Page 350 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

328 CONSEQUENCES OF CORROSION

significant role in several military and commercial aircraft incidents and accidents.
For example, fretting corrosion in electronic components is because of the flaking of
tin oxide film from a mated surface on tin-containing contacts. The problem becomes
more serious and frequent as tin is used in place of gold for purely economic reasons
as tin is less costly than gold. The only available solution for this hard-to-diagnose
and often intermittent problem is to replace the defective component.
A problem discovered by an air force corrosion engineer was the corrosion
of tin-plated electrical connector pins mated with gold-plated sockets. Fretting
corrosion between these very small contacts appears to have been implicated in as
many as six F-16 fighter aircraft crashes when their main fuel shutoff valves closed
uncommanded (17).
Microscopic quantities of corrosion product can cause problems in complex elec-
tronic systems. An example of this is the formation of dendrites across circuit chan-
nels. In the presence of moisture and an electric field, metal ions can migrate to a
negatively charged cathodic surface and plate out forming dendrites. The dendrites
can be silver, copper, tin, lead, or a combination of metals. The dendrites can grow
and eventually bridge the gaps between the contacts, causing an electric short and
possibly arcing and fire. Even a small amount of dissolved metal can result in the
formation of a relatively large dendrite.
Dendrite growth can be very rapid. Failures because of dendrite growth have been
known to occur in less than 30 min or can occur in several months or longer. The
rate of growth of a dendrite depends on the applied voltage, the extent and quantity
of contamination, and surface moisture. The amount of contamination required for
silver dendrites can be extremely small.

5.3.7 Failure
A failure may be defined as an unsatisfactory condition or a deviation from the orig-
inal state or condition that is undesirable or unsatisfactory to a particular user or a
context. The determination that a condition is unsatisfactory, however, depends on
the failure consequences in a given operating context (18).
The exact dividing line of demarcation between satisfactory and unsatisfactory
conditions will depend not only on the function of the item in question, but also on the
nature of the equipment in which it is installed and the operating context in which the
equipment is used. Thus the determination will vary from one operating organization
to another. Within a particular organization, however, it is essential that the boundaries
between satisfactory and unsatisfactory conditions be defined for each item in clear
and unambiguous terms.
The judgment that a condition is unsatisfactory implies that there must be some
condition or performance standard on which the judgment is based. However, an
unsatisfactory condition can range from the complete inability of an item to perform
its intended function to some physical evidence that it will soon be unable to function
properly. For maintenance purposes, failures must therefore be further classified as:
(i) functional failures and (ii) potential failures.