英语论文网

he material because it's a strong acid) is consumed by the SOB and convert it to sulfuric acid (H2SO4). These strong acids along with the presence of water and steel form an electrolytic cell at surface of the steel. The presence of SOB and SRB insures that the process will proceed and rusting corrosion is then accomplished, it becomes worse when pitting is involved in the process.

This type of corrosion usually occurs in marine areas; it is recognized by the presence of fouling layer covering the surface, pitting occurrence and corrosion deposits accumulation. There are many factors that affects the ALWC process such as geometry of the steel surface: it was noticed that the type of cross section affects the location of ALWC, for example, Z sections get corroded at thinner layer while U sections at the outpan flange. Also, the orientation has a significant influence, adding to that the degree of stability of water (inversely proportional), sulfide concentrations and SOB and/or SRB presence.

There are many methods that can be used to protect materials from ALWC and increase their life cycle such as:

Coating: operate through three mechanisms: barrier, inhibitive and galvanic.

Cathodic protection: by making the structure a cathode and use another material as the anode and form an electrolytic cell to compensate for the lost material.

Wrapping: by covering the material with a cover to protect it from physical contact with water.

This accident took place in 1999 near the coasts of France which caused 19800 tons to be spilled and damaging the environment severely. The ALWC corrosion which appeared in the form of rust was the main reason behind this catastrophe, records showed that the ship was affected by very bad corrosion, these corrosions caused the leakage through pinholes, watertight doors didn't seal properly and many holes were discovered on the main body of the ship. It was noticed that the safety authorities didn't take this issue seriously though the ship has been struggling with this problem before five years of the accident.

缝隙腐蚀-Crevice Corrosion

Crevice (also Contact) corrosion is the type that is formed due to the connection between two parts to make a metal-metal or metal-nonmetal connections in still (motionless) environments at microscopic sales.

This type of corrosion usually occurs at sand grains, flanges, washers screws' pockets. This type of corrosion is initiated due to chemical changes within the surrounding environment in one of the following forms:

Consuming of the inhibitor material.

Depletion of Oxygen within the crack volume.

Creation of acidic condition in the surrounding environment.

Formation of material attacking ion species within the cavity.

The principle of Crevice corrosion is based on the concentration cell principal, one material becomes cathode and the other anode leading to consume the cathode material and accumulate it on the anode surface in a manner that dilutes the cathode material and makes it a weak and deteriorated. As an example; the oxygen depletion is the most famous form of this corrosion mechanism. Here, the moist air inside the crevice has lower oxygen content than air on the surface, making the material inside anode and the moisture film on the surface a cathode whi