Rust Never Sleeps

This is an article about rust (corrosion) and the steps which can be taken to stop it from happening in electrical interconnect systems. Common rust is a form of oxide that forms on iron or steel as the metal reacts with air and water. In chemical terms, rust in iron is ferrous hydroxide, a compound consisting primarily of iron and oxygen, with an element of hydrogen. Corrosion not only takes place in liquid environments, it can also occur as the result of contact with hot gases, such as the fumes caused by fuel combustion in a vehicle or the stack-gas from a naval ship. A simple definition for corrosion is the deterioration of material by reaction to its environment. For metal materials, corrosion can be thought of as a form of natural degradation. As raw ores are refined into metals suitable for use in manufacturing, the extracted metals take on a higher energy state and become vulnerable to atmospheric attack. As the metals deteriorate via corrosion, they revert back (or oxidize) to their original state of ores and minerals. In this sense, corrosion is the opposite of the metal refining process: the refined metals are simply returning to their original forms as found in nature. Corroding metals are a bit like 60’s hippies, rebelling from refined society to “get back to the land.” Refined metals corrode because we use them in environments which attack their chemically instability. As we said, iron in the presence of moist air reverts to its natural state, iron oxide. Only copper and the precious metals (gold, silver and platinum) are found in nature in their pure metallic state. All other metals, including aluminum—the most common material used in interconnect components—are processed from raw minerals or ores. In our electrical interconnect industry, the problems associated with corrosion are compounded by the need to produce parts which are electrically conductive. As we all know, it is the conductive properties of connectors and their backshells which prevent electromagnetic interference (EMI) from disrupting the flow of data throughout the interconnect system. To prevent EMI from permeating into the system, conductive cable shielding is grounded to plated connectors and accessories to take the unwelcome EMI harmlessly to earth. If metal connectors and accessories could be produced without the need for conductive surface platings, corrosion in interconnect systems would be a much easier problem to resolve. For as we shall see, it is the surface platings themselves which significantly compound the difficulty of preventing interconnect system corrosion. Charge It Figure 1. This picture should be immediately familiar, as we stole it from your high-school science book. It depicts negatively charged electrons orbiting the center, or nucleus, of the atom in much the same way that the planets orbit the sun in our solar system. Note also the neutral neutrons and the positively charged protons. All these particles play a role in electrochemical corrosion. All matter is electrical by nature. Everything—from your body's nervous system to the earth itself—has electrical properties. All matter is made up of atoms which in turn are composed of protons, neutrons, and electrons. The center, or nucleus of the atom, is composed of positively charged protons and neutral neutrons (see Figure 1). The process of corrosion takes place at this most basic molecular level, and with the rare exception of exclusively chemical types of corrosion such as occur when battery acid is spilled on steel, corrosion is a process literally driven by electricity. To be a bit more exact, the process of corrosion is electrochemical in nature; because for the process to occur several specific conditions must be met, and not all of them are solely electrical: There must be a positive or anodic area, referred to as the "anode." There must be a negative or cathodic area, referred to as the "cathode." There must be a path for ionic current flow, referred to as the "electrolyte." There must be a path for electronic current flow, which is normally a "metallic path." The electrical pressure between the two magnetic poles—the anode and the cathode—results in a migration of electrons from one to the other along the metallic path. With the loss of electrons, positively-charged atoms remain at the anode, combining with negatively-charged ions in the environment to form, in the case of steel parts, ferrous hydroxide, or rust. In most interconnect applications the role of the ionic current flow is played by the atmosphere and rain, although in marine applications it is salt water and spray. The rate at which metal is removed by the corrosive process is usually measured in amperes or thousandths of ampere (milliamperes). In interconnect applications the conductor for this electrical current can be the mating point of the various subcomponents—such as fasteners, bands and braids—or the threaded interface between the backshell and connector. But the metallic path for the current flow can also be between the shell of the component and its own metallic surface plating! PHOTO: NASA The metalic path for corrosive current flow is often between the base metal and its own finish plating. As the title of this issue of QwikConnect dramatically reveals, Rust Never Sleeps. The corrosion process described above will continue without rest, until one or more of the four conditions is no longer met. For this reason, the elimination of one of the four conditions is the very heart and soul of corrosion prevention efforts. An unbroken (perfect) protective coating on the surface of a metal part will, for example, prevent the electrolyte from connecting the cathode and anode and so eliminate the ionic current flow. Sacrificial anodes, to site another example, can halt corrosion by integrating an alternative metal material into the metallic path (usually zinc) to halt corrosion of the more valuable, protected equipment. This method of corrosion control is called cathodic or sacrificial protection. Glenair’s GroundControl Earth Bonds are used in an undersea application of this type to mitigate corrosion on oil production equipment. But Glenair’s composite thermoplastic engineer, Tom Young, would argue that the ultimate solution to corrosion is of course to replace the metallic basis materials with engineering plastics, essentially eliminating the anode from the equation altogether. More on this later. Corrosion in the real world occurs in degrees. Which is to say the amount of magnetic force between the anode and cathode, or the potency of the electronic and ionic currents, will affect the rate at which corrosion occurs. An airplane is an aluminum and stainless steel assembly which operates in what is essentially a gaseous electrolyte—the atmosphere. Imagine two different atmospheric conditions, starting with the most corrosion resistant—bone-dry Tucson Arizona, site of the mothballed U.S. Military Aircraft Storage and Disposal Center. Then picture a jet on the deck of the aircraft carrier Nimitz on station in the Persian Gulf. What you are imagining are two conditions at the opposite ends of the corrosion spectrum. The difference is the electrolytic properties of the atmosphere in which the two exist. Water is the most common chemical on the face of the earth. Water consists mostly of water molecules, but it also has low concentrations of H+ ions and OH- ions. The humid, salty air of the Gulf contains more water molecules and ion elements than the desert setting, and is thus the better electrolyte. When electricity is passed through this electrolyte, a chemical reaction called “electrolysis” occurs—which is another of the many forms of corrosion. And as we already know, when electricity flows in the presence of an electrolyte, chemical changes can occur to the metal materials connected by the current. For many different reasons, electrical potentials may vary from one point to another on an individual component or assembly, with the result being that anodic areas and cathodic areas can exist on a single part. The net result of all this activity is the famous electrolytic "corrosion cell," in which the assembly, bathed in electrolytes, becomes a battery and slowly, inexorably, is consumed by corrosion. To restate what we know so far, four conditions must exist before electrochemical corrosion can proceed: There must be something that corrodes (the metal anode), There must be a cathode (the opposite pole in the circuit), There must be a continuous conductive liquid path (the chemical “electrolyte”) usually from salt or other contaminations, and There must be a conductor (the metallic path) to carry the flow of electrons from the anode to the cathode. If you are confused about the role of the anode and cathode, just remember that anode comes before cathode in the alphabet just like oxidation comes before reduction. Anodes oxidize (they rust) while cathodes reduce (they don’t). Heavy Metals Electrical interconnect components are subject to galvanic corrosion, which occurrs due to the incompatible coupling of plating materials and base metals Anyone who has ever worked on a rusty old automobile knows the effect two dissimilar metals can have on one another. The indiscriminate mix of aluminum and cast iron components in older cars led to some truly monstrous corrosion problems due to the phenomenon of “galvanic” corrosion. Galvanic corrosion is the electrochemical action of two dissimilar metals in the presence of an electrolyte and an electron conductive path. Which is to say, it occurs when two different kinds of metals are in physical contact in the presence of an acid or salt—detergent, battery acid or even exhaust are common electrolytes in cars. A fun science experiment is to place two different metals in an electrolyte (like vinegar or salt water) and connect them with a voltmeter. The voltmeter will display an electric current running between the two metals or alloys. And while that current flows, materials are removed from one of the metals and dissolved into the form of an electrolyte—and that in a nutshell is galvanic corrosion. The reason galvanic corrosion is of interest to us is that it is the principal type of corrosion we battle in electrical interconnect systems. More to the point, it is the galvanic corrosion between the base metal of a part and its conductive plating that is the source of all the trouble. In the January 99 issue of QwikConnect we presented the application benefits of composite thermoplastics as alternative materials for connectors and backshells. One of the main benefits is of course their unlimited corrosion resistance when compared to conventional metal materials. Aluminum interconnect components, for example, are immediately subject to galvanic coupling which causes the metal material to be sacrificed to its cadmium/nickel plating. Substituting composite plastic for the aluminum efficiently eliminates one of the four conditions required for corrosion to occur. In this case, it eliminates the anode, leaving the metallic plating (the cathode) with no one to party with in the ionic bath. Glenair has a responsibility to deliver interconnect systems and hardware to its customers without "built-in" corrosion problems. To prevent corrosion problems in backshells, for example, our engineers use their thorough understanding of design and materials science to produce conductive, plated products which both prevent EMI and resist corrosion in harsh application environments. This exacting work takes place in three principal areas: Strict attention to dissimilar metal combinations; The specification of corrosion-resistant materi- als including plastics and stainless steels in severe environments wherever possible; and The use of surface coatings such as nickel, zinc or gold plating to isolate base metals from reactive electrolytes. Let’s look at each approach in turn: Dissimilar Metals Revisited Metal materials used in manufacturing have different electrochemical potentials. Even so-called “pure” metals have inherent differences in electrochemical potential at the microscopic level. This is why a block of steel sitting all by itself corrodes. The order in which metals will corrode is always from the most anodic (active) to the most cathodic (noble). When two dissimilar metals are put together, only the more anodic metal will corrode. The magnitude of potential difference between the two metals, and which of the metals has the more negative potential, will determine which metal will be the cathode, which will be the anode, and the rate at which corrosion will occur. Dissimilar metals are the most frequent cause of unexpected corrosion failures in marine environments, which is why Glenair’s composite thermoplastic junction boxes and accessory fittings are of such value in systems which are subject to salt spray, stack gas and other corrosive electrolytes. Metals exposed to seawater corrode by releasing metal ions into the water around them. This happens at different rates for different metals. The balance between the reaction in which metal ions go into the water (the anodic reaction) and the reaction that uses up the electrons (the cathodic reaction) causes the metal to sit in a specific narrow range of voltages. The measurement of all the voltages for all the metal types found in the system is called a galvanic series (see Figure 2). PHOTO: NASA The corrosion buck stops here: The NASA corrosion test bed at Florida’s Kennedy Space Center is home to the world’s most comprehensive corrosion research and testing facility. Aluminum, bronze and stainless steel are all common material choices in interconnect hardware. Tin-plated copper wire is the most common material for RFI/EMI shielding, together with nickel-plated copper and silver-plated copper wire. Cadmium over nickel plating, the standard “W” finish specification called out in MIL-C-85409, is by far the most common metallic surface finish, followed closely by commercial combinations of zinc, nickel and copper flash. It should be readily apparent that with this broad a range of metal types in use, an interconnect system is a dissimilar metal accident just waiting to happen. The dissimilar metal problem is compounded by the unfortunate porosity of plated surfaces and the potential for protective plating to be scratched or damaged in handling. Both situations allow for electrolytic coupling of the base metal to its cadmium and nickel plating, which eventually results in catastrophic corrosion of the base material. As discussed above, a galvanic battery essentially sacrifices the less noble aluminum backshell to its more noble cadmium and nickel plating. An obvious step to take in this regard would be to plate a more anodic finish metal over a more cathodic base metal. The more anodic metal would then sacrifice itself (corrode first) and protect the metal underneath from damage. But of course the dual purpose of the plating in the interconnect system is both EMI shielding as well as corrosion protection. So the elimination of nickel in favor of some other (less noble) metal is not possible if systems are to maintain their necessary permeability and surface conductivity for H field EMI shielding. The next best approach therefore is to select metal combinations which are at least compatible within an index of .25 volts on the galvanic table. When this maximum is adhered to, the sacrificing of less-noble metals to more-noble metals is controlled, and components will survive the 500 hour salt fog exposure required by military standards. Go For This One, It’s a Stainless Steel We’ve already noted how the use of composite thermoplastic materials is an effective solution to galvanic corrosion. But in certain applications the use of plastics is not yet considered acceptable, for reasons such as strength or because of extremely high temperature requirements. In such situations, the best alternative is to specify an appropriate stainless steel alloy. Stainless steel is a family of metal alloys that contain at least 10.5% chromium and less than 1% carbon. These two criteria make “stainless” steels totally different from their “mild” steel cousins. The presence of chromium creates an invisible surface film that resists oxidation and makes the material "passive" or corrosion resistant. Additional alloying with molybdenum, nickel and nitrogen can alter the material to meet the needs of different corrosion conditions, temperature ranges, and strength requirements. There are more than 60 grades of stainless steel. For example, when nickel is added and the chromium level is increased, a particular alloy family is formed called "Austenitic” stainless steel. Austenitic stainless steels are famous for their extreme corrosion resistance, weldability and exceptional resistance to both high and low temperatures. The various grades of Austenitic alloys include 304 (most used), 310 (for high temperature), 316 (for better corrosion resistance), and 317 (for best corrosion resistance). Glenair’s stainless steel connector accessories and Band-It® Bands are all made from this family of stainless materials. Glenair’s stainless stell connectors and accessories are all made form passivated austenitic stainless steel. As we mentioned before, the chromium content of stainless steel causes the formation of an invisible, corrosion-resisting chromium oxide film on the steel surface. If damaged mechanically or chemically, this film is self healing, providing that oxygen, even in very small amounts, is present. The protective quality of this oxide film layer can be enhanced through a process called passivation. According to ASTM A380, passivation is "the removal of exogenous iron or iron compounds from the surface of stainless steel by means of a chemical dissolution, most typically by a treatment with an acid solution that will remove the surface contamination, but will not significantly affect the stainless steel itself." In addition, it also describes passivation as "the chemical treatment of stainless steel with a mild oxidant, such as a nitric acid solution, for the purpose of enhancing the spontaneous formation of the protective passive film." Passivation is a critical step in the manufacture of stainless steel connector accessories. The process removes "free iron" contamination left behind on the surface of the stainless steel from casting, machining and other secondary operations. These contaminants are potential corrosion sites that can result in premature corrosion and ultimately in the deterioration of the component if not removed. While passivation is a required practice in the fabrication of durable stainless steel components for most industries, it is not completely free of risk. Perhaps the most overlooked variable in the entire passivation equation is the negative impact of substandard base material, as well as poor machining and heat treating practices. In some cases, cross contamination introduced during manufacturing and/or thermal processes can lead to unacceptable products. The use of cutting tools, grinding wheels, sanding materials or wire brushes made of iron, iron oxide, steel, zinc or other undesirable materials can easily cause contamination of the stainless steel. Austenitic stainless steels can also suffer from stress corrosion cracking to various degrees. Stress corrosion cracking occurs without significant metal loss in the presence of a continuously applied load. If a susceptible material fails by cracking and has numerous side cracks besides the one causing the failure, stress corrosion cracking should be suspected. For this reason, extreme care should be taken during all thermal processes to avoid the formation of oxides. Passivation, combined with a high quality grade of stainless steel, can create products which are extremely corrosion-resistant and will provide many years of useful service. Conversely, the use of inferior or poor quality grades of specialty metals, the introduction of free iron from secondary operations, or mistakes in the passivation process itself can lead to catastrophic failure including pitting, etching and/or total dissolution of the component. Adherence to accepted standards such as QQ-P-35C (inactive for new designs but still required on many older programs) and ASTM A-967 and ASTM A-380 (applicable for new applications) is essential. These standards are well-written, well-defined documents that provide guidance on the entire process, from manufacturing to final testing requirements. Blue Plating Special The third leg on the corrosion-prevention stool is the protective coating of potentially corrosive materials with various forms of electro- and electroless platings. Again, this is because an unbroken (perfect) protective coating on the surface of a metal part will prevent an electrolyte from connecting the cathode and anode and thus eliminate the ionic current flow. The origin of the plating process can be traced to Luigi V. Brugnatelli, who first performed the electrodeposition of gold around 1800 using the Voltaic Pile as discovered by his piasano, Allisandro Volta. Interestingly, an insult from Napoleon Bonaparte caused Brugnatelli to confine the publication of his works to his own journal, which resulted in the loss of this information for almost forty years. John Wright, from Birmingham, England, found that potassium cyanide was a suitable electrolyte for gold and silver electroplating. His work, combined with that of the Elkington cousins, resulted in the issue of several patents in 1840. Brugnatelli’s work and these later discoveries and patents are the foundation of modern plating. The term electroplating means the coating of an object with a thin layer of metal by use of electricity. The metals most often used are gold, silver, chromium, copper, nickel, tin, copper, cadmium, and zinc, but many others are also used. The object to be plated, called the “work,” is usually a different metal, but can be the same metal or a nonmetal, such as an engineering thermoplastic. Electroplating usually takes place in a tank of solution containing the metal to be deposited on the work. This metal is in a dissolved form called ions. An ion is an atom that has lost or gained one or more electrons and is thus electrically charged. You cannot see ions, but the solution may show a certain color; a nickel solution, for example, is typically emerald green. Figure 3. Positively charged copper ions are free in the solution, but are being attracted by the negatively charged key. As the ions contact the key, they regain their lost electrons and become copper metal and stick to the key wherever they touch it. This is the basic process of electroplating, and all forms of it work the same way. When certain metallic chemicals dissolve in water, the metal atoms of these chemicals are freed to move about, but lose one or more electrons (negative charges) and, as a result, are positively charged. The object to be plated is negatively charged and attracts the positive metal ions, which then coat the object to be plated and regain their lost electrons to become metal once again. A familiar example of this process is the experiment often performed in grade schools in which a key is plated with copper. The key (the cathode) is connected to the negative terminal of a battery and is placed in a solution of vinegar, a weak acid. The positive terminal of the battery is connected to a piece of copper (the anode), which is placed in the solution. The acid slowly dissolves the wire, making copper ions that are then attracted to the key, regaining their lost electrons to become copper metal again, but now in the form of a thin coating on the key. The battery forces this activity and prevents the deposited copper from redissolving (see Figure 3). There is another plating process, discovered in 1946, called electroless plating. It earned that name because it operates without using electricity; the action is purely chemical and runs by itself, once started. Electroless plating enables metal coating of nonconductive materials, such as plastics, glasses and ceramics. Unlike electroplating, coatings are usually very uniform. The deposition is carried out in liquids (solutions), and is based on chemical reactions (mainly reductions), without an external source of electric current. Electroless nickel plating is applied to a broad range of Glenair connectors and accessories. The process is suitable for complex shapes and large-size components as well. The deposited layer of nickel has low porosity and high resistance to wear and corrosion. While other metals may be used in electroless plating, nickel is the only high-temperature metal used in commercial production. Most effective protective coatings used in the interconnect industry use a combination of two or more finish materials in order to place a physical barrier between the electrolyte and the electrodes, and to prevent galvanic corrosion due to dissimilar metals. The approved U.S. Navy finish, cadmium over electroless nickel (see sidebar, left) is the most common finish of this type provided by Glenair. But environmental concerns and known health problems associated with cadmium have increased pressures to find a substitute protective coating which both meets salt spray requirements and provides the necessary conductivity and magnetic permeability. Glenair has developed an alternative process, Zinc Nickel, which promises to be a viable alternative to cadmium with improved salt spray performance (see sidebar, above). The new Zinc Nickel plating provides a harder, more scratch resistant exterior than cadmium, without the environmental and health hazards. Fighting corrosion is a constant battle in electrical interconnect systems. And Glenair has developed a broad range of technologies and techniques to win the battle in even the harshest application environments. Our objective is always to build parts which meet industry standards and provide years of useful service in the field. Corrosion-proof composite thermoplastic solutions, stainless steel technologies, and innovative finish platings are at the forefront of our efforts to solve corrosion problems before they can affect the safe operation of high-reliability interconnect systems.

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