The inter-relationship between scale and corrosion and their control in oil field production systems has been well documented in relation to increased localized "under deposit" corrosion and the potential for detrimental performance interferences between scale and corrosion inhibitors. The impact of various chemical interferences (impact on corrosion inhibitor performance by the presence of scale inhibitor) is routinely assessed in laboratory pre-screening tests. However, although the importance of iron carbonate scales and other passivating scales is well documented, the impact on corrosion control due to the deposition of more conventional inorganic scales and their removal is not generally assessed in detail. In addition, the presence of scaling ions in test brines can significantly impact the recorded corrosion rates in conventional electrochemical tests owing to the build up of deposits on the various electrode surfaces. Omitting the scaling ions can lead to a dramatic change in test conditions (for example much lower test pH's in CO2 saturated systems if bicarbonate ions are omitted from the test brine) therefore reducing the appropriateness of the test.
The work reported describes a range of laboratory tests conducted to assess the impact of inorganic precipitates (scales) and their removal on both inhibited and uninhibited corrosion rates. Initial work summarises the impact of the inclusion or exclusion of scaling ions from test brines on the corrosion rates recorded using conventional electrochemical laboratory techniques. Further tests are then reported which examine the impact on general corrosion rates and corrosion inhibition using partially pre-scaled samples and those in which scale deposits have been chemically removed.
In summary the presence of scale deposits can have a significant impact on the recorded corrosion rates and the performance of conventional corrosion inhibitors which may have important implications for chemical screening.