Chemical Fundamentals of Corrosion PDF

Chemical Fundamentals of Corrosion Corrosion Most of the corrosion encountered by aircraft maintenance engineers is the result of a complex electrochemical (galvanic) action that causes metals to be transformed into their salts and oxides. These powdery substances replace the metal and cause severe...

Chemical Fundamentals of Corrosion Corrosion Most of the corrosion encountered by aircraft maintenance engineers is the result of a complex electrochemical (galvanic) action that causes metals to be transformed into their salts and oxides. These powdery substances replace the metal and cause severe loss of strength in the structure. While corrosion is complex in its nature, its actual mechanics are relatively simple and straightforward. © Aviation Australia - Image by David Kingshott Main undercarriage leg corrosion For electrochemical corrosion to occur, four components must be present: A metal anode – a conductive material that is eaten away by corrosion. A cathode – a different conductive material, e.g. a less reactive metal (or carbon). An electrolyte – a continuous conductive liquid in electrical contact with both metals that carries current by providing a path for movement of ions. Direct electrical contact between the dissimilar metals – conducts a flow of electrons that is equal and opposite to the +ve ion flow. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 144 of 229 Electrochemical corrosion Corrosion is a natural process, and its prevention is almost impossible, but it can be controlled. The aviation technician must prevent or remove one or more of the requirements for corrosion. Doing this goes a long way towards adding longevity to the structure of the airplane. Maintaining surface cleanliness is one of the best ways to control corrosion. When moisture is held in contact with the metal surface by an accumulation of dirt or grease, corrosion is sure to start. If the surface can be kept perfectly dry and clean, however, corrosion has little chance of getting started. The essence of corrosion control is therefore prevention rather than removal. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 145 of 229 Dry Corrosion Dry corrosion is a slow chemical reaction, not electrochemical in nature. At room temperature, most metals carry a thin oxide layer as a result of the reaction of metals with oxygen in the atmosphere. An increase in temperature may cause formation of a heavier layer, or the layer may detach. Zinc and zinc coatings carry a fairly protective zinc hydroxide or carbonate layer (zinc patina) which increases in thickness very slowly. Aluminium carries a thin, highly protective oxide layer. Some corrosion takes place even under completely dry conditions. Dry corrosion 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 146 of 229 Wet Corrosion Wet corrosion is an electrochemical process. It takes place in environments where the relative humidity is high. The corrosion may be uniform destruction of the metal surface or localised destruction (pitting, stress corrosion cracking). Wet corrosion is most efficient in water containing salts. Corrosion is most efficient in waters containing salts due to the high conductivity of the solution 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 147 of 229 Direct Chemical Attack If an alkaline or acidic liquid comes into contact with metal, the result is a form of corrosion known as direct chemical attack. It is also fundamentally electrochemical in nature. However, no current flow is detectable, nor are there any definite anodic or cathodic areas. The theoretical rate of a chemical attack can be affected by the formation of a protective film on the metal surface; secondary reactions involving the products of corrosion; the mechanical removal of protective films, such as by erosion or flexing of the metal surface; or temperature changes. For corrosion to form on a metal, there must be an electrode potential difference and an electrolyte. Sulphuric acid as found in batteries is especially active in corroding aluminium, while a weak solution of chromic or phosphoric acid is actually used as a surface treatment when preparing a metal for painting. Aluminium is more vulnerable to strong alkaline solutions than to acids. Aluminium structure, for instance, can be severely corroded if allowed to remain on a concrete floor. Water will leach out enough lime from the cement to form an alkaline solution that will corrode the aluminium. Phosphate ester hydraulic fluids (Skydrol) can cause corrosion. Direct chemical attack 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 148 of 229 Mercury Although it is not commonly found in any quantity around aircraft, there is a possibility that mercury could be spilled in an aeroplane. Mercury attacks aluminium by a chemical reaction known as amalgamation. In this process, the mercury rapidly attacks along the grain boundaries of the aluminium, and in an exceedingly short time will completely destroy it. Mercury corrosion Relevant Youtube link: Mercury corrosion (Video) 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 149 of 229 Types of Corrosion Electrochemical Series If the anode is smaller than the cathode, the anode will give up electrons more easily and the corrosion will be more rapid. Electrochemical series for metals 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 150 of 229 Galvanic Corrosion © Aviation Australia - Image by David Kingshott Galvanic Corrosion Galvanic corrosion can take place where dissimilar metal skins are riveted together or where aluminium inspection plates are attached to a structure with steel screws. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 151 of 229 Galvanic corrosion 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 152 of 229 Stress Corrosion Stresses may come from a fitting that has been pressed into a structural part with an interference fit. Cracks grow rapidly as the corrosive attack concentrates at the end of the crack rather than along its sides as in intergranular corrosion. Stress corrosion 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 153 of 229 Surface Corrosion Where an area of unprotected metal is exposed to an atmosphere containing battery fumes, exhaust gases or industrial contaminants, there will be a rather uniform attack over the entire surface area. This dulling of the surface is caused by microscopic amounts of the metal being converted into corrosion salts. If these deposits are not removed and the surface protected against further action, such a rough surface will develop that corrosion pits will form. Surface corrosion can go undetected until it breaks through the metal, when it is too late to save the affected parts. © Aviation Australia - Image by David Kingshott Skin Surface Corrosion 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 154 of 229 Pitting Corrosion A logical progression from uniform surface corrosion, if left untreated, is pitting. Pits form as localised anodic areas. Corrosive action may continue until an appreciable percentage of the metal thickness is converted into salts, which in extreme cases may eat completely through the metal. Pitting may be detected by the appearance of clumps of white powder on the surface. Corrosive action causing pitting 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 155 of 229 Aviation Australia - Dave Kingshott Fuel filter bowl - pitting corrosion © Aviation Australia - Image by David Kingshott Cylinder wall pitting 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 156 of 229 Intergranular Corrosion Micro-photographs of aluminium alloys show them made up of extremely tiny grains held together by chemical bonds, that is, by the interaction of the atoms of the various elements. In the process of heat treatment, the metal is heated to such a temperature that these alloying agents go into solution with each other. When this temperature has been reached uniformly throughout, the metal is removed from the furnace and immediately quenched in water to solidify these elements into extremely small grains. If quenching is delayed for even a few seconds, these grains will grow, and when the metal is finally quenched, the grains will have reached such a size that the areas of dissimilar metals will provide efficient cathodes and anodes for corrosion formation. This defect is hard to detect as it is inside the metal but often shows up as a blister on the surface. Intergranular corrosion 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 157 of 229 Aviation Australia - David Kingshott Example of intergranular corrosion on an aircraft wing spar 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 158 of 229 Filiform Corrosion Filiform corrosion is a particular case of oxygen cell corrosion in a protective organic coating. In certain conditions, corrosion can be initiated at the defect and can propagate beneath the organic coating: the front of the propagation acts as the anode and the defect neighbourhood acts as the cathode. The unusual characteristic of this type of corrosion is that it forms in areas where there is a deficiency of oxygen. Filiform corrosion 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 159 of 229 Fretting Corrosion When two surfaces fit tightly together but can move relative to one another, they may be eroded. These surfaces are normally not close enough together to shut out oxygen, so they do develop the desired protective film. However, the film is destroyed by the continued rubbing action. This wear is fretting corrosion. When movement between the two surfaces is small, the debris between them does not have an opportunity to escape, and it acts as an abrasive to further erode the surfaces. By the time this type of corrosion appears on the surface, the damage is usually done and the parts must be replaced. Fretting corrosion may occur around rivets in a skin. This will be indicated by dark deposits around the rivet heads, streaming out behind. It gives the appearance of the rivet smoking. Rivets showing this sign of fretting must be replaced as soon as possible. Fretting corrosion 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 160 of 229 Exfoliation Corrosion Exfoliation corrosion, despite its lofty-sounding name, is simply an extreme case of intergranular corrosion. It occurs chiefly in extruded materials, such as channels or angles, where the grain structure is more laminar (layer-like) than in rolled sheets or castings. This type of corrosion occurs along the grain boundaries and causes the material to separate or delaminate. As with other types of intergranular corrosion, by the time it is evident on the surface, the strength of the metal has been greatly decreased. In this state, the metal no longer has any strength. The only remedy is complete removal of the affected sections. Exfoliation corrosion 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 161 of 229 Aviation Australia - David Kingshott Wing skin exfoliation Aviation Australia - David Kingshott Inspection Panel exfoliation 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 162 of 229 Microbiological Corrosion Aircraft use fuel that holds water in suspension. They fly in high-altitude, low-temperature flight conditions, the water entrained in the fuel will condense out and collect in the bottom of the tanks. Microbiological growths This water contains microbes, which are simply microscopic life forms and can be fungus. These organic bodies live in the water and feed on the hydrocarbon fuel. The dark insides of the fuel tanks promote their growth, and these tiny creatures multiply and produce a scum in the tank. The scum holds water in contact with the tank structure, where corrosion of the concentration cell type will inevitably form. If the scum forms along the edge of a seal in an integral fuel tank, the sealant may pull away from the structure, causing a leak and an expensive resealing operation. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 163 of 229 Fungus growth in lower skin of a fuel tank 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 164 of 229 Material Types and Susceptibility to Corrosion Metal Corrosion Susceptibility No metal has perfect environmental integrity and is totally resistant to corrosion. As a result, all metals will corrode sooner or later. The following paragraphs discuss the corrosion characteristics of commonly used aircraft metals. Aluminium Aluminium and its alloys are the most widely used materials for aircraft construction. In addition to their uses in aircraft structure, they are widely used in equipment housings, chassis, mounting racks, supports, frames and electrical connector shells. Aluminium is highly anodic, as evidenced by its position in the galvanic series table. It is anodic to most other metals, and when it comes into contact with them, galvanic corrosion of the aluminium will occur. Aluminium alloys are subject to pitting, intergranular corrosion and stress corrosion cracking. In some cases, the corrosion products of a metal in contact with aluminium are corrosive to aluminium. However, the formation of a tightly adhering oxide film offers increased resistance under mild corrosive conditions. The corrosion product of aluminium is a white-to-grey powdery material (aluminium oxide or hydroxide) which can be removed by mechanical polishing or brushing with abrasives. Grey powder of aluminium surface corrosion 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 165 of 229 Anodised Aluminium Some aluminium parts are protected with an electrochemically applied oxide coating called anodise. Aluminium oxide film on aluminium is a naturally occurring protective film, and anodising merely increases the thickness of the oxide film. When this coating is damaged in service, it can be only partially restored by chemical surface treatment. Unnecessary destruction (e.g. nicks and scratches) of the anodised surface must be avoided. Anodised coating on an aluminium substrate 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 166 of 229 Magnesium Magnesium alloys are the lightest structural metals used for aircraft and missile airframes. They are used extensively throughout avionics systems as antennas, structures, chassis, supports and frames (radar). Magnesium is also used extensively for transmission and gearbox housings. These alloys are highly susceptible to corrosion when the metal surface is exposed to the environment without a protective finish. Magnesium corrosion on wheel assembly Magnesium corrosion deposits tend to raise slightly and the corrosion spreads rapidly. When the white, puffy areas are discovered, they require prompt treatment or the corrosion will penetrate entirely through the structure. The natural oxide-carbonate film formed on magnesium alloys does not provide sufficient corrosion protection even in the mildest environment. The rate of corrosion of a magnesium alloy increases when the alloy is immersed in water or periodically subjected to moisture. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 167 of 229 Titanium Titanium and titanium alloys have numerous uses in aircraft due to its low weight and high strength. At high temperatures titanium readily absorbs gases from the surrounding air and becomes very brittle. Under certain conditions, chlorides and some chlorinated solvents may induce stress corrosion cracking of certain titanium alloys. Titanium and its alloys are highly corrosion resistant. An oxide film forms on their surfaces almost immediately upon contact with air. This film is extremely adherent to the surfaces and provides a protective barrier. Since titanium is the less active member of the electrochemical series (cathodic) of most dissimilar metal couples, it can greatly accelerate corrosion of a dissimilar metal coupled to it. Electrical insulation between titanium and other metals is necessary to prevent galvanic corrosion of the other metal. Titanium in contact with a corroding metal can absorb hydrogen and become brittle. Frequent inspection of such areas is required to ensure that insulation failure has not allowed corrosion to begin. SR71 Blackbird 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 168 of 229 Cadmium Cadmium is used as a coating to protect steel hardware, such as bolts, washers and screws, and as plating on electrical connectors. It is used as a plating on high-strength steel parts (e.g. landing gear) to improve resistance to corrosion fatigue. Cadmium may also be used to provide a compatible surface when a part is in contact with other materials. When coupled with steel, it is anodic and protects the steel by galvanic action. Corrosion on cadmium is evident by white-to-brown-to-black mottling of the surface. When cadmium plate shows mottling and cracks in the coating, the plating is still performing its protective function as an anodic material. Cadmium plated surface conditions 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 169 of 229 Chromium Chromium is used as a protective plating. Chromium plating is also used to provide a smooth, wearresistant surface and to reclaim worn parts. The degree of protection depends on plating thickness. Chromium forms a continuous oxide coating that can be polished to a high lustre and still protect not only itself but any underlying metal. Chromium coatings contain cracks, and corrosion may originate at the base metal below these separations. Aviation Australia - David Kingshott Chrome plating failure on an Oleo Strut 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 170 of 229 Nickel Nickel is important as a plating metal, an additive to stainless steel, and a base for nickel alloys. Pure nickel is used as an electroless coating and is subject to pitting corrosion. Flaking of the nickel coating can also occur when an underlying metal corrodes. When it is added to stainless steel alloys, its stress corrosion resistance increases with nickel contents above 10%. Nickel-based alloys are used in hightemperature areas (engines, afterburners), but they are subject to hot corrosion attack and embrittlement when sulphur-containing gases are present. Nickel corrosion engine cylinder 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 171 of 229 Copper and Copper Alloys Copper and copper-based alloys (brass and bronze) are considered corrosion resistant, with corrosion usually limited to staining and tarnishing. Copper and copper-based alloys are generally used in avionics systems as contacts, springs, leads, connectors, printed circuit board (PCB) conductors and wires. Generally, changes in surface conditions are not dangerous and have no effect on the part. Copper corrosion is evidenced by the accumulation of blue or blue-green corrosion products on the corroded part. Corroded circuit board Sometimes copper or copper alloy surfaces tarnish to a dull grey-green colour (patina) and the surface will remain relatively smooth. This discoloration is the result of the formation of a fine grained, airtight copper oxide crust. This crust offers good protection for the underlying metal in ordinary situations. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 172 of 229 Silver Silver is used as a plating material over copper in waveguides, miniature and microminiature circuits, wires, contacts, high-frequency cavities, tank circuits and RF shielding. Silver does not corrode in the ordinary sense, although it tarnishes in the presence of sulphur. The tarnish is silver sulphide, appears as a brown-to-black film, and may or may not be detrimental to circuit electrical characteristics, depending on the application. When silver is plated over copper, there can be an accelerated corrosion of the copper. This occurs through galvanic action at pinholes or breaks in the silver plating. Radar waveguide showing tarnishing 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 173 of 229 Gold Traditionally considered the best coating for corrosion resistance and solderability, gold is used on printed circuits, semiconductors, leads and electrical contacts. It is usually applied in a thin layer over nickel, silver or copper. Gold is a noble metal (pure metal in nature) and does not normally corrode; however, a slight deposit will appear as a darkening of reflecting surfaces. Tarnish removal is critical on gold components because of the very thin coatings used. Gold plated over silver or copper in thin layers accelerates corrosion over the less noble metals (silver or copper). The corrosion occurs at pores or pinholes in the gold and is readily identified as darkening of the silver or blue-green deposits on the copper. ‘Purple plague’ is a brittle gold-aluminium compound formed when bonding gold to aluminium. The growth of such a compound can cause failure in microelectronic interconnection bonds. Aviation Australia Electrical connector contact with gold plating Tin The use of tin in solder is a well-known application. However, tin plating is also common on avionic RF shields, filters, crystal covers and automatic switching devices. Tin offers the best combination of solderability and corrosion resistance of any metallic coating. However, it tends to grow ‘whiskers’ on tin-plated wire and other plated applications. Tin corrosion 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 174 of 229 Effects of Corrosion on Metals Alloys Type of Attack to which Alloy is Susceptible Appearance of Corrosion Product Aluminium Alloy Surface pitting, intergranular, exfoliation, stress corrosion and fatigue cracking, and fretting White to grey powder Magnesium Alloy Highly susceptible to pitting White, powdery, snow-like mounds, and white spots on surface Titanium Alloy Highly corrosion resistant; exposure to solvents may promote corrosion. Cadmium plated tools can cause embrittlement Surface oxides develop at high temperatures Cadmium (used as a protective plating for steel) Uniform surface corrosion From white powdery deposit to brown or black mottling of the surface Chromium (plate) Pitting (promotes rusting of steel where pits occur in plate) No visible corrosion products; blistering of plating due to rusting and lifting Nickel-base Alloy (Inconel, Monel) Good corrosion resistant qualities; susceptible to pitting in sea water Green powdery deposit Copper-base Alloy, Brass, Bronze Surface and intergranular corrosion Blue or blue-green powdery deposit Silver Will tarnish in presence of sulfur Brown to black film Gold Highly corrosion resistant Deposits cause darkening of reflective surfaces Tin Subject to whisker growth Whisker-like deposits 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 175 of 229