, comprising approximately 8%.

• It is always present in a combined form , usually a hydrated oxide, of which bauxite is the primary ore.

• Metallic aluminum is very thermodynamically active and seeks to return to the natural oxidized state through the process of corrosion.

• Aluminum is second to iron in terms of production and consumption and is the most important metal of commerce in the United States.

• As the resources of other metals are depleted further , the relative position of aluminum will increase.

• Aluminum can be extracted from many resources.• Even though such extraction may be more

expensive than from bauxite, it will still be economically feasible. The increased cost of extraction will be

• offset substantially by increased recycling of aluminum, which saves95% of the energy required for extraction, and by any new smelting processes that Require less energy.

• In addition to being low in cost and having a high strength-to-weight ratio, they are readily fabricated and joined by most of the commonly used methods.

• The aluminum alloys possess a high resistance to corrosion by most atmospheres and waters, many chemicals, and other materials.

• Their salts are nontoxic—allowing applications with beverages , foods , and pharmaceuticals; they are also white or colorless—permitting applications with chemical and other materials without discoloration—

• Other desirable properties of aluminum and its alloys include a high electrical conductivity , high thermal conductivity , high reflectivity , and non catalytic action.

• They are also nonmagnetic.

• Classifications and Designations• Wrought aluminum and aluminum alloys are

classified based on their major alloying element via a four-digit numbering system, as shown in Table19.1.

• These alloy numbers and their respective tempers are covered by the American National Standards Institute(ANSI) standard H35.1.

• In the 1XXX group, the second digit indicates the purity of the aluminum used to manufacture this particular grade.

• The zero in the10XX grade indicates that the aluminum is essentially of commercial purity, while second digits of 1–9 indicate special control of one or more individual impurity elements.

• In the 2XXX–7XXX alloy groups , the second digit indicates an alloy modification.

• If the second digit is zero , the alloy is the original alloy , while numbers 1–9 are assigned consecutively as the original alloy becomes modified.

• The last two digits serve only to identify the different alloys in the group and have no numerical significance.

• THE ELECTROCHEMICAL BASIS OF METAL CORROSION

• Corrosion of metals is caused by the electrochemical reaction between a metal(or an alloy) and an aqueous phase.

• It proceeds according to a complex electrochemical process that is related to the atomic structure of matter.

• Matter is built up from elementary particles that carry electrical charges, namely ions and electrons, and from particles that are electrically neutral, namely atoms and molecules.

• In metals, the electrical environment of atoms is made up of free electrons capable of moving throughout the metal.

• Types of Corrosion on Aluminum• Different types of corrosion , more or less visible

to the naked eye , can occur on aluminum, such as uniform (generalized) corrosion, pitting corrosion, stress corrosion, etc.

• The predominant type of corrosion will depend on a certain number of factors that are intrinsic to the metal, the medium and the conditions of use.

• There is no form of corrosion that is specific to aluminum and its alloys.

• UNIFORM CORROSION• This type of corrosion develops as pits of very small

diameter , in the order of a micrometer, and results in a uniform and continuous decrease in thickness over the entire surface area of the metal.

• With aluminum, this type of corrosion is observed especially in highly acidic or alkaline media , in which the solubility of the natural oxide film is high(FigureB.1.18).

• The dissolution rate of the film is greater than its rate of formation; however, the ratio of both rates can change over time.

• As an example, in sodium hydroxide solutions the dissolution rate has been found to be lower for long term exposure , in the order of 40 or 80 days , than for tests over 20 days (Figure B.2.1).

• PITTING CORROSION• This localised form of corrosion is characterised 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, the medium and service conditions.

• Aluminium is prone to pitting corrosion in media with a pH close to neutral, which basically covers all natural environments such as surface water, seawater, and moist air.

• TRANSGRANULAR AND INTERGRANULAR (INTERCRYSTALLINE) CORROSION

• Within the metal, at the level of the grain, corrosion may propagate in two different ways:

• It spreads in all directions corrosion indifferently affects all the metallurgical constituents ; there is no selective corrosion .

• This is called transgranular or transcrystalline corrosion because it propagates within the grains.

• It follow preferential paths (FigureB.2.8):• Corrosion propagatesatgra in boundaries.• Unlike transgranular corrosion, this form of

intercrystalline corrosion consumesonly a very small amount of metal, which is why mass loss is not a significant parameter for assessment of this type of corrosion.

• It is not detectable with the naked eye but requires microscopic observation, typically at a magnification of 50.

• When penetrating into the bulk of the metal, intercystalline corrosion may lead to a reduction of mechanical properties , especially of elongation , and may even lead to the rupture of components.

• Filiform corrosion• always starts at coating defects, such as

scratches, and weak points:• beards, cut edges or holes. It can be seen after

several years of service.

• CREVICE CORROSION• Crevice corrosion is a localised corrosion in

recesses: overlapping zones for riveting, bolting or welding , zones under joints , and under various deposits (sand , slag , precipitates, These zones , also called crevices , are very tiny and difficult to access for the aqueous liquid that is covering the rest ofthe readily accessibl surfaces.

• This type of corrosion is also known as deposit attack.

• CAVITATION• Cavitation occurs when the hydrodynamic pressure

exceeds the vapour pressure of a moving liquid. • Gas bubbles form within the liquid, which thus

becomes a two-phase system. • These bubbles will be crushed against the metal

surface at high speed, an attack that leads to cavities with rounded contours.

• This degradation is caused by the combination of a mechanical effect and corrosion of the metal.

• The natural oxide film is destroyed and the aluminium is attacked; there is a competition between tearing off the film and reforming it.

• It is not possible to estimate the individual contributions of the mechanical effect and corrosion.

• EROSION• Corrosion by erosion occurs in moving media. This

type of corrosion is related to the flow speed of the fluid.

• It leads to local thinning of the metal, which results in scratches, gullies, and undulations , which are always oriented in the same direction , namely the flow direction.

• Avoiding erosion corrosion on aluminium does not require any specific precautions, only those commonly applied to other metals.