Aircra& Clean & Corrosion Controls Chapter 6 Iron Mine Bauxite Mine Return to Natural State •  Elements want to return to their natural state !
Pure metals don’t exist in nature •  Corrosion is the metal returning to its natural state Return to Natural State Return to Natural State Return to Natural State Return to Natural State Return to Natural State Corrosion •  Corrosion -­‐ an electrochemical acCon on metals that changes some of the metal into its salts Called galvanic corrosion !  Dissimilar metals creaCng a baHery !
•  Cathode – the negaCve side of a baHery or circuit !
The electrons leave an cathode of a baHery •  Anode – the posiCve side of a baHery or circuit !
The electrons enter the anode of a baHery •  Electrolyte -­‐ A chemical, either a liquid or a gas, which conducts electrical current by releasing ions that unite with ions on the electrodes. Corrosion •  Ion -­‐ an atom of a chemical element that does not have the same number of electrons spinning around it as there are protons in its nucleus. A posiCve ion has fewer electrons than protons, and a negaCve ion has more electrons than protons. !  An ion is electrically charged either posiCve or negaCve !  An ion will aHract an oppositely charged ion !
•  Electrolytes enable ions to move and find their opposite Corrosion •  BaHery is made up of: !
Anode •  PosiCve side/electrode !
Cathode •  NegaCve side/electrode !
Electrolyte •  The chemical soluCon enables electrons to move •  If a baHery is created, corrosion will start BaHery When electrons leave the zinc can, chloride ions from the electrolyte replace them and change some of the zinc into zinc chloride, a salt of corrosion.
Corrosion Requirements •  There must be an electrical potenCal difference (a voltage difference) within the metal. •  There must be a conducCve path between the areas of potenCal difference. •  There must be some form of electrolyte, or electrically conducCve liquid or gas, covering the areas of potenCal difference. Rain contains pollutants that create an electrolyte !  Salt water is an excellent electrolyte !  CausCc cleaners are electrolytes !
Prevent Corrosion •  We can minimize the electrical potenCal difference that exists within the metal. !
Separate dissimilar metals •  We can eliminate the conducCve path by placing an insulaCng barrier between the areas of electrical potenCal difference. !
PlasCc or rubber insulators Prevent Corrosion •  We can cover the surface of the metal with some form of oxide or organic coaCng to keep the conducCve electrolyte from covering the areas in the metal that have a difference in potenCal. Paint !  Oil !  Clad cover !
•  Summary: No electron flow; No corrosion Prevent Corrosion Chemically AcCve Table Galvanic Series of Metals Aluminum Corrosion •  Pure aluminum protects itself by forming a passive layer of aluminum oxide •  Aluminum oxide is a white powder that acts as an insulator Aluminum Corrosion •  Pure aluminum is very weak •  Aluminum alloys are used to gain strength Copper, zinc, manganese and magnesium !  The dissimilar metals lead to corrosion !
•  A clad layer of pure aluminum is added to prevent corrosion !
The clad layer forms a layer of aluminum oxide insulator Return to Natural State Cathodic ProtecCon •  Also called sacrificial (offered for destrucCons) corrosion or sacrificial layer •  A sacrificial material that is more acCve than the material being protected is aHached to or plated on the material. The sacrificial material corrodes, while the part is being protected. !
Zinc plaCng is a good example •  The sacrificial material is more anodic !
It corrodes •  The part is more cathodic !
It is protected Corrosion Appearance of corrosion on various metals.
Corrosion Types •  Surface corrosion White powder shows up on the surface !  Usually not a structural problem !  Time to take acCon !
•  Remove corrosion •  Apply primer Surface Corrosion Corrosion Types •  Intergranular Corrosion Cause by improper metal treatment !  Corrosion has reach inside the metal !  May be impossible to see from the surface !  Use NDI on suspected parts !  Damaged part should be replacement !  Prevent by stopping surface corrosion !
•  ExfoliaCon Corrosion Extreme version of intergranular corrosion !  Surface will start to buckle/bubble !
Intergranular Corrosion Intergranular corrosion can start on the surface and progress through the metal, following the grain boundaries.
Intergranular Corrosion Intergranular corrosion can form between sheets of aluminum alloy that are spot welded together.
ExfoliaCon Corrosion ExfoliaCon Corrosion Corrosion Types •  Stress Corrosion A form of intergranular corrosion !  Caused by the combinaCon of stress and corrosive environment !  Shot peening is used on new or repaired parts to improve the resistance to stress corrosion !  If you suspect stress corrosion, NDI the part !
Stress Corrosion Stress corrosion is a form of intergranular corrosion that progresses along the grain boundaries in a material that is under a constant tensile stress.
Shot Peening Shot Peening Corrosion Types •  Fre_ng Corrosion Caused by two surfaces that are rubbing against each other !  Black marks around a row of rivets (smoking rivets) is an indicaCon !
•  Remove rivets •  Remove corrosion •  Apply new paint •  Re-­‐rivet •  Filiform Corrosion Corrosion underneath a painted surface !  Looks like the paint is bubbling or buckling !
Smoking Rivets Filiform Corrosion Where to find Corrosion •  Everywhere – always keep your eyes and hands looking for corrosion •  Anywhere water collects BoHom of the fuselage and wing !  Pockets !
•  Flaps and ailerons •  Seams and lap joints !
Fayed surfaces – surfaces in contact •  Hinges •  Jet engine air inlets !
Debris causes scratches Where to find Corrosion •  Engine exhausts Exhaust fumes are usually acidic and promotes corrosion !  The exhaust heaCng and cooling cycle promotes corrosion !  Water is a by product of combusCon !
•  Where dissimilar metals meet Engine mounts !  Landing gear !  Hinges !  Bolts and screws !
Where to find Corrosion •  Wheel wells and landing gear !
Debris and water is sprayed into this area •  Scratched metal surfaces and water don’t mix well High stressed components !  Dissimilar metals !
•  Fuel tanks !
Jet fuel is notorious for collecCng water and destrucCve microbes Where to find Corrosion •  BaHery compartment and venCng system BaHery venCng system release causCc vapors !  Lead-­‐acid baHeries use acid as electrolyte !
•  Wash down with water and baking soda (sodium bicarbonate) !
Nickel-­‐cadmium baHeries use alkaline (a base) as electrolyte •  Wash down with ammonia or a boric acid and water •  Steel control cables •  Lavatories and food service areas !
Seats •  Welded joints Removing and TreaCng Corrosion •  Removing corrosion from aluminum Natural or nylon fiber brushes !  “Scotch-­‐Brite” pads !  Aluminum wool !  Abrasive wheels made for aluminum !  BlasCng with glass beads or natural abrasives !
•  Never mix with glass beads used on iron or steel Sand with fine sandpaper !  Don’t use !
•  Steel brush •  Steel wool Removing and TreaCng Corrosion •  Acid wash aluminum a&er corrosion removal Usually a sulfuric, phosphoric or chromic acid soluCon that is diluted with water !  Removes small amounts of corrosion !  Neutralizes corrosion salts !  Microscopically roughens the surface up !
•  Help coaCngs adhere to surface Rinse off with clean water !  Follow up immediately with treatment for corrosion !
Aluminum ProtecCve CoaCngs •  Anodizing !
ElectrolyCcally formed oxides •  Similar to the oxides that form naturally on aluminum Provides a protecCve layer against corrosion !  Component is submerged in an acid electrolyte and DC current is applied !
•  The process caused a thicker oxide layer to form Dyes are added to provide different colors !  Provides a good surface for painCng !  Forms an electrical insulaCon layer !
•  Anodized layer must be removed, by sanding it off, if component surface is electrically bonded Aluminum ProtecCve CoaCngs •  Anodizing !
Hardens surface •  Care must be taken with structural components !
Can cause surface cracking that will lead to component failure •  Alodining®/Alodizing (generic name) A passive chromate conversion (chemical process) !  Process !
•  Perform a water break test and leave wet !
Water break test insures the surface is clean •  Brush or spray on alodizing soluCon !
May be dunked in a alodizing tank •  Let stand for 2 to 5 minutes •  Rinse with clean water Aluminum ProtecCve CoaCngs •  Alodining®/Alodizing (generic name) !
Process •  Let surface dry/harden before touching •  If steaks form, the item was too dry before applying alodizing soluCon •  If a powder appears a&er alodizing, the coaCng is bad and must be recoated Usually a yellowish-­‐brown coaCng !  Rough surface good for painCng !  Rags or sponges that dry with the alodizing chemical in them can consCtute a fire hazard !
•  Thoroughly wash a&er use Aluminum ProtecCve CoaCngs •  Alodining®/Alodizing (generic name) !
It is a coaCng •  Not plaCng like cadmium plaCng on bolts Electrically conducCve !  Can be used to repair an anodized part !
•  Paint Referred to as organic finishes !  Primer coat is the most important coat for corrosion protecCon !
•  Zinc chromate provides an inhibiCve layer !
Follow the manufacturers direcCons exactly!!! Treatment of Magnesium Alloys •  Use nylon or natural fiber brushes and pads to remove corrosion !
No metal brushes or pads •  Glass beads may also be used to remove corrosion •  Can be anodized, alodized or painted •  Very corrosive, must be coated Iron and Carbon Steel Corrosion •  Iron oxide (rust) promotes more corrosion •  Corrosion can be removed with wire brushes or power driven wire wheels •  Sanding and blasCng/beading can also be used •  Minimize scratches on highly stressed steel parts !
Use fine-­‐grit aluminum oxide •  Apply protecCve coaCng immediately a&er removing corrosion •
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Iron and Carbon Steel Treatments Cadmium plaCng Nickel or chrome plaCng Galvanizing Metal Spraying !
Molten aluminum is sprayed on part •  PainCng FAA QuesCons •  Why is it important not to rotate the cranksha& a&er the corrosion prevenCve mixture has been put into the cylinders on engines prepared for storage? a. Fuel may be drawn into one or more cylinders and dilute or wash off the corrosion prevenCve mixture. !  b. The seal of corrosion prevenCve mixture will be broken. !  c. Engine damage can occur from hydraulic lock. !
FAA QuesCons •  Why is it important not to rotate the cranksha& a&er the corrosion prevenCve mixture has been put into the cylinders on engines prepared for storage? !
b. The seal of corrosion prevenCve mixture will be broken. FAA QuesCons •  Galvanic corrosion is likely to be most rapid and severe when a. the surface area of the cathodic metal is smaller than surface area of the anodic metal. !  b. the surface areas of the anodic and cathodic metals are approximately the same. !  c. the surface area of the anodic metal is smaller than the surface area of the cathodic metal. !
FAA QuesCons •  Galvanic corrosion is likely to be most rapid and severe when !
c. the surface area of the anodic metal is smaller than the surface area of the cathodic metal. FAA QuesCons •  The interior surface of sealed structural steel tubing would be best protected against corrosion by which of the following? a. Charging the tubing with dry nitrogen prior to sealing. !  b. EvacuaCng moisture from the tubing before sealing. !  c. A coaCng of linseed oil. !
FAA QuesCons •  The interior surface of sealed structural steel tubing would be best protected against corrosion by which of the following? !
c. A coaCng of linseed oil. FAA QuesCons •  (1) In the corrosion process, it is the cathodic area or dissimilar cathodic material that corrodes. (2) In the Galvanic or Electro-­‐Chemical Series for metals, the most anodic metals are those that will give up electrons most easily. Regarding the above statements, a. only No. 1 is true. !  b. only No. 2 is true. !  c. both No. 1 and No. 2 are true. !
FAA QuesCons •  (1) In the corrosion process, it is the cathodic area or dissimilar cathodic material that corrodes. (2) In the Galvanic or Electro-­‐Chemical Series for metals, the most anodic metals are those that will give up electrons most easily. Regarding the above statements, !
b. only No. 2 is true. FAA QuesCons •  Spilled mercury on aluminum a. greatly increases suscepCbility to hydrogen embriHlement. !  b. may cause impaired corrosion resistance if le& in prolonged contact. !  c. causes rapid and severe corrosion that is very difficult to control. !
FAA QuesCons •  Spilled mercury on aluminum !
c. causes rapid and severe corrosion that is very difficult to control. •  PrecauCons: Cleaning Follow the manufacturer’s recommendaCons !  Don’t use a power washer !
•  Gets dirt into aluminum joints •  Washes lubricants out of propeller and bearings !
Use approved cleaners •  MIL-­‐C-­‐43616 – emulsion type cleaners !
Separate dirt from the surface •  Don’t use corrosive cleaners !
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Don’t use 409 Wrong cleaner can cause hydrogen embriHlement (a type of corrosion) in metal structures Take care with chemical exposure and vapors •  PrecauCons: !
Cleaning Don’t let cleaner get in sensiCve parts •  Any thing that is lubricated !
Use so& nylon or natural bristle brushes •  If you must use metal brush, use stainless steel Be careful with cleaners on plasCcs and rubbers !  Use natural fiber rags with petroleum (flammable) cleaners !
•  Man made fiber can generate staCc electricity and start a fire •  PrecauCons: !
Cleaning Follow local environmental laws •  California and Santa Clara County have the toughest in the world •  Start a wash by rinsing/flushing surface with clean water !
The goal is to remove dirt and grit before touching the surface •  Especially important with windshields. Wash grit away to reduce scratches Cleaning •  Magnesium parts need special cleaners •  Follow the manufacturers recommendaCons •  Can be decarbonize, and scrape or grit blast •  Wipe oil/grease off a Cre with a dry cloth and then wash with mild soap and water •  Cleaners !
MEK (methyl ethyl ketone) – very strong cleaner that should be used as a last resort •  Destroys plasCcs and rubbers •  Dissolves fabric covering glues !
Mineral spirits (Stoddard solvent) – good for removing grease before painCng •  Cleaners !
Cleaning AliphaCc naphtha -­‐ petroleum product similar to gasoline and kerosene •  Medium strength cleaner •  Safe for plasCcs and rubber !
AromaCc naphtha -­‐ a coal tar derivaCve (like benzene) •  Very strong cleaner •  Dangerous •  Destroys plasCcs and rubber •  Cleaners !
Cleaning Acetone – nail polish remover •  Good for preparing bare metal for painCng Cleaning for PainCng •  Remove all dirt and oils •  Remove all corrosion •  Perform a water break test !
The surface should be clean enough to support unbroken water •  Mask offs and cover all areas that are not to be stripped !
Don’t get stripper on rubber or plasCc PainCng Strippers •  Pick the stripper that matches your paint •  Apply the stripper and allow it to sit unCl old paint bubbles, swells or wrinkles Brush on stripper in only one direcCon !  Cover with plasCc to prevent it from drying out !
•  Remove paint with a plasCc scrapper and a sCff bristle brush No steel around aluminum !  Pay aHenCon around seams and rivets !
•  Clean off any remaining remover PainCng Strippers •  Repeat if necessary !
Don’t expect a single coat of stripper to get everything •  A&er last coat of stripper, clean metal surfaces with acetone, aliphaCc naphtha or mineral spirits !
Wear gloves a&er final cleaning to prevent oils from contaminaCng surface •  Paint strippers are very dangerous !
Read and follow all safety instrucCons FAA QuesCons •  Fayed surfaces cause concern in chemical cleaning because of the danger of a. forming passive oxides. !  b. entrapping corrosive materials. !  c. corrosion by imbedded iron oxide. !
FAA QuesCons •  Fayed surfaces cause concern in chemical cleaning because of the danger of !
b. entrapping corrosive materials.