Corrosion 101 part I

Dear Reader,

I would like to take the time to explain some of the basic ideas in corrosion to you, now corrosion is a funny thing where it is not wanted it is called corrosion and it is a destroyer of valuable objects much where it is wanted it is called leaching or dissolution and there it is the beneficial liberation of metals and other materials from solids.

One of the things which we need to be aware of is the problem of stress corrosion cracking, now this does not sound as exciting as “metal fatigue” but in many ways SCC is a bigger monster which can cause things to fail.

SCC has two main parts, firstly an object must be under tensile stress for it to occur and secondly a chemically corrosive environment must be present.

The mechanical stress can take many forms, it can be residual stresses in an object which are left behind after it is made or it can be an imposed stress. For example one of the most eye watering and shocking SCC cases was the failure of the stainless steel members holding up a false celling at a Swiss swimming pool (http://www.corrosion-doctors.org/Forms-SCC/swimming.htm). Here the weight of the concrete kept the rods loaded under tension. The corrosive fumes and spray from the swimming pool caused one member to corrode and finally snap. This then imposed a new larger load on other stainless steel members, the overloaded members snapped and a cascading failure then brought the celling falling down.

To my mind a bombardment of concrete from above is about as horrible as someone putting a sea monster in the kid’s pool !

Another mechanical stress can be a thermal stress, if a system is heated up (or cooled down) then the expansion of different parts can set up mechanical stresses in it. For instance in a long pipe line it is common to have a half loop to reduce the impact of the expansion and contraction, but in service it is possible for this half loop to have some parts which are under tension. If the object is under compression then SCC will not occur.

The core of SCC is that metals are ductile while corrosion films tend to be brittle. A crack appears in the corrosion film, this crack will locally raise the stress level and the crack will tend to grow. When the crack reaches the ductile metal it tends to stop, but it will have exposed a new metal surface. If this metal surface then undergoes corrosion and is converted into a brittle corrosion film then the crack will grow again. This process will go on and on until something fails.

It can either be regarded as a mechanically assisted corrosion or as a chemically assisted cracking process. I will get onto another type of corrosion another day soon.