Corrosion in ballast tanks

Corrosion in Ballast Tanks is the deterioration process where the surface of a ballast tank progresses from microblistering, to electroendosmotic blistering, and finally to cracking of the tank steel itself.

Throughout the years the merchant fleet has become increasingly aware of the importance of avoiding corrosion in ballast tanks.

Epoxy and modified epoxy are standard coatings used to provide protective barriers to corrosion in ballast tanks. Exposed, unprotected steel will corrode much more rapidly than steel covered with this protective layer. Many ships also use sacrificial anodes or an impressed current for additional protection. Empty ballast tanks will corrode faster than areas fully immersed due to the thin - and electo conducting - moisture film covering them.

The main factors influencing the rate of corrosion are diffusion, temperature, Conductivity, type of ions, pH, and electrochemical corrosion potential.

Ballast tanks do not corrode uniformly throughout the tank. Each region behaves distinctively, according to it electrochemical loading. The differences can especially be seen in empty ballast tanks. The upper sections usually corrode but the lower sections will blister.

A ballast tank has three distinct sections: 1) upper, 2) mid or "boottop" area and, 3) the "double bottom" or lower wing sections. The upper regions are constantly affected by weather. This area experiences a high degree of thermal cycling and mechanical damage through vibration. This area tends to undergo anodic oxidation more rapidly than other sections and will weaken more rapidly. This ullage or headspace area contains more oxygen and thus speeds atmospheric corrosion, as evidenced by the appearance of rust scales.

In the midsection corrodes more slowly than upper or the bottom sections of the tank.

Double bottoms are prone to cathodic blistering. Temperatures in this area are much lower due to the cooling of the sea. If this extremely cathodic region is placed close to an anodic source (e.g. a corroding ballast pipe), cathodic blistering may occur especially where the epoxy coating is relatively new. Mud retained in ballast water can lead to microbial corrosion.

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Wikipedia