Corrosion Of Copper-Base Alloys In A Geothermal Brine
Authors: Miller Richard L. 
Published: December 2007

The geothermal environment and the experimental procedures and schedules for corrosion tests of copper-base procedures and schedules for corrosion tests of copper-base alloys are described. Corrosive attack on these materials was mostly uniform. Some selective leaching of alloying elements was observed, as was crevice corrosion, but the extent of these forms of corrosion was minor. The results of these tests show a trend toward higher corrosion rates with increasing copper content, for the brass alloys. Commercially pure copper, however, showed corrosion rates 20 to 30% of that suggested by the trend in the data. One copper-nickel alloy was tested to verify earlier test data; this alloy showed a corrosion rate about six times that of a brass of similar copper content. The primary agent of the corrosive attack was hydrogen sulfide, present in the water in trace amounts. present in the water in trace amounts. The primary conclusion from these tests is that copper-zinc alloys are the most economical materials for boiler and preheater construction. The recommendation is made that materials be selected from these brasses: naval brass, yellow brass, admiralty brass, and copper, in this order of decreasing desirability. Aluminum brass and red brass are marginally acceptable. Copper-nickel alloys are unacceptable for boiler and preheater heat exchangers. preheater heat exchangers

Introduction

Corrosion is one of the major problems facing those using geothermal resources for power production or for direct applications such as process heat, agricultural stimulation, and space heating. Results of corrosion testing, in the context of materials selection for a demonstration power plant utilizing geothermal fluids as the heat source, are reported here. Corrosion of selected copper-zinc alloys is given particular emphasis.

The resolution of industrial corrosion problems generally involves a combination of materials selection and corrosion inhibitors. Unfortunately, the volumes of water involved in geothermal applications are usually too large to consider chemical means of corrosion control. Further, one must anticipate that any chemical added to the geothermal fluid for corrosion inhibition must be removed prior to fluid disposal to satisfy environmental quality requirements. Thus, materials selection appears to be the most practical approach to corrosion control in most applications of geothermal fluids.

Materials selection for geothermal service based on experience in other applications has not been successful. For example, although data from seawater and desalination service would seem to be useful, such technology transfer has been unreliable. In addition, data are generally lacking for materials performance in geothermal environments. The absence of performance in geothermal environments. The absence of data prompted the Geothermal Project to conduct a series of screening and short-term corrosion tests at the U. S. Department of Energy (DOE) geothermal test site in the Raft River Valley of Southern Idaho. These are engineering tests whose purpose is to provide a materials data base for design of power plant and direct applications facilities at Raft River. The data ill permit prediction of the corrosion rates of structural materials and will assist in establishing preventive maintenance schedules and procedures. preventive maintenance schedules and procedures. Materials performance in geothermal systems is closely tied to materials costs. Earlier tests at Raft River have suggested that the best materials from a corrosion point of view are also the most costly, and are uneconomical for most applications. The least expensive materials are unacceptable from a corrosion standpoint. A middle ground has to be taken, one which appears to involve brasses and bronzes for many applications.

When the proposed 5 MWe power plant at Raft River received approval from DOE, a review of previous Geothermal Project corrosion data was made. This review resulted in the recommendation that admiralty brass be selected for the tubing of the boilers and preheaters of the power plant. Results of corrosion preheaters of the power plant. Results of corrosion tests conducted to verify the correctness of this material choice support the earlier recommendation.