Monitoring Corrosion in Oil and Gas Production with Iron Counts
|Publication Date:||10 March 2012|
The anomalies experienced when using iron counts as a
The analyst should evaluate available test methods for iron content to determine the most suitable method regarding detection limits, accuracy, precision, and interferences. Specific analytical procedures are not addressed in this standard. The exact method of sampling and sample treatment required to separate and analyze for ferrous, ferric, soluble, and total iron content of a water sample are adequately covered in the analytical procedures described by Rydell and Rodewald,1 Eaton et al.,2 and ASTM(1) D1068.3 If techniques are used to analyze for the individual species of iron, the final report must indicate the form of iron being reported. If only the typical total acid-soluble iron content is determined, the final report should indicate that the result is "total iron."
For the purposes of this standard, it is presumed that iron counts are performed on aqueous samples. Analysis of hydrocarbon samples for iron content is possible and the technique is practiced by some corrosion engineers. One suggested technique for "iron in oil" is described by Rydell and Rodewald.1
The mechanical arrangement, physical conditions, and chemical environment in almost every system or part of a system must be evaluated under comparable conditions before the iron content of each sample can be correctly interpreted. The iron counts measured are not of any value if these variables are not considered in the interpretation.
Monitoring corrosion by the use of iron counts may be done
easily, inexpensively, and quickly in the field. Iron production
rates, unlike test specimen corrosion rates, may give some
indication of corrosion upstream or downhole from the sampling
point. Iron counts are useful when surface-monitoring devices, such
as test specimens, may not reflect downhole conditions, such as
when paraffin forms on test specimens and when downhole conditions
are greatly different from surface conditions. The principal reason
for the historical popularity of iron counts as a standalone
Generally, iron counts from fluids containing dissolved sulfides
or dissolved oxygen are not reliable because of precipitation of
iron sulfide or iron oxide solids that may deposit on metal
surfaces as well as remain suspended in solution. Although the iron
counts may vary over time as temperature, hydrogen sulfide (H2S),
or oxygen levels vary, the iron count value actually represents the
solubility of iron and not the severity of corrosion upstream from
the sampling point. Therefore, the use of iron counts as a
Proper safety precautions when dealing with sour systems are addressed in API(2) RP 54.4 Appendix A (nonmandatory) covers safety considerations when handling H2S, and information on the toxicity of this gas.
(1) ASTM International (ASTM), 100 Barr Harbor Dr., West Conshohocken, PA 19428-2959.
(2) American Petroleum Institute (API), 1220 L St. NW, Washington, DC 20005-4070.