Unalloyed aqueous corrosion

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. 

Transgranular stress corrosion cracking also occurs in solutions of CO-CO2-H2O containing oxygen. Recent results also indicate that unalloyed and low-alloy steels may be sensitive to anodic stress corrosion cracking in aqueous CO2 solutions. The considerable hydrogen permeation has been observed under critical conditions t > 40°C, > 1 Mpa, o > 0.5a therefore, a certain amount... 

Unalloyed and low-alloy steels corrode in neutral aqueous solutions by oxygen corrosion (see Chapter 1, Part I). In the absence of dissolved oxygen the water itself can act as the oxidizing species ... 

Alkaline conditioning of aqueous media is often used technically for unalloyed and low-alloy steels the controlled dose of alkaline substances (Na3P04, NaOH, NH4OH) is a well known means of corrosion protection in heating and steam-producing systems. 

Well established corrosion resistance data are available for the most important steels. In aqueous solutions of 0.1 to 20% NaOH the corrosion rate of unalloyed and low-alloy steels at ambient temperature is less than 0.005 mm year". At higher concentrations - 20 to 50% NaOH - a value of 0.01 mm year" will not be exceeded. These steels show a passive behavior because of the formation of a protective layer. The protective layer of magnetite is built up according to Eq. (1-32) magnetite has the lowest solubility of all iron oxides in alkaline solutions. 

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