Caustic cracking is a type of Stress Corrosion Cracking. (SCC) It is prevalent in systems where high temperature caustic is in contact with carbon steel that has stresses in it.
Caustic SCC is very typical in steam systems where the boiler feed water treatment is not under proper control. In particular, where a lot of NaOH is around. Sometimes NaOH is added as PH control. Sometimes NaOH is formed in the dosing system through the reaction of other additives.
In carbon steel, caustic cracking is typically branched and intergranular.
This cracking would typically occur around welds, as these areas have high residual stresses.
You say that the cracks are running transversely to the pipe. Does this mean that the cracks are mostly associated with the but welds in the pipe? If so, this makes sense. If the cracking is away from a weld in the transverse direction, it sounds strange, because then the stresses causing the cracking should be the stresses induced by the pressure. As the highest applied stresses are hoop stresses, (Assuming the stresses are caused by pressure and not poor installation or supporting.) the cracks should generally run along the length of the pipe.
To identify the cracking as caustic cracking without a doubt, calls for some fancy analyses that can only really be done by a metallurgist. To identify it on a "balance of probabilities" basis is usually quite easy. If you have a number of the following, then it is almost certainly caustic cracking:
1)Cracking is very branched.
2)It is limited to areas of high stress. (e.g. Welds.)
3)There are other instances of similar cracking found elsewhere on the boiler. (especially on boiler feedwater pumps, if they are C/Steel and de-superheaters, if the boiler has these.)
4)Caustic is used in the water treatment regime, or is formed as part of this regime.
There are other possible cracking mechanisms in steam systems, so beware of just assuming it is caustic cracking.
I hope this helps.
Niekie Jooste
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