6-18 SECTION 3. MECHANICAL CORROSION REMOVAL BY BLASTING
6.26 General. Abrasive blasting is a cleaning or finishing process which directs a stream of abrasive particles against a surface, and can be used with metals, plastics, and other materials. Abrasive blasting can remove rust and corrosion and can clean a surface before further processing, such as painting or plating. Use standard blast cleaning practices, to include the following:
• Use any form of blast cleaning equipment, but in-cabinet blasting is preferred.
• Use external gun blasting if adequate abrasive confinement and recovery exists.
6.27 Safety Precautions. Operators should adequately protect themselves with complete face and head covering equipment, and use pure breathing air. Magnesium creates a fire hazard when abrasive blasted. Dry abrasive blasting of titanium alloys and high tensile strength steel creates sparking. Take care that no hazardous concentration of flammable vapors exists. Static ground the dry abrasive blaster and the material to be blasted.
Note: Refer to the OEM maintenance manual for abrasive blasting guidance.
6.27.1 Remove the part from the aircraft for blast cleaning, if possible. Otherwise, mask or protect areas adjacent to the part against abrasive impingement and system contamination. Examples are hydraulic, oil, or fuel systems.
6.27.2 Parts should be clean and dry prior to blast cleaning.
6.27.3 Mask close-tolerance surfaces, such as bushings or bearing shafts.
6.27.4 Blast clean only enough to remove corrosion coating. Proceed immediately with finishing requirement using surface treatments, as required.
6.28–6.39. RESERVED.
6-19 SECTION 4. CORROSION DAMAGE AND REWORK LIMITS
6.40 Discussion. Evaluate corrosion after general inspection and cleaning to determine the nature and extent of repair or rework. Local blending of corroded areas may be required to determine the total extent of the corrosion problem. See Figures 6-1, 6-2, 6-3, and 6-4.
Corrosion damage classifications are defined as follows:
6.40.1 C orrosion Level 1:
1. Damage occurring between successive inspections that is within allowable damage limits;
2. Damage occurring between successive inspections that does not require structural reinforcements, replacement, or new damage tolerance-based inspections;
3. Corrosion occurring between successive inspections that exceeds allowable limits but can be attributed to an event not typical of operator usage of other aircraft in the same fleet; or
4. Light corrosion occurring repeatedly between inspections that eventually requires structural reinforcements, replacement, or new damage tolerance-based inspections.
6.40.2 Corrosion Level 2:
1. Corrosion occurring between any of the two successive corrosion inspection tasks that requires a single rework or blend-out which exceeds the allowable limit; or
2. Corrosion occurring between successive inspections that is widespread and requires a single blend-out approaching the allowable rework limit, i.e., it is not light corrosion as provided for in Corrosion Level 1, definition 3.
6.40.3 Corrosion Level 3: Corrosion occurring during the first or subsequent accomplishments of a corrosion inspection task that the operator determines to be an urgent airworthiness concern.
6-20 Figure 6-8. Severe Corrosion Requiring Trimming Out of Damaged Area Fi gure 6-9. Corrosion at Fastener Holes Removed by Spot Facing
6.41 Removal of Corrosion. Two basic methods of corrosion removal include mechanical and chemical removal. The method used depends upon the type and location of a structure, the type and severity of corrosion, and the availability of maintenance equipment. Mechanical methods of corrosion removal are the most commonly used, and include sanding, buffing, grinding, and section removal. Avoid the use of soft metal wire
6-21 brushes (with copper alloys in the wires) because residual traces of copper on cleaned metal will contribute to future corrosion. If using brushes, they should be stainless steel or nonmetallic. Mechanical methods of corrosion removal are used on all three levels (light, moderate, and severe) of corrosion damage. However, use chemical methods of corrosion removal only with light corrosion and only in areas where the chemicals cannot migrate to other areas.
6.42 Determining Degree of Corrosion Damage. Use the following equipment to decide to measure corrosion damage: depth dial gauge, straight edge, or a molding compound.
Corrosion must be removed before the corroded area can be measured. Before making measurements, see if corrosion is in a reworked area which has previously been repaired.
If corrosion is in a recess of a faired or blended area, measure damage to include the material which has previously been reworked. The following method outlines the procedure for taking measurements with the depth gauge:
1. Remove loose corrosion products, if present.
2. Position depth gauge, as illustrated in Figure 6-10, Corrosion Damage and Rework Measurement Using Depth Dial Gauge, and determine the measurement reading.
Note: T he base of the depth gauge should be flat against the undamaged surface on each side of the corrosion. When taking measurements on concave or convex surfaces, place the base perpendicular to the radius of the surface, as shown in Figure 6-10.
3. Take several additional depth readings.
4. Select the deepest reading as the depth of the corrosion damage.
6.43 Determining Rework Limits. The allowable damage limit chart of the manufacturer’s repair manual documents the maximum amount of material that can be removed from any damaged surface. If no criteria are given, contact the aircraft manufacturer for cleanup limits.
6.44 Determining Material Thickness Reduction After Corrosion Cleanup. Allowable material removal from a part or panel during corrosion cleanup is usually in the manufacturer’s allowable damage limit charts. An accurate measurement should be made of the material removed or material thickness remaining in the reworked area to prevent exceeding allowable limits.
6.44.1 Ultrasonic testers can measure panel thickness after rework. This method requires a qualified nondestructive inspection (NDI) operator and suitable test standards for calibration.
6.44.2 Measurement of the material removed, which is the depth of blended pits, can be made using a depth dial gauge (see Figure 6-10). If the depth dial gauge will not work, clay impressions, liquid rubber, or other similar means of getting accurate results may be used to determine material removed. In the event that material removal limits have been
exceeded, the area or part should be repaired or replaced. If replacement or repair criteria is not contained in the repair manual, contact the manufacturer.
Figure 6-10. Corrosion Damage and Rework Measurement Using Depth Dial Gauge
ADJUSTABLE
DIAL INDICATOR MARKED DIAL FACE LOCK N INCREMENTS OF 0.000€ BASE POINT "A" POINT "B"
CORROSION PIT POINT "C" CONICAL POINT NOTE
• DEPTH DIAL GAUGE O TO 0.500 INCH RANGE. DETAIL I
• TAKE MEASUREMENT READINGS PER-
PENDICULAR TO THE RADIUS OF CONCAVE OR CONVEX SURFACES. SEE DETAIL I. TAKE READINGS AT VARIOUS POINT!
• TAKE MEASUREMENT READINGS ON
EDGES ADJACENT TO CORFOSION
DAMAGE OR BLENDED DEPRESSION (POINTS "A" AND "B"). TARD
SEVERAL READINGS AROUND CENTEF
OF CORROSION DAMAGE OR BLENDED DETAIL II
DEPRESSION (SEE DETAIL II) AND
CONSIDER THE DEEFEST READING TO BE POINT "C." DETERMINE DEPTH AS FOLLOWS:
DEPTH - C - A + B)
6.45-6.49. RESERVED.
6-22
6-23 SECTION 5. ALUMINUM AND ALUMINUM ALLOYS
6.50 Treatment. In general, corrosion of aluminum can be treated more effectively in place than by removing structural parts from the aircraft. Treatment includes the mechanical removal of the corrosion products, the inhibition of residual materials by chemical means, and the restoration of permanent surface coating. Details of treatment vary depending on whether the aluminum surfaces are bare in use or protected by paint coatings.
6.51 Processing of Aluminum Surfaces.
6.51.1 Bare Aluminum Surfaces. While few unpainted aircraft operate in maritime conditions, we include general information on the nature of Alclad surfaces and their treatment. Pure aluminum has greater corrosion resistance than the stronger aluminum alloys.
Manufacturers usually take advantage of this by laminating a thin sheet of relatively pure aluminum, 1 to 5 mils thick, over the base higher-strength aluminum alloy surface.
Corrosion protection is good, and operators can maintain the Alclad surface in a polished condition (see Figure 6-12, Polishing and Brightening Alclad Surfaces). In cleaning such surfaces, however, take care to prevent staining and marring of the exposed aluminum.
More importantly, from a protection standpoint, avoid unnecessary mechanical removal of the protective Alclad layer and exposing the more susceptible base material.
6.51.2 A dditional Processing of Aluminum Surfaces Prior to Paint Finishes . Before applying paint to aluminum surfaces, more severe cleaning procedures and more thorough corrective treatments can be used before painting. Application of a paint finish requires proper prepaint treatment for good paint adhesion.
6.51.3 S pecial Treatment of Anodized Surfaces. Anodizing is the most common surface treatment of aluminum alloy surfaces. Manufactured or reworked parts or components undergo tank processing during and frequently before fabrication from sheet stock. The aluminum sheet or casting becomes a positive pole in an electrolytic bath, in which chromic acid or similar oxidizing agent produces a supplemental protective oxide film on the aluminum surface. Aluminum oxide is naturally protective, and anodizing merely increases the thickness and density of the natural oxide film. When this coating is damaged in service, it can only be partially restored by chemical surface treatment, so avoid unnecessary destruction of the oxide film while processing anodized surfaces.
6.51.3.1 Do not use steel wool, steel wire brushes, copper alloy brushes, or severe abrasive materials on any aluminum surface. Aluminum wool, fiber bristle brushes, and mild abrasives are acceptable tools for cleaning anodized surfaces, but exercise care in any cleaning process to avoid unnecessary breaking of the protective film, particularly at the edges of the aluminum sheet. See Figures 6-11, Removing Oil and Surface Dirt, and 6-13, Cleaning and Stripping Paint.
6.51.3.2 Tampico fiber brushes are preferred and are adequate to remove most corrosion. Producing a buffed or wire brush finish by any means should be prohibited. Take every precaution to maintain as much of the protective
6-24 coating as practicable. Otherwise, treat anodized surfaces in the same manner as other aluminum finishes. Vacuum blasting is an acceptable corrosion removal method to remove surface corrosion. Vacuum blasting should not be used to remove intergranular corrosion.
Figure 6-11. Removing Oil and Surface Dirt Fig ure 6-12. Polishing and Brightening Alclad Surfaces
6-25 Figure 6-13. Cleaning and Stripping Paint
6.51.3.3 C hemical conversion coating, detailed by Military Specification (MIL-SPEC) MIL-C-81706, Chemical Conversion Materials for Coating Aluminum and Aluminum Alloys, is a chemical surface treatment used on aluminum alloys to provide a proper surface for paint finishing.
6.51.4 Special Processing of Intergranular Corrosion in Heat-Tre ated Aluminum Alloy Surfaces.
Intergranular corrosion is usually more severe in heat-treated aluminum alloys and exhibits a corrosive attack along grain boundaries of the alloyed aluminum, where grain boundaries differ from the metal within the grain. When in contact with an electrolyte, rapid corrosion occurs at the grain boundaries. In its most severe form, actual lifting of metal layers, or exfoliation, occurs. Mechanically remove all corrosion products and visible delaminated metal layers to determine the extent of damage and evaluate the remaining structural strength of the component. NDI inspection is necessary to determine the full extent of corrosion. For aluminum, eddy current NDI mapping is used. For stainless steel, penetrant or magnetic particle NDI is used.
6.51.4.1 Use m etal scrapers, rotary files, or abrasive wheels to remove all corrosion products and retain only structurally sound aluminum.
Note: A rotary file is recommended only for severe intergranular or exfoliation corrosion removal.
6.51.4.2 Rotary files should be sharp to cut metal without excessive smearing. A dull cutting tool will smear the metal over corrosion cracks or fissures, giving the appearance that it has removed the corrosion, but has not.
6.51.4.3 Use carbide-tip rotary files or metal scrapers, since they stay sharp longer.
Do not use abrasive blasting to remove intergranular corrosion.
6-26
6.51.4.4 Inspection with a 5X to 10X magnifying glass or using a dye penetrant will help determine if all unsound metal and corrosion products are removed.
6.51.4.5 When corrosion is completely removed, blend or fair using a length-to-depth ratio of 20:1 in the area of corrosion removal. Blend where required, by using aluminum oxide impregnated, rubber-base wheels.
6.51.4.6 Chemical conversion coat exposed surfaces completely and restore paint coatings in the same manner as on any other aluminum surface. See Figures 6-14, Cleaning and Inhibiting Corroded Aluminum Surfaces;
6-15, Removing Excess Inhibitor Solution; and 6-16, Applying Wax to Cleaned Surfaces.
6.51.4.7 C orrosion damage beyond SRM limits should be repaired per a cognizant engineer or aircraft manufacturer’s FAA-approved instructions.
Figure 6-14. Cleaning and Inhibiting Corroded Aluminum Surfaces
6-27 Figure 6-15. Removing Excess Inhibitor Solution Fi gure 6-16. Applying Wax to Cleaned Surfaces
6.52 Repair of Aluminum Alloy Sheet Metal. After extensive corrosion removal, follow the below procedures:
1. If water can be trapped in blended areas, chemical conversion coat and fill the area with structural adhesive or sealant to the same level and contour as the original skin.
When areas are small enough that structural strength has not been significantly decreased, proceed to applying the protective finish.
2. When corrosion removal exceeds SRM limits, contact a cognizant engineer or the aircraft manufacturer for repair instructions.
3. Where exterior doublers exist, seal and insulate them adequately to prevent further corrosion.
6-28
4. Doublers should be made from Alclad, when available, and the sheet should be anodized (this is preferred) or chemical conversion coated after finishing all cutting, drilling, and countersinking.
5. Drill, countersink, surface treat, and prime all rivet holes prior to installation of the doubler.
6. Apply a suitable sealing compound in the area to be covered by the doubler.
Apply sufficient thickness of sealing compound to fill all voids in the area being repaired.
7. Install rivets wet with sealant. Sufficient sealant should be squeezed out into holes so that all fasteners, as well as all edges of the repair plate, will be sealed against entrance of moisture.
8. Remove all excess sealant after fasteners are installed. Apply a fillet sealant bead around the edge of the repair. After the sealant has cured, apply a protective paint finish to the reworked area.
6.53 Corrosion Removal Around Countersunk Fasteners in Aluminum Alloy. In aluminum alloys, intergranular corrosion often originates at countersunk areas where steel fasteners are used. Corrosion removal in a countersink is impossible with the fastener in place.
6.53.1 When corrosion is found around a fixed fastener head, remove the fastener to ensure corrosion removal, prevent further corrosion, and prevent loss of structural strength. To reduce the reoccurrence of corrosion, the panel should receive a chemical conversion coating, be primed, and have the fasteners installed wet with sealant.
6.53.2 Each time removable steel fasteners are removed from access panels, inspect them for material condition, including the condition of the plating. If mechanical or plating damage is evident, replace the fastener. Upon installation, follow one of the below fastener installation methods:
6.53.2.1 A preferred method is to brush a corrosion preventive compound on the substructure around and in the fastener hole, start the fastener, apply a bead of sealant to the fastener countersink, then set and torque the fastener within the working time of the sealant;
6.53.2.2 Apply the corrosion preventive compound to the substructure and fastener, set and torque the fastener; or
6.53.2.3 Apply a coating of primer to the fastener, and, while wet with primer, set and torque the fastener.
6.54 Examples of Removing Corrosion from Aluminum and Aluminum Alloys.
6.54.1 P repare the Aircraft. Prepare the aircraft for corrosion rework, as provided in paragraph 6.12, and remove corrosion products as follows. Observe the work procedures of paragraph 6.2.
6-29
6.54.2 Id entify the Metal. Positively identify the metal as aluminum.
6.54.3 Clean the Area. Clean the area to be reworked. Strip paint, if required.
6.54.4 Determine the Extent of the Corrosion. Determine extent of corrosion damage, as covered in paragraph 6.42. To remove light corrosion, proceed with paragraph 6.54.5. To remove moderate or severe corrosion, proceed with paragraph 6.54.6.
6.54.5 Light Corrosion Removal. Do not use the chemical removal process at temperatures above 100 °F or below 40 °F. Remove light corrosion by hand rubbing the corroded surface with tools, abrasives, or by chemical means, as follows:
6.54.5.1 P rotectively mask adjacent areas to prevent brighteners from contacting magnesium, anodized aluminum, glass, Plexiglas, fabric surfaces, and all steel. Wear acid-resistant gloves, protective mask, and protective clothing when working with corrosion removing compounds. If corrosion removing compounds accidentally contact the skin or eyes, flush off immediately with plenty of clear water. See the safety procedures in Section 1, Safety Procedures.
6.54.5.2 D ilute a corrosion removing compound (SAE AMS1640, Corrosion Removing Compound for Aircraft Surfaces, Type I) with an equal volume of water. Mix the compound only in wood, plastic, or plastic-lined containers.
Apply the diluted solution of corrosion removing compound by flowing, mopping, sponging, brushing, or wiping.
6.54.5.3 A pply the diluted solution to large areas with a circular motion to disturb the surface film and ensure proper coverage. The diluted solution should be applied starting at the lowest area and working upwards. The solution will be more effective if applied warm, 140 °F maximum, followed by vigorous agitation with a nonmetallic, acid resistant brush or aluminum oxide abrasive nylon mat.
6.54.5.4 Leave the solution on the surface for about 12 minutes. Do not let the solution dry on the surface; streaking will result. On large exterior surfaces, remove solution by high-pressure water rinse.
6.54.5.5 Wipe off solution with a clean, moist cloth. Frequently rinse the cloth in clear water. Wipe the area several times with a fresh cloth, dampened and rinsed frequently in clear water.
6.54.5.6 Dry the area with a clean, dry cloth and inspect for corrosion.
6.54.5.7 Repeat the procedure outlined in paragraph 6.54.5 if any corrosion remains.
Note: If corrosion still remains after the second attempt, mechanically remove corrosion, as detailed in paragraph 6.54.6.