Page 412 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

390 CONSEQUENCES OF CORROSION

Pattern C. Shows gradually increasing failure probability with no wear-out region.
Pattern D. Starts with low failure probability followed by an increase to constant
level.
Pattern E. Shows a constant probability of failure.
Pattern F. Shows initial high mortality followed by constant or very slowly
increasing failure probability. This pattern is particularly applicable to elec-
tronic equipment. The six patterns of failure (18) are depicted in Roberge 16,
Fig. 2.5, p. 43.

The probability of failure depends on two factors, namely: (i) the specific form
of corrosion and its rate; (ii) the possible effectiveness of corrosion inspection or
monitoring.
The input of corrosion experts is useful in identifying the forms of corrosion and
the key factors affecting the propagation of the corrosion rate. The ranking of pro-
cess equipment may be done by internal probability of failure analysis. This method
consists of analysis of equipment process and inspection parameters, and then rank-
ing the equipment on a scale of one to three with “one” as the highest priority. This
procedure requires a fair degree of engineering judgment and experience.
The probability of failure approach is based on a set of rules that are dependent
on inspection reports/histories, knowledge of corrosion processes, and knowledge
of normal and upset conditions. The equipment rankings may change with time and
require periodic updating as a result of gaining additional knowledge, changes in pro-
cess conditions, and aging of equipment. Maximum benefits of the procedure depend
on a fixed equipment inspection schedule that allows the capture, documentation,
retrieval of inspection, maintenance, and corrosion failure mechanism information.
The probability of detection (POD) concept has been known as an important
concept since 1973 and has been used in the design of engineering instrumentation
by NASA. The POD concept and methodology are widely used in nondestructive
evaluation(NDE)methods,inparticular,“fracturecontrol”ofengineeringhardwareand
systems.

5.5 HISTORICAL IMPLICATIONS OF CORROSION

In 1761, the 32-gun frigate – HMS Alarm – had its hull covered with a thin cop-
per sheathing to reduce the damage caused by the wood-boring shipworm and the
speed-killing barnacle growth on ship hulls. After 2 years of operation, the sheathing
became detached from the hull in many places because the iron nails used to fas-
ten the copper to the timbers were corroded to such an extent that the corroded iron
nails were hardly visible. Closer inspection showed that the iron nails wrapped in
brown paper remained intact. This observation shows that iron should not be in con-
tact with copper in a seawater environment if severe corrosion is to be avoided. This
type of corrosion by two dissimilar metals in contact is known as galvanic corrosion
or bimetallic or dissimilar metal corrosion.