Abstract |
Geothermal brines contain a variety of corrosive dissolved species, such as chloride and sulfate
anions that can damage various system metallurgies via a number of corrosion processes. Several
forms of corrosion can occur, the most common being uniform and pitting corrosion. The
combination of low water quality with high-temperature and high-pressure conditions usually
accelerate these corrosion processes, with undesirable results to the system’sintegrity. On the other
hand, in addition to the above-mentioned conditions, the different corrosion products (eg. Fe
oxides) can form films on the metal surface that influence the corrosion rate.
Among the several metallurgies that can be used in system components, carbon (mild) steel is
one that is widely used in industry, due to its compatibility with other metals present, and,
primarily, its comparatively low cost. The electrochemical reactions taking place during carbon
steel corrosion promote formation of insoluble compounds of oxidized forms of Fe on the metal
surface, such as iron oxides (magnetite, hematite) and hydroxides (lepidocrocite).
In this thesis, we furnish comparative results from corrosion inhibition experiments based on
two quantifying methods: (a) gravimetric measurements based on the mass loss of each metallic
specimen and (b) total iron quantification based on an established photometric methodology (ironphenanthroline complex). In an effort to understand the corrosion behavior of carbon steel and the
factors that influence it, a large number of experiments were carried out under several conditions,
by studying the effect of certain variables (temperature, water quality, stirring speed, etc.). Finally,
the anti-corrosion efficiency of several chemical additives was evaluated. The inhibitors were
categorized as phosphonates and non-phosphonate additives. Emphasis was given to phosphonatebased corrosion inhibitors and their potential synergy with metal cations present in the brines.
The first step was the identification of the corrosion products under the experimental conditions.
Lepidocrocite appear to form most often at ambient temperature and at 60 °C, while hematite and
magnetite appear to form at higher temperatures. In addition, if bicarbonate is present, the
formation of iron carbonate was observed. This study showed that the formation of different
corrosion products can affect the corrosion rate.
Overall, the phosphonate inhibitors showed better performance in high salinity water.
Furthermore, phosphonates exhibit great inhibition in medium salinity water under high pressurehigh temperature conditions. Regarding the non-phosphonate inhibitors, each chemical additive
was affected in a different way by the change in the experimental conditions. Two of the nonphosphonate inhibitors exhibited their highest anti-corrosion efficiency in deionized water, while
the performance of the other two was best in medium salinity water. This research concluded that
the optimum concentration depends on several conditions, such as the temperature and the water
quality, regardless of the nature of the inhibitor. Finally, the results of the total iron determination
method showed some variance from those of the mass loss measurements, especially in the
presence of phosphonates.
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