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TO 1-1-700 - Robins Air Force Base

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<strong>TO</strong> 1-1-<strong>700</strong><br />

9.2.4.1.1 Restricted Flaw Openings. The penetrate<br />

inspection process depends upon the ability of the penetrant to<br />

enter and exit the flaw opening. Any factor that interferes with<br />

the entry or exit reduces its effectiveness. Organic coatings,<br />

such as paint, oil, grease, and resin, are examples of this interference.<br />

Any coating that covers or bridges the flaw opening<br />

prevents penetrant entry, and even if it does not cover the<br />

opening, material at the edge of the opening affects the mechanism<br />

of penetrant entry and exit and greatly reduces the reliability<br />

of the inspection. Coatings at the edge of the flaw may<br />

also retain penetrant causing background fluorescence. An<br />

inspection method other than penetrant must be used if the<br />

organic coating cannot be stripped or removed from the surface<br />

in the area to be inspected.<br />

9.2.4.1.2 Smeared Metal. Mechanical operations, such as<br />

abrasive blasting, buffing, wire brushing, grinding, or sanding,<br />

can smear or peen the surface of metals. This mechanical<br />

working closes or reduces the surface opening of any existing<br />

discontinuities or flaws. Mechanical working (smearing or<br />

peening) also occurs during service when parts contact or rub<br />

against each other. Penetrant inspection will not reliably indicate<br />

discontinuities or flaws when it is performed after a<br />

mechanical operation or service that smears or peens the surface.<br />

Chemical etching per procedures in <strong>TO</strong> 33B-1-1 and/or<br />

system specific equipment manual is recommended prior to<br />

penetrant operations to improve test sensitivity when smeared<br />

metal is present.<br />

9.2.4.1.3 Porous Surfaces. Penetrate inspection is impractical<br />

on porous materials with interconnected subsurface<br />

porosity. The penetrant rapidly enters the pores and migrates<br />

through the network. This results in an overall fluorescence or<br />

color that masks any potential discontinuity or flaw indications.<br />

In addition, removal of the penetrant after the inspection<br />

may be impossible. If it is necessary to use an eddy current or<br />

an ultrasonic inspection for an unusual requirement, see <strong>TO</strong> 1-<br />

1-691, Chapter 4.<br />

9.2.5 Coin Tap-Test (Non-Destructive Inspection<br />

Method). The coin tap-test method is a non-destructive<br />

inspection method used to determine delaminations of the<br />

inside surfaces of tactical shelter panels (foam-beam and honeycomb)<br />

along and between the structural members. A tap<br />

hammer (see Chapter 2 of <strong>TO</strong> 35E4-1-162) shall be used in<br />

the inspection. The inspector shall be an experienced tapper.<br />

The tapping procedure consists of tapping lightly over the surface<br />

of the shelter wall. A hollow sound indicates a proper<br />

bond and a dull-thudding sound indicates areas of moisture or<br />

delamination. All shelters shall be non-destructively inspected<br />

on the inside of the shelter panels for corrosion. See <strong>TO</strong> 35E4-<br />

1-162 for field level and depot instructions of the coin-tap<br />

inspection method.<br />

9.3 EVALUATION OF CORROSION DAMAGE.<br />

Visually determine if the corrosion is present in an area which<br />

has previously been reworked. If the corrosion damage is in a<br />

previously reworked area, measure the damage to include the<br />

material which has previously been removed. A straight edge<br />

and a 10X magnifying glass may be used to assist in determining<br />

if an area has previously been reworked. Place the straight<br />

edge across the area being examined at various angles and<br />

check for irregularities, low spots, or depressions (see Figure<br />

4-5 in Chapter 4 of <strong>TO</strong> 1-1-691). If any irregularities, low<br />

spots, or depressions are found, and a visual determination<br />

cannot verify previous rework, closely examine the suspected<br />

area and the surrounding area using the 10X magnifying glass.<br />

After determining that the area has been previously reworked,<br />

evaluate the depth of the previous rework (grind-out) to determine<br />

if further metal removal will exceed grind-out limits,<br />

affect structure integrity, and require replacement of the structure.<br />

Depth measurements can also be made using the depth<br />

gages as described in Chapter 4 of <strong>TO</strong> 1-1-691.<br />

9.4 DEGREES OF CORROSION.<br />

Corrosion must be evaluated after the initial inspection and<br />

cleaning to determine the nature and extent of repair or rework<br />

needed. It is difficult to draw a distinct and specific dividing<br />

line among the degrees of corrosion, so reliable evaluation<br />

requires sound maintenance judgment. Use the following categories<br />

in reporting degrees of corrosion:<br />

9.4.1 Light Corrosion. This degree of corrosion is indicated<br />

by the protective coating being scrapped, gouged, or chipped<br />

to bare metal or showing the tracks of filiform corrosion in the<br />

film and the bare metal showing is characterized by discoloration<br />

of surface corrosion and/or pitting to a depth of approximately<br />

one mil (0.001 inch) maximum. This type of damage<br />

can normally be removed by light hand sanding.<br />

9.4.2 Moderate Corrosion. This degree of corrosion looks<br />

somewhat like light corrosion except some blisters or evidence<br />

of scaling and flaking of the coating or paint system is present,<br />

and the pitting depths may be as deep as 10 mils (0.010 inch).<br />

This type of damage is normally removed by extensive hand<br />

sanding or light mechanical sanding.<br />

9.4.3 Severe Corrosion. The general appearance of this<br />

degree of corrosion is similar to moderate corrosion in the<br />

appearance of the coating system but with severe intergranular<br />

corrosion cracks and blistering exfoliation with scaling or<br />

flaking of the metal surface. The pitting depths are deeper than<br />

10 mils (0.010 inch). This damage must be removed by extensive<br />

mechanical sanding or grinding and may require a patch<br />

type repair or component replacement.<br />

9-3/(9-4 blank)

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