Page 58 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

36 INTRODUCTION AND FORMS OF CORROSION

SCC in weldments because of residual stresses introduced during welding is rare.
Brazed joints in aluminum alloys have good resistance to corrosion. Soldered joints
may be useful in milder environments but not in aggressive media (49).

1.5.1.23 Corrosion Resistance of Nickel-Based Alloys There are three types of
nickel-based alloys: (i) CRA; (ii) heat-resistant alloys; and (iii) high-temperature
alloys or super alloys. The corrosion performance can change because of the presence
of a weld seam as a second phase.
Common failures are because of: oxidation, carburization, metal dusting, sulfida-
tion, chlorination, and nitridation. The most common high-temperature degradation
mode is oxidation, and the protection against oxidation is achieved by the formation
of chromium oxide scale. Small amounts of aluminum or silicon may improve the
resistance of chromia alloy. The attack by chlorine and sulfur depends strongly on
the partial pressure of oxygen in the system.
Industrial environments can be oxidizing or reducing in nature in terms of elec-
trode potential that the alloy experiences, which is controlled by the cathodic reaction.
Uniform corrosion occurs under reducing conditions and localized corrosion such as
pitting and crevice corrosion occur under oxidizing conditions. SCC can occur in any
potential range (50).
Corrosion-resistant alloys of nickel are Ni–Cu, Ni–Mo, Ni–Cr–Mo, and Ni–Cr–Fe
alloys. The cast nickel alloys do not have the same degree of corrosion resistance as
the corresponding wrought alloys because of the higher carbon and silicon contents
and the anisotropic structure of the cast alloys (50).
The performance of cast nickel-based alloy depends on the microstructure quality
such as the amount of interdendritic segregation, secondary carbides, and intermetal-
lic phases. The corrosion rate of a nickel alloy can vary by several orders of magnitude
depending on the microstructure. The two most important metallurgical factors are
second-phase precipitation because of thermal instability and the presence of cold
work. Cold work is important where SCC is expected (50).

1.6 MICROBIOLOGICALLY INFLUENCED CORROSION (MIC)

1.6.1 Growth and Metabolism
Growth and metabolism depends on the availability of water as microorganisms take
up the nutrients present in water and produce cell material (51). Under favorable
conditions, some bacteria can double in number every 20 min or less. Thus a single
bacterium can produce a mass of over a million microorganisms in <7 h. The bacteria
∘
can survive from −10 to >100 C, pH ∼0–10.5, dissolved oxygen 0 to saturation,
pressure of vacuum to >31 MPa, and a salinity of ppb to 30%. Most of the bacteria
∘
encountered in corrosion grow best at 15–45 C and pH of 6–8 (41).
A majority of these microorganisms can form extracellular polymeric materials
known as polymer or slime. The slime helps glue the organisms to the surface, trap,
concentrate nutrients as food for microbes and shields the organisms from biocides.