PROTECTIVE FINISHES

TM 5-5420-279-23

Fig 5.3 Exfoliation Corrosion

(4) 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.

(5) Impact Fractures. Careless handling and bad stacking generally causes these.

(6) Rusting. This occurs where the protective coating on pins has broken down or worn; it

can also contribute to bi-metallic corrosion.

(7) 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.

(8) 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.

(9) 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.

5-8

PROTECTIVE FINISHES

a. 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 of replacement.

b. 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.

c. The materials used to replace the protective finish are listed in the Expendable and Durable

Supplies and Materials List (in RPSTL TM 5-5420-279-23P), and must be applied in accordance

with TM 43-0139, Painting Instructions for Army Materiel.