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#The type of steel lost to time series#

In some circumstances the attack on the original anodic area is not stifled and continues deep into the metal, forming a corrosion pit.

Stainless Steels 430/304/316 (In the passive state).

In simple terms, the greater the A c/A a ratio, the greater the tendency for bimetallic corrosion. The tendency for bimetallic corrosion is also influenced by the relative surface areas of the cathodic and anodic metals (A c/A a). Alternatively the application of a suitable paint system over the assembled joint is also effective. For greater risk situations, gaskets, sleeves and similar electrically insulating materials should be used. No special precautions are required in most practical building or bridge situations. stainless steel brick support angles attached to mild steel structural sections, the effect on the steel sections is minimal.

Bimetallic corrosion is most serious for immersed or buried structures, but in less aggressive environments e.g.

#The type of steel lost to time series#

The further apart the two metals in the series the greater the tendency.Īnother aspect that influences bimetallic corrosion is the nature of the electrolyte. The tendency of dissimilar metals to bimetallic corrosion is partly dependent upon their respective positions in the galvanic series. zinc) corrode preferentially themselves, thereby protecting the low alloy structural steel. stainless steel) cause low alloy structural steel to corrode preferentially whereas other metals (e.g. When two dissimilar metals are joined together and in contact with an electrolyte, an electrical current passes between them and corrosion occurs on the anodic metal. Various types of localised corrosion can also occur but these tend not to be significant for structural steelwork. In the absence of either, corrosion does not occur. The corrosion process requires the simultaneous presence of water and oxygen. A schematic representation of the corrosion mechanism is shown (above right). In this case, over long periods, the loss of metal is reasonably uniform over the surface, and this is usually described as 'general corrosion'. New, reactive anodic sites may be formed thereby allowing further corrosion. However, after a period of time, polarisation effects such as the growth of corrosion products on the surface cause the corrosion process to be stifled.

(Steel) + (Oxygen) + (Water) = Hydrated ferric oxide (Rust) red rust.) The sum of these reactions can be represented by the following equation: These react with the ferrous ions from the anode to produce ferrous hydroxide, which itself is further oxidised in air to produce hydrated ferric oxide (i.e. Electrons are released from the anode and move through the metallic structure to the adjacent cathodic sites on the surface, where they combine with oxygen and water to form hydroxyl ions. Initial attack occurs at anodic areas on the surface, where ferrous ions go into solution. The corrosion of steel can be considered as an electrochemical process that occurs in stages.