Ballast Tank Repair Assignment Example | Topics and Well Written Essays - 1750 words

Ballast Tank Repair - Assignment Example Mild steel by nature offers poor resistance to corrosion because of its inherent heterogeneous impurities, which facilitate emergence of anodic and cathodic nodes on the surface of the steel. Oxygen and water react with these nodes vigorously causing the metal to lose its electrons to form metallic ions, represented by the anodic reaction: 4 Fe ? 4 Fe++ + 8 e? Iron Ferrous Electrons Atom Ion The above electrochemical corrosion process is feasible only if there is a suitable electron acceptor to combine with the electrons released by the iron atom. Seawater containing dissolved atmospheric oxygen readily serves this purpose. The oxygen is electrochemically reduced to hydroxyl ions in the following cathodic reaction: 2 O2 + 4 H2O + 8 e? ? 8 OH? Oxygen Water Electrons Hydroxyl Ions The heterogeneous character of steel allows for some of its sites to favour the anodic reaction and for others to the cathodic reaction. The ferrous ions and hydroxyl ions formed combine together to produce f errous hydroxide: 4 Fe++ + 8 OH? ? 4Fe (OH) 2 Ferrous Hydroxide The ferrous hydroxide formed reacts with more oxygen to form hydrated ferric oxide, the familiar reddish brown rust – the telltale symptom of corrosion. ... oxygen concentration, water temperature, pH and the presence of metals in the form of dissolved salts such as oxides, chlorides, carbonates, sulphates and sulphites in the mineral state of their stable oxidised condition. In view of the large amount of energy expended for the extraction of a metal by the reduction process, there is a sustained pressure on the metal to revert back to its stable low energy oxidised state in the given environmental conditions. The driving force for corrosion is the energy differential between the pure metal and its oxidised forms in the scenario of ever varying ambient conditions. Pitting Corrosion Penetrative localized attack resulting in formation of deep crevices causing thinning of the parent steel material and revealed by rust is characteristic of Pitting Corrosion. It takes place more often in submerged bottoms, inaccessible edges and corners, and at high temperature locations like in the hull of ships where diffusion is easy. The locations most s usceptible to corrosion in water ballast tanks are: upper surface of face plate of bottom longitudinals, bottom girders and bottom transverses; upper surface of shell and bulkhead longitudinals; Cut edges of slots and lightening holes in horizontal girders; upper surface of horizontal stiffeners and brackets; deck longitudinals; upper part of deck transverses; and, upper part of longitudinal and transverse bulkheads. If left unattended, rust can grow at the rate of 0.22 mm/year, though the assessed average rate is 0.005 mm/year. ASTM D 610 Standard, supported by ISO 4628: 2003 Standard provides further insight into the mechanics and assessment of the degree of rusting. Corrosion Prevention The popular prevention techniques in modern use have been logically derived from the manifestations of