The malfunctioning of controls and switching gear 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 when 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 Problems
Problems 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, an 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 and petrochemical industries have to cope with the corrosive ingredients found in oils and gases, and possibly a salt-laden marine environment, while coal-based power stations and metallurgical and mining industries create corrosive dust in the air while hot metal production involves heat combined with corrosive acids. (Related reading: The 6 Corrosive Components That Can Be Found in Crude Oil.)
Wire Terminals for Automated Equipment
Industrial automated and semi-automated control equipment has a number of wire terminals that connect communication, power supply and control systems to enable their smooth functioning. 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 for 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.
A reliable and sustainable electrical connection has certain prerequisites, such as:
Corrosive Chemicals
Electrically 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 Construction
In 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, clamps and screws are made of brass with nickel plating or steel with a 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 an 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. (To learn more about fretting, read Understanding and Preventing Fretting Corrosion Damage in Electrical Devices.)
Protective Coatings
Besides 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 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 a sacrificial coating. (For more about zinc coatings, read Galvanization and its Efficacy in Corrosion Prevention.)
Protective coatings applied as a barrier provide a physical seal for the metallic substrate, preventing electrochemical and chemical attack. The coating is not 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
(For a quick introduction to metal coatings, read: 5 Most Common Types of Metal Coatings that Everyone Should Know About.)
Medium 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 the 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 component's life.
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 requires specific 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 need 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 be allowed to 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 Advantage
Stainless 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 is 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. (Related reading: Why is Stainless Steel Corrosion Resistant?)
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 attack also increases due to ferrous contaminants on the stainless steel surface acquired during machining. Contaminants are removed by a passivation process.
Brass with Nickel Plating
As an alloy of copper, brass is susceptible to corrosion when exposed to acidic substances such as chlorides and 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 regulatory organizations.
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 as much 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 and other compounds are poor conductors of electricity. 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 plating on current bars, terminals and wires. Tin is able to provide protection against the formation of sulfur compounds and oxides.
Fretting Corrosion of Electrical Connectors
Fretting corrosion is a combination of mechanical wear due to relative movement and vibration, and the 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 that causes severe power loss across joints and connections.
The application of a lubricant on contact surfaces (e.g., 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. A 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:
- Vibration
- Frequency
- Pressure or force
- Air humidity
- Temperature
- Base metal and coating used
Accelerated tests of fretting corrosion are used to determine the longevity of coatings on contact surfaces. In general, electrical components are packed in airtight housings to protect them from corrosion.
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 most preferred plating metals, and it is selected 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, 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 are resistant to crevice corrosion. The application of sealants can also prevent this form of corrosion.
Steel Conduit Corrosion
Strong 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. Conduit systems include rigid steel conduits, intermediate metal conduits, electric metal tubing, and elbows, couplings and nipples. On the outside diameter surface, steel conduit is given a zinc alloy coating, while the inside diameter surface is coated with zinc alloy or a non-metal coating such as polyvinyl chloride (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 Conduit
PVC 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 necessary for 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 corrosion protection.
Corrosion of Low Voltage Systems and Homes
Electrical panels and other equipment accessories in low-voltage applications become damaged due to corrosion caused by moisture ingress. Circuit breakers may malfunction due to this condition if water enters 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 connectors 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 connectors.
Other areas affected may include:
- Panel surfaces
- Base of panels
- Bus bar connectors
- Service entry cables
- Terminals of the main fuse block
Reasons for moisture ingress may include:
- Collection of moisture at panel bottom
- Moisture condensation on panel components
- Damp climate
- Roof leaks
- Wind ingress
- Water from worn fabric cable covers
The solution to this moisture ingress lies in frequent inspections and the elimination of the root cause. Panel components should be coated with anti-corrosive paints or sprays, and damaged parts should be restored by metallic sprays of zinc or aluminum.
Besides the moisture, other components like battery terminals corrode due to acid spillage. Conductive greases and contact greases are used for rusting of contacts and battery terminals.
Control Room Corrosion Protection
Control 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
Corrosion protection of control rooms, switches, computers, instrument controllers and other equipment can be ensured by:
- Keeping the control room pressurized with clean air
- Purifying incoming air using activated carbon
- Humidity and temperature control
- Removal of air contaminants
Conventional air conditioning cannot deal with industrial contaminants. Corrective/preventive measures to address this include:
- 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