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Corrosion (part 2 of 4)

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:

a. 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.

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

c. 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

d. Atmospheric – attach by sulphur dioxide / sulphur trioxide / hydrogen sulphide or chloride

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

So how do chemical elements help combat corrosion, and which elements are “good” against particular types of corrosion:

Nickel – resists reducing & caustic acids, and improves resistance to stress corrosion cracking in chlorides & caustic acids.

Chromium – resists oxidising (especially high temperature oxidation and sulphurisation, and enhances resistance to pitting & crevice corrosion.

Molybdenum – resists reducing acids, and to pitting & crevice corrosion in aqueous chloride environments.

Iron – improve resistance to high temperature carburising environments

Copper – resists reducing acids (particularly non-aerated sulphuric & hydrofluoric), and salts. Copper additions to nickel-chromium-molybdenum-iron alloys provide enhanced resistance to mineral acids (hydrochloric, phosphoric & sulphuric)

Aluminium – improves resistance to oxidation at elevated temperatures.

Titanium – (with carbon) reduces susceptibility to intergranular corrosion due to carbide precipitation resulting from heart treatments.

Niobium – (with carbon) reduces susceptibility to intergranular corrosion due to carbide precipitation resulting from heart treatments, improves resistance to pitting & crevice corrosion.

Tungsten – resists reducing acids and localised corrosion.

Nitrogen – improves pitting & crevice corrosion resistance

Cobalt – improves resistance to carburisation & sulphurisation.

In the next blog, we will look at the standard tests that can be carried out to check for corrosion in metals. If the suspension gets too much and you can’t wait until then, please contact our Special Process Department on ++44 114 232 9241.

The Broder Blogger

Tea and Chocolate at Broder Metals Sheffield Office

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