Low Total Dissolved Solids Research Articles

Corrosion behaviour and microstructure of two thermal spray coatings

Many of the state of the art thermal spray coatings (e.g. plasma, HVOF) have been developed with wear resistance as a primary aim. However, these coatings are increasingly being required to function in aqueous environments and, to date, their corrosion behaviour is not well defined. This paper comprises a description of a study of the corrosion behaviour of two types of thermal sprayed cermet coatings applied to a superduplex stainless steel (UNS S32760) substrate. Both coatings (86WC-10Co-4Cr and 50WC-50Ni-Cr-B-Si) were applied using a proprietary HVO F process. Each generic coating was studied in three forms: unsealed, sealed using a polymeric sealant, and with an interlayer of Inconel 625 applied between coating and substrate. Surfaces were tested in both the as sprayed condition and after abrasion and polishing to 1üm. The coatings were exposed to seawater and a low total dissolved solids (TDS) water at 18 and 50°C. The corrosion behaviour and detailed corrosion mechanisms were investigated using electrochemical monitoring techniques supported by precise post-test microscopical examination using light microscopy, scanning electron microscopy, and X-ray microanalysis. The two coatings studied are shown to be susceptible to corrosion after relatively short periods in both the low TDS water and seawater. Distinct differences in the extent and mechanisms of attack are demonstrated. In addition, the findings from electrochemical accelerated tests are shown to correlate well with the results of detailed examination of specimens after longer term exposure to the aqueous environments.

Aqueous geochemistry of major constituents in the Alph River and tributaries in Walcott Bay, Victoria Land, Antarctica

Two geochemical surveys of the major constituents of the Alph River, situated in Walcott Bay, Victoria Land, were undertaken in the austral summer of 1987–88. At the same time, tributaries and the runoff from various glaciers were investigated. The Alph River has an average total dissolved solids (TDS) concentration of 63.5 mgl−1, approximately half that of average world river water. The chemical composition is dominated by Na+ and HCO−3. Glacial melt waters have very low TDS but chemical weathering over the course of a few kilometres causes solute concentrations in the tributaries to exceed those of the Alph River. The composition of the streams is variable, but often Ca2+ is the principal cation. Enrichment factor and mass balance calculations indicate that the salts in the Alph River and its tributaries have a substantial non-marine component. Chemical weathering of calcite, mirabilite, gypsum and halite contribute solutes to the aquatic system. A “Gibbs Plot” [TDS versus Na:(Na+Ca) weight ratio] indicates that water samples from direct glacial runoff fall outside the world water envelope. They have low solute levels but enhanced Ca2+ concentrations, resulting from the aeolian deposition and subsequent dissolution of calcitic material.

Groundwater level and hydrochemistry in the San Jacinto basin, riverside county, California

The San Jacinto basin, structurally a sediment-filled graben (4 km by 40 km) formed by right-stepping en echelon faulting in the right-slipping San Jacinto fault zone, provides most of the water demand in the San Jacinto valley. Pumping of groundwater in the past 80 yeas had caused water level to drop at some locations by 90 m. Analysis of seasonal and long-term fluctuations of water levels suggests at least four hydrologic subregimes in the basin. Stiff and Piper diagrams indicate four major types of subsurface water. Major ion concentrations appear stable over the recorded period from 1951 to 1986 despite repeated changes of water levels in the past 48 years. Hydrologic discontinuities are recognized from contour maps of four-year mean winter water elevations and four-year mean concentrations of total dissolved solids (TDS). Some discontinuities coincide with faults while others probably signify sedimentary facies boundaries. A trough of low TDS (reflecting ancient channel deposits of high hydraulic conductivity) lies southwest of the present channel of the San Jacinto River. This implies the San Jacinto River has migrated northeast probably in response to tectonic tilting of the basin. That TDS trough also suggests that most of the subsurface water leaves the basin about 7 km south of the present river exit. Because of its mixing with high-TDS water from the northwest, the northwest moving subsurface water appears to have been displaced southward before it exists the basin. Contours of sulfate, which is almost absent in the water from the northwest, provide a better indication of exit location.