The malfunctioning of controls and switchgear results in more than 50% of electrical system downtime, causing a huge loss to the U.S. economy and the economies of other countries. How does this happen? The answer is corrosion. Harsh environments can cause electrochemical deterioration and chemical attack on metallic wire terminals. Overall, the corrosion of current-carrying parts and switching, as well as controlling equipment, affects the reliability and uninterrupted operation of all kinds of electrical components and production facilities.
Designers of electrical equipment and systems need to apply sound judgment in prescribing material specifications, designing equipment and applying coatings to minimize the deterioration of metallic surfaces in electrical components. Field experience, combined with accelerated test methods, help maintain a balanced approach. Here we'll look at corrosion in electrical systems and how it may be prevented.
The Nature of Corrosion ProblemsProblems resulting from the corrosion of electrical contacts include unintended power disruption and failure to disconnect or connect, as well as intermittent connection due to deteriorated contact surfaces.
In automated applications, intermittent connection is unacceptable. Therefore, industries need to analyze the risk of corrosion, whether it's from moisture, or from ingredients like reactive agents. Petroleum as well as petrochemical industries have to cope with the corrosive ingredients found in oils and gases, as well as a salt-laden marine environment, while coal-based power stations as well as metallurgical and mining industries create corrosive dust in the air while hot metal production involves heat combined with corrosive acids.
Wire Terminals for Automated EquipmentIndustrial automated and semi-automated control equipment has a number of wire terminals that connect communication, power supply, and control to enable the smooth functioning of automatic control. In such cases, a large number of individual metallic wires are connected to wire terminals with the capacity to distribute electrical power to processing equipment such as heaters and motors. Therefore, the electrical connectors working in a corrosive environment also need mechanical properties in order to maintain low electrical resistance metal-to-metal contact. Metal must be protected from corrosion to ensure low electrical contact resistance, high conductivity, and the mechanical capabilities required for accurate application of force.
Reliable and sustainable electrical connection has certain prerequisites such as:
- Low contact resistance
- High corrosion resistance
- Sustained mechanical stability
Corrosive ChemicalsElectrically operated automatic controls often have to work in corrosive chemical environments that may contain sulfur compounds and ammonia, which is known to cause stress-cracking in brass and other electrical metal parts. Chemicals in synergistic combination, along with heat and moisture, promote rapid corrosion of most metals.
Chlorine gas and chlorides are commonly present in paper and pulp-making processes, as well as food and beverage plants. Sodium chloride is present in seawater as well as the coastal atmosphere; it is also used in the food processing industry. Sodium carbonate is used in chemical processes, such as detergents, paper-making, and glass manufacturing processes. Nitrates are present in fertilizer processing, while sodium hypochlorite is used in the beverage industry. Sulfur dioxide is present in oil-and-gas handling and processing, cement, rubber and metal processing, as well as power plants and tire manufacturing.
Terminal ConstructionIn order to meet requirements for the electrical and mechanical functioning of systems, wire terminals are constructed as screw terminals or spring clamp terminals. In the clamping system of wiring connection, clamps and screws are made up of brass with nickel plating or steel with zinc chromate coating.
Copper with tin plating is generally used as conductor ferrules and as conductors and bar-shaped conductors. Sometimes nickel-plated bars are also used as conductors at interconnections. As the terminals are exposed to vibration, mechanical shock, and cyclical heat, along with a potentially corrosive environment, the clamping design must ensure airtight connection by maintaining a measured clamping force. A screw terminal may use an anti-vibration, self-locking arrangement to prevent gradual loosening. The application of correct tightening torque helps reduce fretting corrosion of the joint.
Protective CoatingsBesides stainless steel, every other metal used in electrical wiring terminals requires protective coatings. These coatings are classified as barrier coatings and sacrificial coatings. If the coatings get consumed during the normal operation, the coating is of the sacrificial type. Corrosion can begin when the sacrificial coating is fully removed and the substrate metal is fully exposed to a corrosive environment. Zinc coating on steel (galvanizing) is a typical example of sacrificial coating.
Protective coatings applied as a physical barrier provide a physical seal for the metallic substrate, preventing electrochemical and chemical attack. The coating does not get consumed during corrosive processes, but may fail due to porosity, mechanical wear-and-tear, or a defective coating process.
Materials used as barrier coatings include:
- Chromates as passivators, with zinc as a primer
- Additional top coats over a chromate layer
- Tin plating applied over copper, mainly for conduction
- Nickel-coated brass
Metallic Steel as Clamps in Screw TerminalsMedium carbon steel is normally preferred for screw terminals with a zinc layer and a chromate layer, along with an additional top coat. When the zinc layer starts to corrode due to environmental factors, a whitish powder is produced on the surface. When zinc is fully consumed, the reddish layer formed on the surface indicates the corrosion of steel. A thicker layer of zinc, therefore, can prolong the life of a component.
Because zinc is less noble than steel in the galvanic series, zinc corrosion is slow in most cases. An additional chromate layer with an additional top barrier coating further enhances the life of the protective system. Zinc alloy coatings such as zinc cobalt, zinc nickel, tin-zinc, and zinc-iron are widely used for their enhanced corrosion protection. Zinc nickel produces three times higher corrosion resistance as compared to a pure zinc layer of corresponding thickness when the substrate is exposed to salt water and other corrosive substances.
A barrier layer of hexavalent chromate on top of zinc or zinc alloys helps improve corrosion protection, but the material has now been replaced by trivalent chromate due to environmental concerns. Trivalent chromate needs certain top coats in order to be as effective as hexavalent chromate, which has some self-healing properties. Certain silicates are used as top coats above trivalent chromates. These top coats resist water and reactive chemicals. However, the conductivity of top coats needs to be checked when the coats are used on conductive surfaces, because in some terminals, the current-carrying conductor is directly clamped on the current bar of the terminal. The sealing coat on the clamp or current bar should not affect conductivity of the connection.
Some sealing materials are formulated with self-healing capabilities. Nano-composite techniques also provide plating formulations that can replace hexavalent chromates as a coating solution for corrosion protection.
The Stainless Steel AdvantageStainless steel produces a passive oxide film with moisture, which is sustained by the chromium in the steel. This film is considered to be the most effective option, because it's nonporous and self-healing. Stainless steel is the material of choice for corrosive environments for insulation displacement type, as well as spring clamp types of wire terminals.
However, under high mechanical stresses, and temperatures ranging from 150°F to 170°F (66°C to 77°C), the stainless steel of certain grades may fail, particularly due to stress corrosion cracks in typical acidic environments, such as chloride environments. Susceptibility to corrosive attacks also increases due to ferrous contaminants on the stainless steel surface acquired during machining. Contaminants are removed by a passivation process.
Brass with Nickel PlatingAs an alloy of copper, brass is susceptible to corrosion when exposed to acidic substances such as chlorides, as well as compounds of nitrogen. Stress corrosion cracking can occur due to a chemical environment and high mechanical stresses. But copper also has advantages, including high electrical conductivity and low overall cost.
Nickel plating is used to protect brass from corrosion. The plating process is prone to defects, such as pinholes and pores, giving highly corrosive substances access to the copper substrate in certain conditions. Nickel is also considered a health risk to those exposed to it. Nickel is even considered a material of concern according to European legislation.
Conductors and Current Bars of Copper with Tin Plating
Copper is primarily chosen for its very high current carrying capability and the fact that it doesn't generate heat due to resistance. Electrochemical reactions with oxygen and moisture cause the formation of copper oxide, as well as chlorides, sulfides, and other compounds depending on the chemical contaminants. Oxides as well as other compounds are poor conductors of current. As a result, they increase electrical resistance and heat, and fail to connect or cause flickering. Barrier-type protective coatings are often applied to copper in the form of tin plate on current bars, terminals, and wires. Tin is able to provide protection against the formation of sulfur compounds and oxides.
Fretting CorrosionFretting corrosion is a combination of mechanical wear due to relative movement and the vibration and electrochemical reaction of heavily loaded metallic surfaces. It occurs as an electrochemical and chemical reaction of metals used in wire terminal connections and contact surfaces. Tin plated contacts are typically affected by fretting corrosion. The resulting reduction in the contact area can lead to the unreliable operation of contacts and breakers, and an excessive voltage drop causing severe power loss across joints and connections.
The application of lubricant on contact surfaces, such as battery terminals, reduces fretting corrosion as well as wear and deterioration. However, the reduced friction at the joint can potentially reduce the stability of a tight connection and contact pressure of the surfaces. In electrical contacts, a conductive surface has to fulfill the function of conducting electricity without heating. This property is drastically reduced by corrosion, which raises contact resistance and increases the voltage drop and heat generation. Accumulated metallic oxidation products and the micro motions of surfaces can cause resistance value spikes and very high effective contact resistance. Apart from the vibration, differences in thermal elongation between surfaces also results in relative motion. High contact resistance can cause malfunctioning of signaling, controlling, and communication systems, particularly in miniaturized systems, as signaling power is badly impacted.
To prevent corrosion, copper-based alloys like bronze may be used as a base metal for contacts with tin and tin lead coating on top. Platinum and silver metals are also used as top coatings. In the case of tin coatings, oxides of tin cause high electrical resistance, even when contacts appear to be in good condition. Silver coating is excellent in corrosion resistance for tips of contacts. Plating, electroplating, dip coating, and swabbing are the techniques used for silver coating. The disadvantage here is the likelihood of galvanic corrosion and reactivity with high concentrations of sulfur compounds in the air and water. A larger amplitude of relative micro-movement causes very severe corrosion, along with higher contact pressure. The final signs of degradation show up as severe arcing, melting, delaminating, adhesion, and abrasion.
The rate of fretting corrosion depends on:
- Pressure or force
- Air humidity
- Base metal and coating used
A cross-section of an electrical contact is illustrated below:
Apart from a very high corrosion resistance, plating metal needs to be resistant to wear and fretting corrosion. It should also be highly conductive. While nickel and tin are affected by fretting corrosion under severe loads and corrosive environments, other alloys like palladium nickel are prone to cracking. Silver has a high corrosion resistance and is not subject to fretting corrosion, but its wear resistance is lower so a thicker layer—a minimum of 3 microns—is needed to ensure longevity.
When it comes to fretting corrosion, gold is one of the preferred plating metals, and it's chosen for high corrosion resistance as well as stable electrical performance. Passive metal coatings like tin and nickel may suffer from porosity. This can be overcome by increasing the thickness of the coating, but gold is resistant to corrosion agents such as hydrogen sulfide, sulfur dioxide, chlorine, ammonia, nitrogen dioxide, and salt fog.
Fretting corrosion can be controlled and prevented by using a contact lubricant to prevent contact with air and moisture. The ball and roller bearings of electric motors also fail due to fretting corrosion, due to severe loading, heat, and other corrosive agents unless they are protected by frequent relubrication.
Screws and fasteners used in wiring terminals also deteriorate due to crevice corrosion, mainly at threaded joints. Molybdenum containing stainless steels is resistant to crevice corrosion. The application of sealants can also prevent this form of corrosion.
Steel Conduit CorrosionStrong metallic conduits ensure effective physical protection for the electrical cables and conductors within. Due to the variability of corrosive working environments in which a conduit must function, it's impossible to predict the longevity of the conduit. The conduit systems include rigid steel conduits, intermediate metal conduits, electric metal tubing, as well as elbows, couplings, and nipples. On the outside diameter surface, steel conduit is given zinc alloy coating, while the inside diameter surface is coated with zinc alloy or a non-metal coating such as PVC.
A galvanized surface is protected against chemicals with a pH between 4 and 12. Steel conduits are primarily protected by galvanization. In addition to zinc, conduits are given additional coatings of a non-metallic substance. Other substances used as additional coating on top of zinc, are zinc-rich acrylic, urethane, epoxy-based coatings, and PVC. Proper surface preparation is important for effective corrosion protection. Surfaces must also be cleaned without scratching, blasting, or abrasion to keep the zinc coating intact. Metal sprays are used for repairing aluminum and zinc coats.
PVC Coated ConduitPVC is a non-metallic coating, which is evaluated for suitability-based flame propagation, compatibility to primary coat, fit of couplings, and electrical continuity with couplings. It is also sometimes salt spray tested and tested under UV light. A PVC-coated conduit is a necessity in applications where corrosion and abrasion can damage a standard electrical system and wiring. The process of applying the coating on conduits and fittings ensures good adherence to the metal as well as superior protection from corrosion.
Corrosion of Low Voltage Systems and HomesElectrical panels and other equipment accessories in low-voltage applications get damaged due to corrosion caused by moisture ingress. Circuit breakers may suffer malfunction due to this condition, as water may enter into panels at the service wiring entry point. This ingress can be due to storm water and wet winds. Corrosion of wiring connections, terminals, cable joints, contacts of breakers, and contactors may result in increased contact resistance, overheating and, eventually, complete system failure. Corrosion can also affect the grounding system, fuse terminals, ground bus bars, breaker terminals, and contactors.
Other areas affected may include:
- Panel surfaces
- Base of panels
- Bus bar connectors
- Service entry cables
- Terminals of the main fuse block
- Collection of moisture at panel bottom
- Moisture condensation on panel components
- Damp climate
- Roof leakages
- Wind ingress
- Water from worn fabric cable covers
Besides the moisture, other components like battery terminals get corroded due to acid spillage. Conductive greases and contact greases are used for rusting of contacts and battery terminals.
Control Room Corrosion ProtectionControl room corrosion shows up as:
- Corrosion of contacts, color changes, high resistance
- Corrosion and breakage of very fine wires, conductors
- Defective push buttons
- Broken conductors near junctions, bus stabs, motor starters, and terminals
- Keeping the control room pressurized with clean air
- Purifying incoming air using activated carbon
- Humidity and temperature control
- Removal of air contaminants
- For enhancing component life, choose thicker conductors and corrosion-resistant substances and inhibitors
- Use activated carbon to filter air and obtain pure air that's free from corrosive gases
- Coat power connectors with silver, tin, cadmium, or nickel
- Coat aluminum and copper bus bars against corrosion