Application of flux monitoring to sour corrosion scenarios.
Carbon steel is used to convey and contain vast quantities of sometimes corrosive hydrocarbon fluids, because it lends its physical attributes without failure, for a minimum time of service of at least a few year, at a relatively low cost of installation.
The cost of failure of steel in corrosive service is generally rated at many times the cost of replacement of a steel pipe or vessel, especially when failure causes an unscheduled shutdown in the mining of hydrocarbons, a treatment facility, or refinery process. However, even if unscheduled failure is not a risk, the extension of corrosive service of steel often offers a very attractive saving against the cost of the corrosion control. The cost benefit is particularly keen where equipment, built decades previously, is turned into the service of more corrosive oil and gas feedstock for which it was not designed.
The primary cause of failure of steel in sour service is the loss of steel wall to corrosion over a number of years. Severe sour corrosion can also promote very severe hydrogen permeation through steel, resulting in HIC.
Corrosion mitigation is often achieved by adjustment to process parameters, but to a large extent, chemical inhibitors are used to suppress corrosion. The intention is sometimes to extend the lifetime of steel in sour service by many years, or indefinitely. However, inhibitors may also be used to suppress more severe, short lived corrosion, or indeed, the hydrogen induced cracking (HIC) of steel that may arise therefrom. The verification of effective corrosion control by hydrogen flux measurements is in itself very attractive, in that it can provide a very early indication that an inhibitor has been effective in preventing corrosion, and even assist in the appropriate inhibitor dosing.
The scheme below offers a summary view of sour corrosion chemistry.
The hydrogen flux attending sour corrosion is contingent upon there being no passivating corrosive scale on steel. It should be recognised that a corrosive scales comprising non-stoichiometric pyrrhotite does not prevent corrosion and therefore a flux measurement where this scale prevails is not meaningful, but then nor, probably, is the application of an inhibitor as effective. This in effect means that using measurements of flux to indicate sour corrosion should be confined to circumstances in which the progress of sour corrosion itself is contingent upon there being no corrosive scale on the steel. Typically, such corrosion scenarios are well identified: the lack of scale and hence progress of corrosion may be due to the equipment being in commission, or due to the action of cyanides or acids in removing sulfide scale, for example. Such corrosion scenarios benefit from the application of inhibitors, and so naturally flux monitoring finds synergy with inhibitor based corrosion control.
Another important example of where corrosion is contingent on the removal of scale is in refinery process streams containing naphthenic acid, a term used loosely to describe a range of cyclic carboxylic acids. The corrosion occurs upwards of 200 oC, 392 oF. Here removal of scale is facilitated by changes in the acid and sulfur compound constituency of the process stream, and is controlled by both adroit use of specialised inhibitors and deft blending of refinery feedstock. Again, flux offers a prospective indicator of such corrosion in near real time.
Generally, corrosion resistant steels are much more expensive, and used only in corrosive service where corrosion inhibitors are not an viable and cost effective option, and carbon steel fails within timescales less than a few years.