Hardness, aqueous corrosion

In aqueous corrosion, raising the temperature increases the dissolution of zinc in water. A marked increase occurs up to around 60°C followed, by a decrease at higher temperatures due to the decrease in oxygen supply and the formation of more compact and adherent scale. Intergranular corrosion of zinc casting alloys is a risk above 70 C in wet or humid conditions, such as in steam, when no protective layer can form and selective dissolution of the structure occurs. In hot hard waters, scale forms at water temperatures above 55 C. This scale has a coarse grain structure and does not adhere well to the zinc surfece. Corrosion of the zinc occurs locally because of the discontinuities in the scale or local electrochemical action. Above 60°C, zinc usually becomes cathodic to steel therefore, the steel will corrode to protect the zinc coating. 

Immersion in aqueous media open to air Solutions in which tin is cathodic to steel cause corrosion at pores, with the possibility of serious pitting in electrolytes of high conductivity. Porous coatings may give satisfactory service when the corrosive medium deposits protective scale, as in hard waters, or when use is intermittent and is followed by cleaning, as for kitchen equipment, but otherwise coatings electrodeposited or sprayed to a sufficient thickness to be pore-free are usually required. 

These sort of problems make it difficult to obtain reliable high temperature data on the aqueous chemistry of transition metal ions. Unfortunately the necessary timescales for even the simpler experimental studies are frequently too long for a Ph.D. student to make reasonable progress in 3 years from scratch or for industrial researchers to make much reportable progress before the patience of those supporting the work is exhausted. Results can be reported far more rapidly from, for example, corrosion experiments and since corrosion theories are in general of so little predictive value, each relevant alloy/electrolyte combination needs its own study. In such circumstances it is hardly surprising that thermodynamic studies have been (with a few notable exceptions) relatively poorly supported, while corrosion data continue to be amassed without any reliable thermodynamic framework within which to understand them. 

La. The material is hardly affected by hot aqueous solutions and is particularly effective in absorbing corrosion-product ions derived from metal surfaces. Kraus, Carlson and Johnson (1956) prepared a column from precipitated zirconium tungstate for the separation of Na+, K+, Rb+ and Cs+. The eluant was aqueous NH4CI whose concentration was increased as the elution proceeded. 

Electrocatalysis in metallic corrosion may be classified into two groups Adsorption-induced catalyses and solid precipitate catalyses on the metal surface. In general, the bare surface of metals is soft acid in the Lewis acid-base concept and tends to adsorb ions and molecules of soft base forming the covalent binding between the metal surface and the adsorbates. The Lewis acidity of the metal surface however may turn gradually to be hard as the electrode potential is made positive, and the bare metal surface will then adsorb species of hard base such as water molecules and hydroxide ions in aqueous solution. Ions and molecules thus adsorbed on the metal surface catalyze or inhibit the corrosion processes. Solid precipitates, on the other hand, are produced by the combination of hydrated cations of hard acid and anions of hard base forming the ionic bonding between the cations and the anions on the metal surface. 

The magnesium ion is a hard acid and the fluoride ion is a hard base. The two ions then combine with each other to form a precipitate film of ionic bonding. In the similar way, benzotiazole, BT AH, which dissociates into deprotonated benzotiazole ion, BTA, and proton, H+, inhibits the corrosion of copper in aqueous solution by forming an insoluble precipitate film of cuprous polymer complexes [85] ... 

In aqueous systems the dilemma often facing the water chemist is whether or not a particular industrial water is likely to be scale forming or corrosive. The reason for this diffiiculty may be attributed to the presence of CaCO (see Chapter 8). A simple test that may be applied to a water to provide a qualitative assessment of the problem is to add powdered CaCO to the water. If the water is supersaturated in respect of CaCO the addition of the solid particles will cause precipitation from the solution. Under these conditions there is a reduction in the pH of the water. It may be said that the water has a positive saturation index. If however the water is not saturated in respect of CaCO (such waters are corrosive), some of the added solid CaCO will enter the solution thereby increasing the hardness, alkalinity and pH. Water displaying these properties may be said to display a negative saturation index. 

HIC differs from sulfide stress cracking (SSC), which also results from the absorption of hydrogen during corrosion in aqueous H2S environments [127, 128]. Whereas HIC does not require an apphed stress and occurs in low-strength steels, SSC does require an external tensile stress and occurs in high-strength steels or in hard areas associated with the heat-affected zones adjacent to welds. However, SSC cracks in the heat-affected zone can lead to SOHIC in the adjacent base metal. 

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