ARMY TM 5-5420-212-23
MARINE CORPS TM 08676A-23/2
FIGURE 2-3 Exfoliation corrosion
Fatigue Cracking. This type of cracking is unlikely to appear, but may occur in indicator outer jaws where a
highly stressed component is approaching the end of its useful life.
Impact Fractures. These are generally caused by careless handling and bad stacking.
Rusting. This occurs where the protective coating on pins has broken down or worn; it can also contribute
to bi-metallic corrosion.
Pin Hole Wear. This is the result of frequent building with dirty panel pins. In extreme cases fatigue
cracking may occur through excessively loose pins.
Distortion. This is evident especially as elongation of components, this may occur during the later stages
of bridge life. Excessive loads or accident damage can also cause distortion. All these may result in the loss of
interchangeability of parts.
Weld Cracking. This is usually caused by weld defects and internal stresses. The incidence of cracking is
likely to decrease with component usage or aging.
2-8 PROTECTIVE FINISHES
Protective finishes are not indestructible, but they are replaceable. Even when properly applied on well prepared
surfaces they will gradually deteriorate and eventually fail, although the rate of deterioration is slowed when the right
procedures are carried out. The life of a paint system depends on the correct use of appropriate materials skillfully
applied and dried under controlled conditions. Inspectors and persons responsible for the condition of protective finishes
must be familiar with the signs of various stages and types of deterioration and be able to determine the time and extent
It is important to avoid delay in repairing paint over protective metal coatings. A broken down paint film retains
water and can cause rapid and serious corrosion of a metal coating beneath it. In most cases the most economical policy
is to make regular inspections of the structure and to start re-painting as soon as the first signs of paint breakdown
become evident. This breakdown, in order of increasing seriousness, generally takes the form of chalking, cracking,
blistering and rust staining. As a rule breakdown should not be allowed to proceed beyond the chalking stage because at
this point the only preparation required before painting is washing down and drying off.