Page 360 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

338 CONSEQUENCES OF CORROSION

TABLE 5.7 Failure Modes in 2002
Failures (%)

Mode Engineering Components Aircraft Components
Corrosion 29 16
Fatigue 25 55
Overload 11 14
High-temperature corrosion 7 2
SCC/Corrosion fatigue/hydrogen 6 7
embrittlement
Wear/abrasion/erosion 3 6
Brittle fracture 16
Creep 3

in the closed position. Stainless steel shims were fitted between catches and the
aluminum plate.
The outer surface of the panel and door after stripping paint and removing the
catches is shown in Figure 5.13b. There appears to be no damage to the plate and the
door. The examination of the inner surface showed extensive exfoliation corrosion on
the panel and the door in the catch positions (Fig. 5.14). Cracking originating from the
catch position was observed in the stiffening rib. A cross section of plate containing
an extensively corroded area is shown in Figure 5.15.
Exfoliation corrosion occurs when the attack occurs along the grain boundaries, in
particular, when they are elongated and form thin platelets. The voluminous corrosion
product causes splitting of layers of uncorroded material.
The material conformed to the specification, and the aluminum alloy is known to
be subject to exfoliation corrosion. The corrosion was extensive at the catch posi-
tion, which is attributed to the stainless steel shims fitted below the catches. The
paint between aluminum and stainless steel shims deteriorated, resulting in galvanic
corrosion with the stainless steel acting as the noble metal.

Example 5.3 A bolt from an aircraft flap control unit fractured in the threaded
region of the shank near the shoulder with the head on installation after a major
service. The bolt was made from cadmium-plated high-strength steel. The bolt con-
formed to the specifications and had ultimate tensile strength of ∼1400 MPa.
SEM examination showed ductile features (Fig. 5.16) on the center of the bolt and
intergranular features on the outer circumference (Fig. 5.17). Both sides of cracking
were caused by static overload failure with the ductile features at the center present
throughout. The intergranular appearance at the edge is suggestive of embrittlement,
leading to premature failure at loads below those expected.
The embrittlement is attributed to cadmium plating (21) on the bolts applied to
protect them from corrosion. During cadmium plating, hydrogen is absorbed by steel,
and cadmium acts as a barrier for the escape of hydrogen. In high-strength steels