Corrosion

Beginners Guide to Corrosion
This document has been prepared by Bill Nimmo and Gareth Hinds of NPL’s Corrosion Group from various source material. It is intended to give an introduction to corrosion and its control in non-technical terms. More technical information is available on other areas of the NPL NCS website.

Bill Nimmo and Gareth Hinds NPL February 2003

Beginners Guide to CorrosionFebruary 2003

Page 1 of 10

CORROSION – BASICS What follows is a simple explanation of how corrosion occurs, what the different types are how problems can be solved. It is intended to be used by the non-expert to gain an initial appreciation of the subject before exploring further.

1

HOW DOES IT HAPPEN?

1.1

WHAT WE ALL KNOW

We have all seen corrosion and know that theprocess produces a new and less desirable material from the original metal and can result in a loss of function of the component or system. The corrosion product we see most commonly is the rust which forms on the surface of steel and somehow Steel → Rust

1.2

TWO REACTIONS

For this to happen the major component of steel, iron (Fe) at the surface of a component undergoes a number of simplechanges. Firstly, Fe → Fen+ + n electrons

the iron atom can lose some electrons and become a positively charged ion. This allows it to bond to other groups of atoms that are negatively charged.

We know that wet steel rusts to give a variant of iron oxide so the other half of the reaction must involve water (H2O) and oxygen (O2) something like this O2 + 2H2O + 4e- → 4OH-

Beginners Guide toCorrosion

February 2003

Page 2 of 10

This makes sense as we have a negatively charged material that can combine with the iron and electrons, which are produced in the first reaction are used up. We can, for clarity, ignore the electrons and write 2Fe + O2 + 2H2O → 2Fe(OH)2 Iron + Water with oxygen → Iron Hydroxide dissolved in it

1.3

THE NEXT STEP

Oxygen dissolves quite readily inwater and because there is usually an excess of it, reacts with the iron hydroxide. 4Fe(OH)2 + O2 → 2H2O + 2Fe2O3.H2O Iron hydroxide + oxygen → water + Hydrated iron oxide (brown rust)

1.4

THE PROCESS (Five facts)

This series of steps tells us a lot about the corrosion process.

(1) (2) (3) (4)

Ions are involved and need a medium to move in (usually water) Oxygen is involved andneeds to be supplied The metal has to be willing to give up electrons to start the process A new material is formed and this may react again or could be protective of the original metal

(5)

A series of simple steps are involved and a driving force is needed to achieve them

The most important fact is that interfering with the steps allows the corrosion reaction to be stopped or slowed to amanageable rate.

2

UNIFORM CORROSION

30% of failures

Beginners Guide to Corrosion

February 2003

Page 3 of 10

Uniform corrosion, as the name suggests, occurs over the majority of the surface of a metal at a steady and often predictable rate. Although it is unsightly its predictability facilitates easy control, the most basic method being to make the material thick enough tofunction for the lifetime of the component. Uniform corrosion can be slowed or stopped by using the five basic facts;

(1)

Slow down or stop the movement of electrons (a) (b) Coat the surface with a non-conducting medium such as paint, lacquer or oil Reduce the conductivity of the solution in contact with the metal an extreme case being to keep it dry. Wash away conductive pollutants regularly.(c) Apply a current to the material (see cathodic protection).

(2)

Slow down or stop oxygen from reaching the surface. Difficult to do completely but coatings can help.

(3)

Prevent the metal from giving up electrons by using a more corrosion resistant metal higher in the electrochemical series. Use a sacrificial coating which gives up its electrons more easily than the metal being...