Environmental Engineering Reference
In-Depth Information
tion in high-strength aluminum alloys in marine and industrial environments
causes some blocks of grains to lift up from the metal surface because of the
pressure of the voluminous corrosion product. Such attack is traditionally referred
to as
exfoliation corrosion
. Exfoliation may be minimized by the use of extended
aging cycles for Al-Cu alloys, use of organic and sprayed metal coatings, by
avoiding graphite-bearing lubricants that act as cathode, and by promoting an
equiaxed grain structure at the surface or throughout the alloy.
Titanium and some of its alloys show intergranular corrosion in red fuming
nitric acid at room temperature. Addition of 1% NaBr inhibits attack. Commer-
cially pure titanium is intergranularly attacked in methanol-halide solutions. A
small addition of water acts as inhibitor.
3.6.5 Practical Examples
The brake pedal support plate made of Al-8Si-3.5Cu alloy from an aircraft refuel-
ing tanker truck fractured in service [9]. The contaminating liquid was found to be
alkaline, containing borax. Metallographic examination revealed an intergranular
attack along the needle-like copper-rich phase (Fig. 3.22).
Figure 3.23 shows two impellers made of 53Ni-16Cr-16Mo alloy put in ser-
vice in 25% hydrochloric acid with 100 ppm chlorine at ambient temperature
[6]. The impeller on the left was solution-annealed and the one on the right was
sensitized before being placed in service. The sensitized one shows severe inter-
granular corrosion with grain dropping occurring at the tip of the blades.
Figure 3.22
Intergranular corrosion along the needle-like copper-rich phase of an Al-
Si-Cu alloy casting.
