Titrations, corrosion measurements

Corrosion measurements by titration (CMT) were first introduced in 1990. The method is based on the following considerations When a metal corrodes with no applied current in an aqueous solution, the anodic dissolution reaction... 

Naphthenic acid is a collective name for organic acids present in some but not all crude oils. In addition to true naphthenic acids (naphthenic carboxylic acids represented by the formula X-COOH in which X is a cycloparaffin radical), the total acidity of a crude may include various amounts of other organic acids and sometimes mineral acids. Thus the total neutralization number of a stock, which is a measure of its total acidity, includes (but does not necessaiily represent) the level of naphthenic acids present. The neutralization number is the number of milligrams of potassium hydroxide required to neutralize one gram of stock as determined by titration using phenolphthalein as an indicator, or as determined by potentiometric titration. It may be as high as 10 mg KOH/gr. for some crudes. The neutralization number does not usually become important as a corrosion factor, however, unless it is at least 0.5 mg KOH/gm. 

Recently a new pH-stat instrument (PHM290 with autoburet ABU901) was introduced by Radiometer International A/S. This instrument can be connected directly to a personal computer, so that titration data and pH can be recorded in a Windows-based program. A recording of corrosion potential must then be made by another instrument, but both titration data and potential measurements can be recorded simultaneously using two inputs. 

A different pattern of dissolution was seen with a Zn-Sn alloy containing 26% zinc. In this case the stable dissolution situation established after ca. 90 min showed a ratio of EC to CMT measurements of 1 4. As seen in Fig. 3, this remained fairly constant, though the corrosion potential increased by more than 50 mV. Only selective zinc dissolution took place, and analysis by atomic absorption spectroscopy of the amount of dissolved zinc agreed within 10% with the value according to the titration. This pattern is still difficult to understand. The ratio of ca. 1 4 between EC and CMT measurements could be interpreted in terms of formation of the low-valent zinc species ZnJ, which seems unlikely, or in terms of dissolution of divalent zinc ions accompanied by loss of chunks consisting of precisely three zinc atoms, each time a zinc ion is dissolved. The latter alternative seems to require a more discrete mechanism of dissolution than... 

Hazel et al. were the first to study the strong interaction between metal ions, specificaUy Fe, and silicate species [69]. Later, when studying corrosion inhibition of zinc by silica. Hazel et al. [70] measured the interaction of Zn ion and silicates and observed slight, but definite, pH value shifts in pH titration inflection points. They suggested that hydroxyl ion was being adsorbed, or occluded, by the amorphous sihceous precipitates. Chemisorption of sihca at a zinc hydroxide surface appeared to occur independent of the surface charge or the exact chemical composition of the surface. Falcone [71] measured the activities of Ca, Mg" and Cu" in the presence of soluble silica. Polymerized sihcate anions also shifted the pMe vs. pH curves to lower pH values... 

Chloride content. Samples for determining the chloride level in concrete are collected in the form of powder produced by drilling or by the extraction of cores, sections of which are subsequently crushed. The latter method can provide a more accurate chloride concentration depth profile. The chloride ion concentration, used as a measure of the risk of corrosion damage and degree of chloride penetration, is subsequently determined by potentiometric titration. Two distinctions are made in chloride ion concentration testing Acid-soluble chloride content (ASTM C 114) refers to the total chloride ion content, while the water-soluble content represents a lower value. 

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