Page 47 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

METALLURGICALLY INFLUENCED CORROSION 25

impedance spectra for pits in stainless steels and magnesium are different from those
of aluminum (18, 29).

1.4.14 The SRET
This technique has enabled the measurement of localized corrosion current densities
in the vicinity of pits in stainless steel in natural water. Novel potentiodynamic pitting
scans were obtained for localized areas adjacent to accurately defined regions of the
electrode surface. The SRET has been applied by Isaacs for studying pitting and IGC
in stainless steels. Scanning vibrating electrode technique has also been proposed for
studying localized corrosion (39). It has also been used in localized measurement
of electrochemical impedance spectra (39). The electrochemistry background of the
method is given in the literature (35).
A new microelectrochemical technique using microcapillaries as electrochemical
cells has been developed. Small surface areas a few micrometers or even nanometers
in diameter are exposed to the electrolyte. This leads to current resolution of the order
of picoamperes. Microelectrochemical techniques combined with statistical approach
of the data evaluation may result in greater understanding of the mechanism involved
in these processes (29).

1.5 METALLURGICALLY INFLUENCED CORROSION

Very pure single crystals have defects that can effect corrosion, but impurities and
alloying elements, grain boundaries, second phases, and inclusions often have serious
effects. Welded structures invariably corrode first at the welds because of metallur-
gical heterogeneities that are present at the welds. It is an obvious fact that the most
susceptible site or defect in a metal will be the first point of attack on exposure to
a corrosive environment. Sometimes such attack can result in innocuous removal of
susceptible material leaving a surface with improved corrosion resistance (4).
Metallurgically influenced corrosion is because of chemical composition (such
as alloying elements, metalloids, and impurities), metallurgical properties (metallic
phases, grain joints), and fabrication procedures (thermal treatments, lamination, and
welding). Metallurgically influenced corrosion consisting of (i) weld decay; (ii) deal-
loying; (iii) exfoliation; and (iv) internal attack is shown in Figure 1.11.
The selective corrosion of cast iron known as graphitization, the preferential cor-
rosion of the steel welding known as grooving corrosion, sensitization and knife-line
attack of welded stainless steels are typical examples of corrosion influenced by met-
allurgical parameters.

1.5.1 The Influence of Metallurgical Properties in Aqueous Media
1.5.1.1 Chemical Composition and Microstructure When an alloy composed
of various elements corrodes, usually one or more elements dissolve preferentially