R 19 mm 0.74 in
2. Pastille dia. 30 mm sur antistatique recouverte de finition pour controle eventuel de la couche antistatique.
1.18 in dia. finish-painted masking patch placed over anti-static coat for subsequent inspection.
Finish-painted masking patches after application of anti-static coat on wing tips
Figure 204
rty : S / N 1 - 9999 A alidi 20-00-03Page 222
(BA) DEC 98
METAL TREATMENT AND PROTECTION
DESCRIPTION AND OPERATION
1. GENERAL
The purpose of metal treatment and protection is to substract the various aircraft components to agression caused by agents that may produce a change in the mechanical properties of the metal.
Any repair process which breaks the surface of original structure requires a treatment, and protection as required, before assembly.
Corrosion implies alteration against which equipment should be protected and the rate of which should be checked (preventive maintenance) so as to be remedied if possible in order to keep the quality of equipment above a minimum allowable limit set for considerations of flight safety and operation cost—effectiveness (reconditioning).
CAUTION : INSUFFICIENT PREVENTIVE CHECKS OF CORROSION ATTACK ON STRUCTURAL MEMBERS MAY NOT ONLY REQUIRE EXPENSIVE REPAIRS BUT ALSO AIRCRAFT DOWNTIME FOR SEVERAL MONTHS, AND ABOVE ALL QUESTION THE SAFETY OF FLIGHTS.
This section describes corrosion so that maintenance personnel can identify the various types of corrosion and apply preventive measures to minimize corrosion growth.
Further information about corrosion control is available in FAA Advisory Circulars AC 43-4A "Corrosion Control for Aircraft" and AC 43.13-1B "Acceptable Methods, Techniques and Practices — Aircraft Inspection and Repair".
2. CORROSION GROWTH
Metals corrode by direct chemical or electrochemical reaction with their environment. The steps listed below describe electrochemical reaction.
Electrochemical type of corrosion can best be compared to a battery cell. Four conditions must exist before electrochemical corrosion can occur.
— There must be a metal that corrodes and acts as the anode.
— There must be a less corrodable metal that acts as the cathode.
— There must be a continuous liquid path between the two metals which acts as the electrolyte, usually condensation and salt or other contaminations.
— There must be a conductor to carry the flow of electrons from the cathode to the anode. This conductor is usually in the form of a metal—to—metal contact (rivets, bolts or welds).
The elimination of any one of the four conditions described above will stop the corrosion reaction process.
One of the best ways to eliminate one of the four described conditions is to apply an organic film (such as paint, grease or plastic) to the surface of the metal affected. This will prevent the electrolyte from connecting the cathode to the anode and prevent corrosion reaction as current cannot flow.
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