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Corrosion could be described as the destructive attack of a metal either by chemical or electrochemical reaction with its environment. Deterioration which occurs as a result of physical causes is not called corrosion, but could be best described as erosion, galling, or wear. In some instances, chemical attack accompanies physical deterioration, as described by the following terms: corrosion-erosion, corrosive wear, or fretting corrosion. Non-metals are not included in this definition of corrosion. Plastics may swell or crack, wood may split or decay, granite may erode, and Portland cement may leach away, but the term corrosion is restricted to chemical attack of metals.
"Rusting" applies to the corrosion of iron or iron-base alloys with formation of corrosion products consisting largely of hydrous ferric oxides. Nonferrous metals therefore corrode but they do not rust. (Ref 1)
Corrosion as described in the first paragraph is most likely to occur or occurs in metals. With a few exceptions, metals are unstable in ordinary aqueous environments. Metals are usually extracted from ores through the applications of a considerable amount of energy.
The corrosion resistance of metals and alloys is a basic property that can be related to the ease with which these materials react within an environment.
The end result of corrosion involves a metal atom being oxidized where by it loses one or more electrons and leaves the bulk metal.
Corrosion of metals normally occurs or could occur as a result of this materials been exposed or been in contact with water (and moisture in the air), acids, bases, salts, oils, aggressive metal polishes and other solid liquid chemicals.
In the case of heavy goods vehicles corrosion could occur as a result of different factors and examples of this could be the materials used in the design of this vehicles, nature of use of the vehicle, operating conditions, micro structural changes, fatigue initiation, thermodynamic and environmental issues.
AIMS & OBJECTIVES
There are a few factors which will have to be investigated before being able to identify a cost effective corrosion protection for this heavy goods vehicles. Below are some areas that would be looked into in this project:
Understanding corrosion and different forms of corrosion
Identifying different types of materials that are affected
Understanding corrosion and oxidation chemically
How can corrosion be measured physically
How can we experiment with corrosion
Current experimental procedures or methods set in place
Typical type of rust proofing this heavy goods vehicles could get
What is the process of rust proofing already in place
How are these methods typically tested
ANALYSIS AND REVIEW OF THE PROBLEM
Rusting of iron consists of the formation of hydrated oxide, Fe (OH)3, FeO (OH), or even Fe2 O3 H20. It is an electrochemical process which requires the presence of water, oxygen and an electrolyte. In the absence of this substances, rusting does not occur to any significant extent. In air, a relative humidity of over 50% provides the necessary amount of water and at 80% or above corrosion of bare steel is worse.
When a droplet of water containing a little dissolved oxygen falls on a steel, the solid iron on Fe(s) under the droplet oxidizes:
Fe(s) â†’ Fe 2+ (aq) + 2e-
The electrons are quickly consumed by hydrogen ions from water (H20) and dissolved oxygen or O2 (aq) at the edge of the droplet to produce water:
4e- + 4H+ (aq) + O2 (aq) â†’ 2H2O (l)
More acidic water increases corrosion. If the PH is very low the hydrogen ions will consume the electrons anyway, making hydrogen gas instead of water:
2H+ (aq) + 2e- â†’ H2 (g)
Hydrogen ions are being consumed by the process. As the iron corrodes, the PH in the droplet rises. Hydroxide ions (OH-) appear in water as the hydrogen ion concentration falls. They react with the iron (II) ions to produce insoluble iron (II) hydroxides or green rust:
Fe2+ (aq) + 2OH- (aq) â†’ Fe (OH) 2 (s)
The iron (II) also reacts with hydrogen ions and oxygen to produce iron (III) ions:
4Fe 2+ (aq) + 4H+ (aq) + O2 (aq) â†’4Fe3+(aq) + 2H2O (l)
The iron (III) ions also react with hydroxide ions to produce hydrated iron (III) oxides (also known as iron (III) hydroxides):
Fe3+ (aq) + 3OH- (aq) â†’ Fe (OH)3 (s)
The loose porous rust or Fe (OH)3 can slowly transform into a crystallized form written as Fe2 O3 H2O the familiar red-brown stuff that is called "rust" forming tubercles. Since these processes involve hydrogen ions or hydroxides ions, they will be affected by changes in PH.
The formation of rust can occur at some distance away from the actual pitting or erosion of iron as illustrated below. This is possible because the electrons produced via the initial oxidation of iron can be conducted through the metal and the iron ions can diffuse through the water layer to another point on the metal surface where oxygen is available. This process results in an electrochemical cell in which iron serves as the anode, oxygen gas as the cathode, and the aqueous solution of ions serving as a "salt bridge" as shown below.
The involvement of water accounts for the fact that rusting occurs much more rapidly in moist conditions as compared to a dry environment such as a desert. So there as mentioned earlier the operating condition or environment where this vehicles are been operated do affect their resistance to corrosion. Many other factors affect the rate of corrosion. For example the presence of salt greatly enhances the rusting of metals. This is due to the fact that the dissolved salt increases the conductivity of the aqueous solution formed at the surface of the metal and enhances the rate of electrochemical corrosion. This is one reason why iron or steel tend to corrode much more quickly when exposed to salt (such as that used to melt snow or ice on roads).
SOLUTIONS ALREADY IN PLACE
Recommended corrosion test methods for commercial vehicles components offers procedures to validate acceptable corrosion performance of metallic parts and assemblies used in medium trucks, heavy trucks, buses and trailers. Test methods incorporate recurring conditions such as corrosive chemicals, drying, humidity and abrasive exposure, as well as variations in the environment.
Currently vehicle corrosion standards do not provide suitable coverage for new de - icing chemicals and application methods, nor do they adequately account for aggravating effects such as temperature extremes, abrasion and mechanical stress.
Aluminium as well as its alloys, has been extensively used in industry because of its particular properties such as low density, good appearance and corrosion resistance. However, experiences demonstrate that Al and its alloys are susceptible to pitting corrosion when they are exposed to an aggressive environment such as chloride media. Nanocrystallization or microcrystallization is in favour of the enhancement of pitting resistance of metals or alloys. This is significant for preventing stress corrosion cracks because the corrosion pits might be the most common sites for crack initiation. However, the fundamental understanding about effect of the fine grain size on the initiation and growth of pitting corrosion is still limited.
Aluminium alloys when exposed to chloride media shows that a large numbers of pits are formed. However, the vast majority of these pits which nucleate do not propagate indefinitely, but repassivate after a very short period of growth. These pits are described as metastable pits. The other pits, however, continue growing for a longer time to form stable pits with greater depth. These stable pits are fatal when considering the effects of corrosion damage. Metastable pits and stable pits are thought to be initiated in the same manner, and stable pits are considered a subset of metastable pits. In order to predict when stable pits will develop, it is important to determine when metastable pits occur, and subsequently the likelihood of growing into stable pits from metastable pits. The study of metastable pits is therefore an important issue in the prediction of the development of pitting corrosion. It provides unique opportunities for understanding pitting corrosion. Especially, the repassivation of metastable pits provides information on the stability criteria for pit growth.
Pure aluminium usually experiences low corrosion rates as its surface is covered with a protective oxide scale. But the flaws in the oxide scale are unable to be repaired immediately when exposed to a chloride media. These "pit promoting" ions interfere with the oxide formation chemicals preventing local repassivation of the metal. Then the pits at these locations will develop into stable pits. Unlike stable pits, metastable pits only grow to a very small size (less than 10 Î¼m in diameter) at which stage their growth is arrested. The only possible reason for these pits to cease growing in light of the reactivity of the metal is that the protective oxide scale is regenerated.
The figure below shows TEM images of the sputtered Al film. Fig 1a indicates that the sputtered Al consists of equiaxed crystals. The grain sized was approximately 400nm. Moreover, the fine grain size is further demonstrated by the almost continuous ring character of the electron diffraction pattern in Fig 1b.
Fig. 1 (a) TEM image and (b) electron diffraction pattern of microcrystalline aluminium coating.