Synthetic sea water

Immersed Molybdenum has good resistance to synthetic sea water, the rate of attack up to 60° C being less than 0-1 mm/y" and it is only slightly corroded when exposed to synthetic sea water spray at 60°C for periods of 10, 20 and 30 days. 

Work by the US Bureau of Mines" involving galvanic couple experiments showed that the normally low corrosion rates of molybdenum were reduced further by contact with aluminium, SAE 1 430 steel or magnesium in aerated solutions of synthetic sea water or 3% sodium chloride. 

Contact with copper in sodium chloride solutions reduces the corrosion of molybdenum, but in synthetic sea water the corrosion rate is somewhat higher. In 1 % sodium hydroxide, molybdenum corrodes slightly faster when coupled with SAE 1 430 steel with or without the presence of air. It is protected by contact with copper in aerated 0-46% sulphuric acid. It is not significantly affected by contact with 316 or Carpenter 20 stainless steels in sulphuric acid solutions. 

The US Bureau of Mines found the chemical and galvanic corrosion behaviour of both the TZM and Mo-30W alloy to be generally equal or superior to that of unalloyed molybdenum in many aqueous solutions of acids, bases and salts. Notable exceptions occurred in 6-1 % nitric acid where both alloys corroded appreciably faster than molybdenum. In mercuric chloride solutions the TZM alloy was susceptible to a type of crevice corrosion which was not due to differential aeration. The alloys were usually not adversely affected by contact with dissimilar metals in galvanic couple experiments, but the dissimilar metals sometimes corroded galvanically. Both alloys were resistant to synthetic sea water spray at 60°C. 

The most widely used accelerated tests are based on salt spray, and are covered by several Government Specifications. BS 1391 1952 (recently withdrawn) gives details of a hand-atomiser salt-spray test which employs synthetic sea-water and also of a sulphur-dioxide corrosion test. A continuous salt-spray test is described in ASTM B 117-61 and BS AU 148 Part 2(1969). Phosphate coatings are occasionally tested by continuous salt spray without a sealing oil film and are expected to withstand one or two hours spray without showing signs of rust the value of such a test in cases where sealing is normally undertaken is extremely doubtful. 

Fig. 19.11 Effect of specimen position on corrosion in salt-spray tests specimens of cold-rolled steel, (a) 20% NaCl, (i>) synthetic sea-water...

The results will also be influenced by the concentration of NaCl solution sprayed —some metals are affected more by one concentration than another — for example, zinc is corroded most by a concentrated brine (20%), while iron is corroded most by a dilute brine (3%) synthetic sea-water is less corrosive to these metals than either brine. In view of the many other ways by which the conditions within a salt-spray box differ from those of exposure to a natural sea-coast environment, there seems to be no great advantage in making-up complicated synthetic sea-waters for use in salt-spray testing. However, tablets for this purpose are commercially available. 

Favretto and co-workers [198,206-208] have described direct spectrophoto-metric methods for non-ionic surfactants based on the formation of a sodium picrate surfactant adduct. This method has been applied to seawater. A mean value of 93 1% was obtained in recovery experiments on C12E9 (at an aqueous concentration of 0.10 mg/1) extracted from synthetic sea water by means of this... 

A mixture of 300 mL of the oil under test is stirred with 30 mL of distilled water or synthetic sea water, as required, at a temperature of 140°F (60°C) with a cylindrical steel specimen completely immersed therein. It is customary to run the specimen for 24 hours however, the test period may, at the discretion of contracting parties, be for a shorter or longer period. The specimen is observedfor signs of rusting and, if desired, degree of rusting. 

Often, this method is further defined as ASTM D-665 A or ASTM D-665 B. Method A requires the use of distilled water. Method B requires the use of a synthetic sea water solution. The composition of this solution is found in APPENDIX 5. 

Appendix 5. Composition of Synthetic Sea Water Utilized in ASTM D-665-B... 

Natural vs. Synthetic Sea Water. It is not always realized that natural sea water is a completely different medium from synthetic sea water from the corrosion standpoint. A primary factor is the presence of biological effects in natural sea water that are not present in the synthetic mixture. 

The relative order of resistance of metals as determined by their corrosion rates in natural sea water is not readily simulated with synthetic sea-water preparations in the laboratory. 

The effect of the presence of water in oil in terms of alkaline reserve of oil, pH (extract) and TBN value was investigated (Fox et al., 1990 Pawlak et al., 1985). The soluble acid/base content has been determined by extraction into either DI water, 7% synthetic sea water solution or aqueous ethanol mixture (1 1, v/v). Extraction of oil samples by aqueous ethanol mixture (1 1, v/v) was applied to new and used samples of an SAE 30 oil. The relationship between pH extracted in aqueous ethanol and TBN value is shown in curve I, Fig. 6.8. 

Hansson, I. Determination of the acidity constant of boric acid in synthetic sea water media, Acta Chem. Scand. 27, 924-930 (1973). 

Stewart and Munjal (20) determined the solubility of CO2 in distilled water, a S3mthetic sea water, and in three and five-fold concentrations of the synthetic sea water. The Henry s Law constants for CO2 computed for these systems at different temperatures were found to be linearly related to temperature and solution molality. The concentration of CO2 (aq) in moles per total moles of solution (mole fraction) may be written... 

Stewart, P. B., and Munjal, P. K. The solubility of carbon dioxide in distilled water, synthetic sea water and synthetic sea water concentrates. Water Resour. Center, Univ. 

The matrix matching method attempts to duplicate the sample matrix by adding the major matrix constituents to the standard and blank solutions. For example, in the analysis of sea water samples for a trace metal, the standards can be prepared in a synthetic sea water containing Na, K, Cl , Ca, Mg " and other components. The concentrations of these species are well known in sea water and fairly constant. In some cases, the analyte can be removed from the original sample matrix and the remaining components used to prepare standards and blanks. Again, we must be careful that added reagents do not contain the analyte or cause extra interference effects. 

Vanadium was one of the elements determined. Standard solutions in a synthetic sea water matrix were prepared and determined by ICP-MS. The following results were obtained ... 

Protection against ferrous corrosion by sea water can be assessed by IP 135B in which a mild steel pin is suspended in a mixture of oil and standardised synthetic sea water for 24 h at 60 C. Other tests used include a static water drop test and a... 

Corrosion. Fink 16) of Battelle Memorial Institute has presented the results of a literature study combined with views of experts on the corrosion of metals by sea water. The study revealed a paucity of data on corrosion at elevated temperatures. The Cl ion is the chief culprit in causing corrosion, but an important factor is dissolved oxygen and it is probable that oxygen-free sea water would have very little corrosive action, at least at ordinary temperatures. Natural sea water may have very different corrosion effects from synthetic sea water because of the organic content. Fouling of the surface by organic deposits can lead to severe pitting due to concentration-cell effects. Consequently corrosion by actual water is not readily simulated in the laboratory by synthetic sea water. 

Fonlupt et al. [115] investigated the role of the second phase in the SCC of a nickel-aluminum-bronze alloy using strain rate tests in synthetic sea water. 

Karasek et al. [66] conducted drop-weight impact tests on IM7 carbon fibre/ epoxy resin laminates after sea water exposure. Four resin systems (Table 7.2) were used, three of them being based on the same Shell Epikote 828 with different hardeners and/or impact modifiers. The volume fractions were in the range 60-69% fibre and the lay-up was [0/ 45/90°]js. Samples were about 6.35 mm thick for all materials (different cured ply thicknesses necessitated the use of differing numbers of plies) and cut to 100 x 100 mm squares for testing. The samples were saturated with ASTM D1141-52 Formula A synthetic sea water at 95°C, and then conditioned at ambient... 

Electrode response is affected by ionic strength variations because of the associated effect on activity coefficients [9]. This makes an assessment of ionic strength on the test solution desirable. However, because of the powered term in /x = SC, conductance monitoring may not always be satisfactory, unless, of course, the individual ionic contributions (concentrations, C, and valence, z) are known from previous analyses. Such information is useful for preparing calibration standards of the same ionic strength range as the sample under examination and is the basis of synthetic sea water samples as calibrants. Calcium chloride standards in 0.150 mol dm sodium chloride is a similar ploy used in calibrating... 

Moore studied the removal of Cu(II) ions from synthetic sea water by adsorption on different activated carbons and observed that the adsorption was strongly influenced by pH and the nature of the carbon. The adsorption of Cu(II) ions increased with increase in the ionic concentration of the solution on addition of NaCl. This increase in adsorption was attributed to the formation and adsorption of negatively charged Cu(ll) chloride complexes. The effect of parameters such as contact time, pH and the concentration of the solution in the range 10 to 1000 ppm on the adsorption of Cu(ll) ions has also been reported. Low et al. found that the dye modification of coconut husk enhanced its ability to absorb Cu. 

Annex 3, Acidified Synthetic Sea Water (Fog) Testing, is commonly referred to as the SWAAT test and is often used to test aluminum alloys. Rather than a 5 % salt solution, a synthetic seawater solution is used with the pH adjusted to 2.8-3.0. This aimex includes recommendations regarding the chamber operating temperature for different substrates. The chamber must be equipped with a device to conduct the following cycle 30-min seawater spray (fog), followed by a 90-min period at a relative humidity greater than 98 %. The high relative humidity is achieved by maintaining approximately 1 in. of water in the bottom of the chamber [4]. 

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