Griffiths, A J; Turnbull, A (2003) Assessment of the role of the acetic acid/acetate buffer in corrosion/stress corrosion testing of modified 13 Cr stainless steels and carbon steels. NPL Report. MATC(A)131

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Abstract

Acetic acid and acetate anions are present in oil production fluids and are also used as a buffer system in laboratory testing in simulated produced waters. In the latter case, there has been controversy as to the possible direct effect of this buffer on the stability of the passive film on modified 13 Cr martensitic stainless steel outwith its role in maintaining constancy of pH. Also, there have been investigations into the role of this buffer in the corrosion of carbon steel with the intention of better reflecting its known enhancement of corrosion rates in service. The most recent literature has been evaluated.
For the modified 13 Cr martensitic stainless steel, the initial indication that the buffer can have a direct detrimental effect on the stability of the passive film when testing in NaCl solutions at pH 3.5 (with 1% H2S/bal. CO2) at 20 ºC was not supported by a later more controlled study. In view of the later findings, the reduction in acetate concentration by a factor of 10 from the previously used value of 4 g/L, incorporated into the most recent EFC17 guidelines 1 , may not have been prudent. Maintenance of the solution pH in acidified H2S-containing environments at ambient temperature, when this steel is in the active state, may then become a problem.
In relation to the role of this buffer system in corrosion of carbon steel the indication from rotating cylinder testing at ambient temperature is that the acetic acid/acetate buffer can act as a weak anodic inhibitor when testing in a relatively innocuous environment (0.2 M NaClO4). In more relevant laboratory testing at 50 º C or 60 º C with bubbling CO2, bicarbonate and chloride, the presence of this buffer enhanced the corrosion rate. The enhancement was not related to the bulk solution. pH. The most likely explanation is that dissociation of acetic acid is the predominant process in buffering the rise in pH at the cathodic regions. Provided the supply of acetic acid to the surface is sustained, then, correspondingly, the dissociation of CO2 or bicarbonate, the alternate buffering mechanisms, would be much reduced. This would then limit the formation of bicarbonate and carbonate respectively, thus limiting the formation of protective deposits. In addition, acetic acid causes dissolution of existing iron carbonate films. Technically, a problem for simulated service testing is sustaining the acetic acid concentration without the need for regular refreshment. A gas mixture comprising CO2 with acetic acid vapour is proposed but recirculation through the test cell from a large reservoir would seem to be required also for more direct relevance to service.
The contrasting effects of the acetic acid/acetate buffer on the modified 13 Cr steel in the active state at pH 3.5 at 20 º C and on the carbon steel in the bicarbonate solutions at 50 º C is due to the fact that in the former the acid conditions and lower temperature constrain the formation of protective carbonate films.

Item Type:	Report/Guide (NPL Report)
NPL Report No.:	MATC(A)131
Subjects:	Advanced Materials Advanced Materials > Corrosion
Last Modified:	02 Feb 2018 13:16
URI:	http://eprintspublications.npl.co.uk/id/eprint/2576

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