Corrosion Causes and Types

8-1

Cleaning & Corrosion Control

Chapter 8

Corrosion

Many aircraft structures are made of metal, and the most insidious form of damage to those structures is corrosion. From the moment the metal is manufactured, it must be protected from the deleterious effects of the environment that surrounds it. This protection can be the introduction of certain elements into the base metal, creating a corrosion-resistant alloy, or the addition of a surface coating of a chemical conversion coating, metal, or paint. While in use, additional moisture barriers, such as viscous lubricants and protectants, may be added to the surface. The introduction of airframes built primarily of composite components has not eliminated the need for careful monitoring of aircraft with regard to corrosion. The airframe itself may not be subject to corrosion; however, the use of metal components and accessories within the airframe means the aviation maintenance technician (AMT) must be on the alert for the evidence of corrosion when inspecting any aircraft.

This chapter provides an overview to the problems associated with aircraft corrosion. For more in-depth information on the subject, refer to the latest edition of the Federal Aviation Administration (FAA) Advisory Circular (AC) 43-4, Corrosion Control for Aircraft. The AC is an extensive handbook that deals with the sources of corrosion particular to aircraft structures, as well as steps the AMT can take in the course of maintaining aircraft that have been attacked by corrosion. Metal corrosion is the deterioration of the metal by chemical or electrochemical attack. This type of damage can take place internally, as well as on the surface. As in the rotting of wood, this deterioration may change the smooth surface, weaken the interior, or damage or loosen adjacent parts.

Water or water vapor containing salt combines with oxygen in the atmosphere to produce the main source of corrosion in aircraft. Aircraft operating in a marine environment, or in areas where the atmosphere contains industrial fumes that are corrosive, are particularly susceptible to corrosive attacks. [Figure 8-1] If left unchecked, corrosion can cause eventual structural failure. The appearance of corrosion varies with the metal. [Figure 8-2] On the surface of aluminum alloys and magnesium, it appears as pitting and etching and is often combined with a gray or white powdery deposit. On copper and copper alloys, the corrosion forms a greenish film; on steel, a reddish corrosion byproduct commonly referred to as rust. When the gray, white, green, or reddish deposits are removed, each of the surfaces may appear etched and pitted, depending upon the length of exposure and severity of attack. If these surface pits are not too deep, they may not significantly alter the strength of the metal; however, the pits may become sites for crack development, particularly if the part is highly stressed. Some types of corrosion burrow between the inside of surface coatings and the metal surface, spreading until the part fails.

Factors Affecting Corrosion Many factors affect the type, speed, cause, and seriousness of metal corrosion. Some of these factors that influence metal corrosion and the rate of corrosion are: 1. Type of metal 2. Heat-treatment and grain direction 3. Presence of a dissimilar, less corrodible metal 4. Anodic and cathodic surface areas (in galvanic corrosion) 5. Temperature 6. Presence of electrolytes (hard water, salt water, battery fluids, etc.) 7. Availability of oxygen 8. Presence of biological organisms 9. Mechanical stress on the corroding metal 10. Time of exposure to a corrosive environment 11. Lead/graphite pencil marks on aircraft surface metals Pure Metals Most pure metals are not suitable for aircraft construction and are used only in combination with other metals to form alloys.

Most alloys are made up entirely of small crystalline regions called grains. Corrosion can occur on surfaces of those regions that are less resistant and also at boundaries between regions, resulting in the formation of pits and intergranular corrosion. Metals have a wide range of corrosion resistance. 8-2 The most active metals (those that lose electrons easily), such as magnesium and aluminum, corrode easily. The most noble metals (those that do not lose electrons easily), such as gold and silver, do not corrode easily. Climate The environmental conditions that an aircraft is maintained and operated under greatly affects corrosion characteristics.

In a predominately marine environment (with exposure to sea water and salt air), moisture-laden air is considerably more detrimental to an aircraft than it would be if all operations were conducted in a dry climate. Temperature considerations are important, because the speed of electrochemical attack is increased in a hot, moist climate. Geographical Location The flight routes and bases of operation expose some airplanes to more corrosive conditions than others. The operational environment of an aircraft may be categorized as mild, moderate, or severe with respect to the corrosion severity of the operational environment. The corrosion severity of the operational environments in North America are identified in Figure 8-3. Additional maps for other locations around the world are published in AC 43-4.

The corrosion severity of any particular area may be increased by many factors, including airborne industrial pollutants, chemicals used on runways and taxiways to prevent ice formation, humidity, temperatures, prevailing winds from a corrosive environment, etc. Suggested intervals for cleaning, inspection, lubrication, and preservation when located in mild zones are every 90 days, moderate zones every 45 days, and severe zones every 15 days. Foreign Material Among the controllable factors that affect the onset and spread of corrosive attack is foreign material that adheres to the metal surfaces. Such foreign material includes: • Soil and atmospheric dust • Oil, grease, and engine exhaust residues • Salt water and salt moisture condensation • Spilled battery acids and caustic cleaning solutions • Welding and brazing flux residues Micro-organisms Slimes, molds, fungi and other living organisms (some microscopic) can grow on damp surfaces. Once they are established, the area tends to remain damp, increasing the possibility of corrosion.

Manufacturing Processes Manufacturing processes, such as machining, forming, welding, or heat-treatment, can leave stresses in aircraft parts. The residual stress can cause cracking in a corrosive environment when the threshold for stress corrosion is exceeded. It is important that aircraft be kept clean. How often and to what extent an aircraft must be cleaned depends on several factors, including geographic location, model of aircraft, and type of operation.

Types of Corrosion

There are two general classifications of corrosion that cover most of the specific forms: direct chemical attack and electrochemical attack. In both types of corrosion, the metal is converted into a metallic compound, such as an oxide, hydroxide, or sulfate. The corrosion process involves two simultaneous changes: the metal that is attacked or oxidized suffers what is called anodic change, and the corrosive agent is reduced and is considered as undergoing cathodic change. Direct Chemical Attack Direct chemical attack, or pure chemical corrosion, is an attack resulting from direct exposure of a bare surface to caustic liquid or gaseous agents. Unlike electrochemical attack where anodic and cathodic changes take place a measurable distance apart, the changes in direct chemical attack occur simultaneously at the same point. The most common agents causing direct chemical attack on aircraft are: spilled battery acid or fumes from batteries; residual flux deposits resulting from inadequately cleaned, welded, brazed, or soldered joints; and entrapped caustic cleaning solutions. [Figure 8-4] With the introduction of sealed lead-acid batteries and the use of nickel-cadmium batteries, spilled battery acid is becoming less of a problem. The use of these closed units lessens the hazards of acid spillage and battery fumes.

Many types of fluxes used in brazing, soldering, and welding 8-3 Alloy Type of attack to which alloy is susceptible Highly susceptible to pitting Surface oxidation and pitting, surface, and intergranular Surface pitting, intergranular, exfoliation stress– corrosion and fatigue cracking, and fretting Highly corrosion resistant; extended or repeated contact with chlorinated solvents may result in degradation of the metal’s structural properties at high temperature Uniform surface corrosion; used as sacrificial plating to protect steel Crevice corrosion; some pitting in marine environments; corrosion cracking; intergranular corrosion (300 series); surface corrosion (400 series) Generally has good corrosion resistant qualities; susceptible to pitting in sea water Surface and intergranular corrosion Pitting (promotes rusting of steel where pits occur in plating) Will tarnish in the presence of sulfur Highly corrosion resistant Subject to whisker growth Magnesium Low alloy steel (4,000–8,000 series) Aluminum Titanium Cadmium Stainless steels (300–400 series) Nickel–base (Inconel, Monel) Copper–base Brass, Bronze Chromium (Plate) Silver Gold Tin Appearance of corrosion product White, powdery, snow-like mounds and white spots on surface Reddish–brown oxide (rust) White–to–grey powder No visible corrosion products at low temperature.

Colored surface oxides develop above 700 °F (370 °C) From white powdery deposit to brown or black mottling of the surface Rough surface; sometimes a uniform red, brown, stain Green powdery deposit Blue or blue–green powdery deposit No visible corrosion products; blistering of plating due to rusting and lifting Brown–to–black fi lm Deposits cause darkening of reflective surfaces Whisker–like deposit are corrosive, chemically attacking the metals or alloys that they are used with. Therefore, it is important to remove residual flux from the metal surface immediately after the joining operation. Flux residues are hygroscopic in nature, absorbing moisture, and unless carefully removed, tend to cause severe pitting.

Caustic cleaning solutions in concentrated form are kept tightly capped and as far from aircraft as possible. Some cleaning solutions used in corrosion removal are, in themselves, potentially corrosive agents. Therefore, particular attention must be directed toward their complete removal after use on aircraft. Where entrapment of the cleaning solution is likely to occur, use a noncorrosive cleaning agent, even though it is less efficient. Electrochemical Attack Corrosion is a natural occurrence that attacks metal by chemical or electrochemical action, converting it back to a metallic compound. The following four conditions must exist before electrochemical corrosion can occur. [Figure 8-5] 1. A metal subject to corrosion (anode) 2. A dissimilar conductive material (cathode) that has less tendency to corrode 3. Presence of a continuous, conductive liquid path (electrolyte) 4. Electrical contact between the anode and the cathode (usually in the form of metal to metal contact, such as rivets, bolts, and corrosion) Elimination of any one of these conditions stops electrochemical corrosion.

NOTE: Paint can mask the initial stages of corrosion. Since corrosion products occupy more volume than the original metal, painted surfaces must be inspected often for irregularities, such as blisters, flakes, chips, and lumps. An electrochemical attack may be likened chemically to the electrolytic reaction that takes place in electroplating, anodizing, or in a dry cell battery. The reaction in this corrosive attack requires a medium, usually water, that is capable of conducting a tiny current of electricity. When a metal comes in contact with a corrosive agent and is also connected by a liquid or gaseous path that electrons flow through, corrosion begins as the metal decays by oxidation. [Figure 8-5] During the attack, the quantity of corrosive agent is reduced and, if not renewed or removed, may completely react with the 8-4 CORROSION SEVERITY ZONE MILD MODERATE SEVERE ANCHORAGE FAIRBANKS MONTREAL NEW YORK ATLANTA MIAMI HAVANA JAMAICA HAITI ST. DOMINGO PUERTO RICO CHICAGO DENVER SEATTLE LOS ANGELES MEXICO HOUSTON DALLAS metal becoming neutralized. Different areas of the same metal surface have varying levels of electrical potential and, if connected by a conductor such as salt water, sets up a series of corrosion cells and corrosion will commence.

All metals and alloys are electrically active and have a specific electrical potential in a given chemical environment. This potential is commonly referred to as the metal’s “nobility.” [Figure 8-6] The less noble a metal is, the more easily it can be corroded. The metals chosen for use in aircraft structures are a studied compromise with strength, weight, corrosion resistance, workability, and cost balanced against the structure’s needs. The constituents in an alloy also have specific electrical potentials that are generally different from each other. Exposure of the alloy surface to a conductive, corrosive medium causes the more active metal to become anodic and the less active metal to become cathodic, thereby establishing conditions for corrosion. These are called local cells. The greater the difference in electrical potential between the two metals, the greater the severity of a corrosive attack if the proper conditions are allowed to develop.

8-5 Anodic area Cathodic area Simplified corrosion cell showing conditions that must exist for electrochemical corrosion. Elimination of corrosion by application of an organic film to metal surface. Anodic area Cathodic area No contact between electrolyte and anode and cathode Continuous liquid path (electrolyte) Current flow Continuous liquid path (electrolyte) Electron flow Electron conductor metal Unbroken paint film The conditions for these corrosion reactions are the presence of a conductive fluid and metals having a difference in potential. If, by regular cleaning and surface refinishing, the medium is removed and the minute electrical circuit eliminated, corrosion cannot occur. This is the basis for effective corrosion control. The electrochemical attack is responsible for most forms of corrosion on aircraft structure and component parts.

Forms of Corrosion There are many forms of corrosion. The form of corrosion depends on the metal involved, its size and shape, its specific function, atmospheric conditions, and the corrosion producing agents present. Those described in this section are the more common forms found on airframe structures. Surface Corrosion General surface corrosion (also referred to as uniform etch or uniform attack corrosion) is the most common form of corrosion. Surface corrosion appears as a general roughening, etching, or pitting of the surface of a metal, frequently accompanied by a powdery deposit of corrosion products.

Surface corrosion may be caused by either direct chemical or electrochemical attack. Sometimes corrosion spreads under the surface coating and cannot be recognized by either the roughening of the surface or the powdery deposit. Instead, closer inspection reveals the paint or plating is lifted off the surface in small blisters that result from the pressure of the underlying accumulation of corrosion products. [Figure 8-7] Filiform Corrosion Filiform corrosion is a special form of oxygen concentration cell that occurs on metal surfaces having an organic coating system. It is recognized by its characteristic worm- like trace of corrosion products beneath the paint film.

[Figure 8-8] Polyurethane finishes are especially susceptible to filiform corrosion. Filiform occurs when the relative humidity of the air is between 78–90 percent, and the surface is slightly acidic. This corrosion usually attacks steel and aluminum surfaces. The traces never cross on steel, but they 8-6 Metal most likely to corrode (anodic) Metal least likely to corrode (cathodic) Magnesium Magnesium alloy Zinc Aluminum (1100) Cadmium Aluminum 2024-T4 Steel or iron Cast iron Chromium-iron (active) Ni-Resist cast iron Type 304 stainless steel (active) Type 316 stainless steel (active) Lead-tin solder Lead Tin Nickel (active) Inconel nickel-chromium alloy (active) Hastelloy alloy C (active) Brass Copper Bronze Copper-nickel alloy Monel nickel-copper alloy Silver solder Nickel (passive) Inconel nickel-chromium alloy (passive) Chromium-iron (passive) Type 304 stainless steel (passive) Type 316 stainless steel (passive) Hastelloy alloy C (passive) Silver Titanium Graphite Gold Platinum cross under one another on aluminum, making the damage deeper and more severe for aluminum. If the corrosion is not removed, the area treated, and a protective finish applied, the corrosion can lead to intergranular corrosion, especially around fasteners and at seams.

Filiform corrosion can be removed using glass bead blasting material with portable abrasive blasting equipment or sanding. Filiform corrosion can be prevented by storing aircraft in an environment with a relative humidity below 70 percent, using coating systems having a low rate of diffusion for oxygen and water vapors, and by washing the aircraft to remove acidic contaminants from the surface, such as those created by pollutants in the air. Pitting Corrosion Pitting corrosion is one of the most destructive and intense forms of corrosion. It can occur in any metal but is most common on metals that form protective oxide films, such as aluminum and magnesium alloys. It is first noticeable as a white or gray powdery deposit, similar to dust, which blotches the surface. When the deposit is cleaned away, tiny holes or pits can be seen in the surface. These small surface openings may penetrate deeply into structural members and cause damage completely out of proportion to its surface appearance. [Figure 8-9] Dissimilar Metal Corrosion Extensive pitting damage may result from contact between dissimilar metal parts in the presence of a conductor. While surface corrosion may or may not be taking place, a galvanic action, not unlike electroplating, occurs at the points or areas of contact where the insulation between the surfaces has broken down or been omitted. This electrochemical attack can be very serious because, in many instances, the action is taking place out of sight, and the only way to detect it prior to structural failure is by disassembly and inspection.

[Figure 8-10] The contamination of a metal’s surface by mechanical means can also induce dissimilar metal corrosion. The improper use of steel cleaning products, such as steel wool or a steel wire brush on aluminum or magnesium, can force small pieces of steel into the metal being cleaned, causing corrosion and ruining the adjoining surface. Carefully monitor the use of nonwoven abrasive pads, so that pads used on one type of metal are not used again on a different metal surface. Concentration Cell Corrosion Concentration cell corrosion, (also known as crevice corrosion) is corrosion of metals in a metal-to-metal joint, corrosion at the edge of a joint even though the joined metals are identical, or corrosion of a spot on the metal surface covered by a foreign material. Metal ion concentration cells, 8-7 oxygen concentration cells, and active-passive cells are three general types of concentration cell corrosion.

Metal Ion Concentration Cells The solution may consist of water and ions of the metal that are in contact with water. A high concentration of metal ions normally exists under faying surfaces where the solution is stagnant and a low concentration of metal ions exist adjacent to the crevice, created by the faying surface. [Figure 8-11] An electrical potential exists between the two points: the area of the metal in contact with the low concentration of metal ions is anodic and corrodes; the area in contact with the high metal ion concentration is cathodic and does not show signs of corrosion. Oxygen Concentration Cells The solution in contact with the metal surface normally contains dissolved oxygen. An oxygen cell can develop at any point where the oxygen in the air is not allowed to diffuse into the solution, thereby creating a difference in oxygen concentration between two points. Typical locations of oxygen concentration cells are under gaskets, wood, rubber, and other materials in contact with the metal surface.

Corrosion occurs at the area of low oxygen concentration (anode). Alloys such as stainless steel are particularly susceptible to this type of crevice corrosion. [Figure 8-12] Active-Passive Cells Metals that depend on a tightly adhering passive film, usually an oxide for corrosion protection, are prone to rapid corrosive attack by active-passive cells. The corrosive action usually starts as an oxygen concentration cell. The passive film is broken beneath the dirt particle exposing the active metal to corrosive attack. An electrical potential will develop between the large area of the passive film and the small area of the active metal, resulting in rapid pitting. [Figure 8-13] Intergranular Corrosion This type of corrosion is an attack along the grain boundaries of an alloy and commonly results from a lack of uniformity in the alloy structure. Aluminum alloys and some stainless steels are particularly susceptible to this form of electrochemical attack.

[Figure 8-14] The lack of uniformity is caused by changes that occur in the alloy during the heating and cooling process of the material’s manufacturing. Intergranular corrosion may exist without visible surface evidence. High-strength aluminum alloys, such as 2014 and 7075, are more susceptible to intergranular corrosion if they have been improperly heat- treated and then exposed to a corrosive environment. Exfoliation Corrosion Exfoliation corrosion is an advanced form of intergranular corrosion and shows itself by lifting up the surface grains of a metal by the force of expanding corrosion products occurring at the grain boundaries just below the surface.

[Figure 8-15] It is visible evidence of intergranular corrosion and is most often seen on extruded sections where grain thickness is usually less than in rolled forms. This type of corrosion is difficult to detect in its initial stage. Extruded components, such as spars, can be subject to this type of corrosion. Ultrasonic and eddy current inspection methods are being used with a great deal of success. Stress-Corrosion/Cracking This form of corrosion involves a constant or cyclic stress acting in conjunction with a damaging chemical environment. The stress may be caused by internal or external loading.

[Figure 8-16] Internal stress may be trapped in a part of structure during manufacturing processes, such as cold- working or by unequal cooling from high temperatures. Most manufacturers follow these processes with a stress relief operation. Even so, sometimes stress remains trapped. The stress may be externally introduced in part structure by riveting, welding, bolting, clamping, press fit, etc. If a slight mismatch occurs or a fastener is over-torqued, internal stress 8-8 High oxygen concentration Low oxygen concentration OXYGEN CONCENTRATION CELL Low metal ion concentration High metal ion concentration METAL ION CONCENTRATION CELL TROUGH PITS SIDEWAY PITS Narrow, deep Wide, shallow Elliptical Vertical Subsurface Undercutting Horizontal is present. Internal stress is more important than design stress, because stress corrosion is difficult to recognize before it has overcome the design safety factor. The level of stress varies from point to point within the metal. Stresses near the yield strength are generally necessary to promote stress corrosion cracking. However, failures may occur at lower stresses.

Specific environments have been identified that cause stress corrosion cracking of certain alloys. 1. Salt solutions and sea water cause stress corrosion cracking of high-strength, heat-treated steel and aluminum alloys. 2. Methyl alcohol-hydrochloric acid solutions cause stress corrosion cracking of some titanium alloys. 3. Magnesium alloys may stress corrode in moist air. Stress corrosion may be reduced by applying protective coatings, stress relief heat-treatments, using corrosion inhibitors, or controlling the environment. Shot peening a metal surface increases resistance to stress corrosion cracking by creating compressive stresses on the surface which should be overcome by applied tensile stress before the surface sees any tension load. Therefore, the threshold stress level is increased.

8-9 Cathode Anode Electrolyte enters through cracks in paint film CladdingCladding 7075-T6 AluminumSteel fastener Paint film Preferential corrosion along grain boundaries Intergranular corrosion Foreign material creates a low oxygen region that prevents the re-formation of passive film ACTIVE – PASSIVE CELL Passive film protects exposed surface Active metal environment. Metals may withstand cyclic stress for an infinite number of cycles so long as the stress is below the endurance limit of the metal. Once the limit has been exceeded, the metal eventually cracks and fails from metal fatigue. However, when the part or structure undergoing cyclic stress is also exposed to a corrosive environment, the stress level for failure may be reduced many times. Thus, failure occurs at stress levels that can be dangerously low depending on the number of cycles assigned to the life- limited part.

Fatigue corrosion failure occurs in two stages. During the first stage, the combined action of corrosion and cyclic stress damages the metal by pitting and crack formations to such a degree that fracture by cyclic stress occurs, even if the corrosive environment is completely removed. The second stage is essentially a fatigue stage where failure proceeds by propagation of the crack (often from a corrosion pit or pits). It is controlled primarily by stress concentration effects and the physical properties of the metal. Fracture of a metal part due to fatigue corrosion generally occurs at a stress level far Fretting Corrosion Fretting corrosion is a particularly damaging form of corrosive attack that occurs when two mating surfaces, normally at rest with respect to one another, are subject to slight relative motion. It is characterized by pitting of the surfaces and the generation of considerable quantities of finely divided debris. Since the restricted movements of the two surfaces prevent the debris from escaping very easily, an extremely localized abrasion occurs. [Figure 8-17] The presence of water vapor greatly increases this type of deterioration. If the contact areas are small and sharp, deep grooves resembling brinell markings or pressure indentations may be worn in the rubbing surface. As a result, this type of corrosion on bearing surfaces has also been called false brinelling. The most common example of fretting corrosion is the smoking rivet found on engine cowling and wing skins. This is one corrosion reaction that is not driven by an electrolyte, and in fact, moisture may inhibit the reaction. A smoking rivet is identified by a black ring around the rivet.

Fatigue Corrosion Fatigue corrosion involves cyclic stress and a corrosive 8-10 extremely vulnerable. Exposure of airframe materials to salts or their solutions is extremely undesirable. • Atmosphere—the major atmospheric corrosive agents are oxygen and airborne moisture. Corrosion often results from the direct action of atmospheric oxygen and moisture on metal, and the presence of additional moisture often accelerates corrosive attack, particularly on ferrous alloys. However, the atmosphere may also contain other corrosive gases and contaminants, particularly industrial and marine salt spray.

• Water—the corrosiveness of water depends on the type and quantity of dissolved mineral and organic impurities and dissolved gasses (particularly oxygen) in the water. One characteristic of water that determines its corrosiveness is the conductivity or ability to act as an electrolyte and conduct a current. Physical factors, such as water temperature and velocity, also have a direct bearing on its corrosiveness.

Preventive Maintenance

Much has been done to improve the corrosion resistance of aircraft, such as improvements in materials, surface treatments, insulation, and modern protective finishes. All of these have been aimed at reducing the overall maintenance effort, as well as improving reliability. In spite of these improvements, corrosion and its control is a very real problem that requires continuous preventive maintenance. During any corrosion control maintenance, consult the Safety Data Sheet (SDS) for information on any chemicals used in the process. Corrosion preventive maintenance includes the following specific functions: 1. Adequate cleaning 2. Thorough periodic lubrication 3. Detailed inspection for corrosion and failure of below the fatigue limit of an uncorroded part, even though the amount of corrosion is relatively small.

Galvanic Corrosion Galvanic corrosion occurs when two dissimilar metals make electrical contact in the presence of an electrolyte. [Figure 8-18] The rate which corrosion occurs depends on the difference in the activities. The greater the difference in activity, the faster corrosion occurs. The rate of galvanic corrosion also depends on the size of the parts in contact. If the surface area of the corroding metal is smaller than the surface area of the less active metal, corrosion is rapid and severe. When the corroding metal is larger than the less active metal, corrosion is slow and superficial.

Common Corrosive Agents Substances that cause corrosion of metals are called corrosive agents. The most common corrosive agents are acids, alkalies, and salts. The atmosphere and water, the two most common media for these agents, may also act as corrosive agents. • Acids—moderately strong acids severely corrode most of the alloys used in airframes. The most destructive are sulfuric acid (battery acid), halogen acids (hydrochloric, hydrofluoric, and hydrobromic), nitrous oxide compounds, and organic acids found in the wastes of humans and animals. • Alkalies—as a group, alkalies are not as corrosive as acids. Aluminum and magnesium alloys are exceedingly prone to corrosive attack by many alkaline solutions unless the solutions contain a corrosion inhibitor. Substances particularly corrosive to aluminum are washing soda, potash (wood ashes), and lime (cement dust). Ammonia, an alkali, is an exception because aluminum alloys are highly resistant to it.

• Salts—most salt solutions are good electrolytes and can promote corrosive attack. Some stainless-steel alloys are resistant to attack by salt solutions but aluminum alloy, magnesium alloys, and other steels are