Corrosion

Corrosion is a universal phenomenon and it exist everywhere. It is defined as the degradation or deterioration of materials due to the reaction with the environment. There are various forms of corrosion and specific countermeasures are necessary for each. One common type is the uniform corrosion. Uniform corrosion is a general type where corrosion occurs uniformly over the entire exposed area of a metal surface. A general thinning of the material forming rust will occur that eventually leads to failure if control steps are not taken. This type of corrosion represents the greatest amount of destruction on a tonnage basis but is also considered as a safe form of corrosion. This is because simple tests; thickness reduction rate and lifetime, can be done and therefore, corrosion is predictable. The common examples are the rusting of a sheet iron roof or steel nuts on a street lamp post. Common countermeasures include use of coatings to isolate the metal form the corrosive environment and cathodic protection; converting the anode to cathode using electric current.

Another popular type of corrosion is galvanic corrosion. Galvanic corrosion occurs when two dissimilar alloys are in contact with each other in the presence of corrosive electrolyte. This connection creates a path for electron flow which creates the driving force for corrosion process; potential difference between materials. Galvanic series can be used to measure the potential difference of materials thus, determining the nobility of the materials. In galvanic corrosion, the less noble or active metals will be the anode and corrosion is likely to occur while the materials at the upper end of the series are noble and act as cathode. The greater the distance between metals in the series, the greater the corrosion effect/rate will be.One example of galvanic corrosion is the ‘lasagna cell’ where lasagne (electrolyte) is stored in steel pan (cathode) and covered with aluminium foil (anode). After some time, when the foil touches the lasagna spots of corrosion can be seen on the foil. Significant countermeasures include use of metals that are closer to each other in the galvanic series, coatings on the more noble metal to prevent electron from being consumed and maintaining small area ratios of anode-cathode combination since smaller anode with respect to cathode corrodes faster.