{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

What is Corrosion - Coatings Technology What is Corrosion...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
Coatings Technology: What is Corrosion? CORROSION As the photograph clearly demonstrates, corrosion can be a major problem. In fact, it is reasonable to say that the single most important reason to paint a ship is to prevent corrosion. the metal at the metal/environment interface. This reaction subsequently results in a deterioration in the mechanical properties of the metal. For marine applications, mild steel remains the number one metal for constructional purposes by virtue of its relatively low cost, mechanical strength and ease of fabrication. Its main drawback is that is corrodes easily in seawater and unless adequately protected, rapidly loses strength which may result in structural failure. In general terms corrosion can be defined as the spontaneous reaction between a material and its environment which results in the degradation of that material. To be more specific, metallic corrosion is the oxidisation of Severe corrosion in a cargo hold The majority of metals in nature are found in their mineral state, i.e. in their stable oxidised condition as oxides, chlorides, carbonates, sulphates, sulphides, etc. The extraction of a metal from the appropriate mineral involves a reduction process in which a great deal of energy is absorbed. As a consequence of this large energy input the metal is in a high energy condition and will endeavour to return to its former stable, oxidised, low energy state as quickly as environmental conditions will allow. It is the energy difference between the pure metal and its oxidised forms which is the driving force for corrosion. Corrosion mechanism Mild steel is an impure, heterogeneous material. This is a major factor contributing to its poor resistance to corrosion. The presence of what are referred to as anodic and cathodic sites on the steel surface and their reaction with oxygen and water results in the transformation of a metal atom to a metal ion by the loss of electrons i.e. Anodic Reaction : 4Fe 4Fe ++ + 8e ¯ Iron Ferrous Electrons Atom Ion This process is referred to as electrochemical corrosion. The reaction can only occur if there is a suitable electron acceptor to combine with the electrons released by the iron atom. Seawater contains dissolved atmospheric oxygen which readily serves this purpose. The oxygen is electrochemically reduced to hydroxyl ions in the cathodic reaction. Cathodic Reaction: 2O 2 + 4H 2 O + 8e¯ 8OH¯ Oxygen Water Electrons Hydroxyl Ions Corrosion mechanism www.international-marine.com
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Coatings Technology: What is Corrosion? (The heterogeneous character of steel allows for some sites to favour the anodic reaction and for others to favour the cathodic reaction).
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.