Page 352 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

330 CONSEQUENCES OF CORROSION

1. Safety-related issues
2. Operational problems
3. Nonoperational consequences
4. Hidden failure consequences.

The most important consideration in the evaluation of corrosion-related failure
is safety, that is, whether the failure causes a loss of life or of function or secondary
damage that could have adverse effect on operating safety. A critical failure is any fail-
ure that could affect adversely the safety of operation of both the equipment and the
operating personnel. The term, direct effect, implies certain limitations. The impact
of the failure must be immediate if it is to be considered direct. Further, the conse-
quences must result from a single failure and not from a combination of the failure
with another that is yet to occur. If a failure has no resultant effect on the system, it
cannot by definition have a direct effect on safety.
It is to be noted that not every critical failure results in an accident. However,
the question is not whether such consequences are inevitable, but whether they are
possible. Safety considerations are conservative and rigid and they are assessed at the
most conservative level. In the absence of proof that a failure cannot affect safety, it
is considered by default as a critical issue that requires immediate consideration.
When possible critical failure is envisaged, it is imperative that all possible effort
must be made to prevent its occurrence. Quite often, redesigning of one or more
vulnerable items is all that is required to avoid potential failure. However, the design
and manufacture of new parts and their subsequent use in service equipment can take
a long time, of the order of a few months, and sometimes as much as years. Thus
temporary measures to rectify the problem are often required.
Once the safety consequences are ruled out, attention turns toward the next set
of consequences such as the effect of the failure on the operational capability of the
system without any difficulty. A failure has operational consequences whenever a
need to correct the failure disrupts the planned operations. The operational conse-
quences consist of the need to abort an operation after a failure occurs, the delay or
cancellation of other usual operations to make unanticipated repairs, or the need for
operational limitations until the necessary repairs are made. A critical failure may be
viewed as a special case of a failure with operational consequences.
In such a case, the consequences are those of economic in nature and consist of
imputed cost of lost operational function. Nonoperational consequences consist of
many kinds of failures that have no direct adverse effect on the operational capability.
This is illustrated by the failure of a navigational unit in a plane equipped with a
highly redundant navigation system. As other units ensure availability of the required
function, the failed unit can be replaced at some convenient time. Thus the costs,
because of such a failure, are limited to the cost of corrective maintenance.
Hidden failures comprise another important class of failures with no immediate
consequences as the failures of hidden function items are responsible for the failures.
By definition, hidden failures have no direct adverse effects, that is, if they did, these
failures would not be hidden. However, the ultimate consequences of hidden failure