Corrosion rate determination chromium

In this oxidizing environment the corrosion rate of unsensitized, properly annealed material is determined primarily by the chromium content. Thus, the base rate for alloy C-276 (UNS N10276) with lower chromium content, for example, is higher than the other alloys. The relative suitability of an alloy for a partieular application is sometimes mistakenly related to the annealed base corrosion rate in the IGA test. This concept is an error. The test is designed to flag out improperly produced material exhibiting a rate above the normal base corrosion rate (Table 2), not a rate higher than some other alloy. 

Localized corrosion propagation rates are highly variable and not normally measured unless the time to initiation of attack is determined visually or by electrochemical monitoring. Pits in nickel alloys such as alloy 400 (UNS N04400), which are usually of the broad and shallow variety, tend to propagate much more slowly than the narrow undercut types found in chromium-bearing nickel alloys. Electrochemical tests are useful to predict pit initiation, while immersion tests evaluate initiation and also provide some indication of the possibility of severe pit propagation. 

Other common transition metal corrosion products typically monitored at various sites within the plant include iron, copper, nickel, zinc, and chromium. More than 80% of BWR plants analyze for iron, nickel, copper, and zinc in reactor water, and nearly all of the BWR plants determine these metals in feed water. In addition, zinc is also an additive used in many plants to control the shutdown radiation dose rate. Nickel and chromium are corrosion products in BWR plants fi-om stainless-steel piping. The best selectivity and sensitivity for achieving low to submicrogram/Liter detection limits for transition metals can be obtained by separating transition metal complexes using pyridine-2, 6-dicarboxylic acid (PDCA) or oxalic acid as chelators in the eluent, followed by postcolumn derivatization with 4-(2-pyridylazo)resorcinol (PAR) and absorbance detection at 520 nm (see Section 8.2.1.2). This approach was successfully used to determine trace concentrations of iron, copper, nickel, and zinc in BWR and PWR matrices [197]. Figure 10.113 compares the chromatograms from the... 

These materials are passive in seawater and, up to temperatures of 333 K (60 °C), practically resistant to uniform surface corrosion. The resistance to pitting and crevice corrosion and the critical pitting and crevice corrosion temperatures are determined mainly by the content levels of chromium and molybdenum and therefore increase with the pitting resistance equivalent. The materials listed in the Table are resistant to stress corrosion cracking in seawater, even at higher temperatures, and less sensitive to corrosion fatigue. They are also suitable for use at higher flow rates up to about 20 m/s [23]. 

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