Corrosion – Part two

May 1, 2017 | Technical Blogs

This is the second in a series of blogs on corrosion and corrosion resistance in our stock grades. The first blog (found on our web site) looked at types of corrosion and the steps we take to minimise it. In this blog we will look at what it is that attacks metal and the factors (particularly chemistry) that affect a metals’ ability to withstand corrosion. The third blog will look at tests for corrosion resistance, and the final blog will rank our stock grades rank in terms of corrosion resistance.

Generally nickel-based alloys give better tolerance to corrosion than stainless steels and other iron-based alloys. However, this also depends upon the type of corrosion that they need to resist, and in turn the conditions under which the metal is placed and what may be attacking the metal. The following are the main form & sub-categories of attacking agents:

Acids, which can be either:

  • Oxidising – such as nitric / sulphuric in concentrated forms [>87% by volume]
  • Reducing – such as hydrochloric / dilute & intermediate strength sulphuric / phosphoric (unless compounded with oxidising elements) / organic acids (formic / acetic / propionic / butyric)
  • Hydroflouric – out on its own because it is highly corrosive – only gold & platinum are completely resistant to attack in aqueous solutions.

Corroding alkalis: usually lead to pitting & other localised attack.

Salts – salts increase conductivity and are able to carry higher corrosion currents (increasing galvanic corrosion seen in salt water solutions). Salts can be sub-divided into:

  • Natural – sodium chloride / potassium chloride / sodium sulphate / potassium sulphate.
  • Neutral & alkaline – sodium hydrochlorate / calcium hydrochlorate / sodium nitrate / potassium permanganate
  • Acid – magnesium chloride / potassium bisulphate, ammonium sulphate,aluminium sulphate
  • Acid – oxidising – cupric / ferric / mercuric / stannic chloride
  • Alkaline – potassium fluoride / sodium ?& potassium phosphates & carbonates
  • Atmospheric – attach by sulphur dioxide / sulphur trioxide / hydrogen sulphide or chloride

Water – mainly sea water – particularly causes pitting, crevice & stress corrosion cracking.

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