1.Nature of the metals:
Metals with higher electrode potentials do not corrode easily. They are noble metals like, gold , platinum, silver.Whereas metals with lower electrode potentials , readily undergo corrosion.e.g. metals like, zinc, magnesium, aluminium.When two metals are in contact with each other,higher the difference in electrode potentials greater is the corrosion.For eg., the potential difference between iron and copper is 0.78V which is more than that between iron and tin (0.3V). Therefore, iron corrodes faster when in contact with copper than that with tin. On this account, the use of dissimilar metals should be avoided wherever possible. For example, bolt and nut or screw and washer should made of the same metal (or alloy) to avoid galvanic corrosion.
2.Ratio of cathodic to anodic region:
The rate of corrosion is influenced by relative size of cathodic to anodic area. If the metal has small anode and large cathodic region, corrosion rate is very high. As the ratio decreases corrosion rate further increases. This is because at anode electrons are liberated. Which are consumed at cathodic region. If cathodic region is larger the liberated electrons are rapidly consumed at cathode. This futher enhances, the anodic raction leading to increase in overall rate of corrosion. When two dissimilar metals are in contact, Corrosion is more rapid and severe, if the anodic area is small and cathodic area is large (e.g., a small steel pipe fitted in a large copper tank), If during plating of tin on iron, some areas are not covered or some pin holes are left, there results a small anode and large cathode . An intense localized corrosion occurs at the exposed small anodic surface. On the other hand, zinc plating on iron gives an anodic coating to iron since zinc is above Fe in the electrochemical series. Even if zinc plating peels off at some points intense corrosion of iron would not occur. This is because of formation of large anodic and small cathodic areas .
3.Nature of the corrosion product:
The corrosion product formed like metal oxide may act as protective film, if it is stable, insoluble, and non porous. If it acts as protective film it prevents further corrosion by acting as barrier between metal surface and corrosion medium. On the other hand if corrosion product is unstable, porous, and soluble, it further enhances corrosion.
Eg. in oxidizing environments, metals like aluminium, chromium, titanium, etc., are highly passive as their oxides as corrosion products form protective films on the metal surface, preventing further corrosion. Metals such as iron, zinc, magnesium etc., do not form any protective film and are highly susceptible for continuous corrosion, when exposed to oxidizing environments.
4. pH of the medium:IN general rate of corrosion is higher in acidic pH tha in neutral and alkaline pH.In case of iron, at very high pH protective coating of iron oxide is formed which prevents corrosion. Where as at low pH severe corrosion takes place. But for metals like Aluminium, corrosion rate is high even at high pH.
Rate of corrosion increases with increase in temperature. This due to the increase in conductance of the medium with increase in temperature and hence an increase in the diffusion rate. As a consequence, corrosion progresses faster at higher temperatures. In some cases, rise in temperature decreases passivity, which again leads to an increase in the corrosion rate.
6 Effect of oxygen:
Oxygen plays an important corroding medium. If the metal is exposed to different concentration of oxygen, differential aeration corrosion takes place. The region which is less exposed to oxygen acts as anode hence undergoes corrosion and region exposed to high oxygen concentration acts cathode hence doesn’t undergo corrosion.
7) Hydrogen over voltage:
The metal with lower hydrogen over voltage on its surface is more succeptible for corrosion, when cathodic reaction is hydrogen evolution type. Since at lower hydrogen over voltage, liberation of hydrogen gas is easy. Therefore cathodic reaction is very fast, which inturn makes anodic reaction very fast. Thus increasing rate of corrosion. Higher the over voltage lesser is the corrosion.
|Guest Author: Polisetti Ramakrisha 28 Mar 2015|