Jane Debbrecht is the Principal of JDebbrecht Consulting and a management consultant specializing in manufacturing company turnarounds and strategy development. She held a number of engineering, manufacturing and leadership positions throughout her career, culminating in the position of President, Sulzer Metco North American division, a global leader of coatings products and services. She subsequently established an independent consulting business, and now leads manufacturing companies to improve performance. She has worked in a number of different industries, but specializes in the coatings area. Corrosionpedia interviewed Debbrecht about electroplating and how it's used in corrosion prevention.
Corrosionpedia: Can you briefly introduce and explain the electroplating process?
Jane Debbrecht: Electroplating is a process for coating a metal part (usually) with a thin layer of another metal, forming an alloy with the part’s surface to bond the coating in place. The process involves the use of an electric current flowing through a circuit consisting of a cathode, an anode and a liquid solution. The part itself is connected to the electricity source, and thus becomes the cathode. The electricity source provides electrons to the part, creating a charge on its surface. If we call the part material Me (metal), its surface could become Me2-, for example. To plate copper onto the surface, the copper ion Cu2+ must be in the solution. This will then be attracted to the metal part, forming MeCu at the surface to create the desired coating.
The solution and the process take place in a tank, which is called the bath. The electrical current and the amount of time during which the part is in the bath undergoing the plating process will determine the coating thickness. There is a practical limit to the coating thickness, and in general, it ranges from a very thin coating of less than 0.1 micrometer thick (.00004 in.) for quick "flash" coatings to about .060" as a maximum for chrome plating and up to .100" for nickel plating. The use of electroplating for decorative purposes only uses quite thin coatings, but we will be addressing those coatings of more use for industrial purposes.
In industrial usage, electroplated coatings are used for corrosion prevention, hardness to prevent scratches on a surface, creating lubricity or a low friction surface, to create a consistent, high-quality surface which will hold subsequent painting, or to create a conductive surface.
Corrosionpedia: The argument for electroplated coatings is always focused around its cost effectiveness, how well does this position hold up next to some of the other approaches?
Jane Debbrecht: Paints and plastics are used to encapsulate a surface to prevent corrosion by preventing the corrosive environment from coming into contact with the substrate. However, the environment that the part is placed in may not only be a corrosive environment - there may be abrasion, wear and impact, or the need for electrical conductivity or resistance, for example. Paints and plastics may more readily chip and so may not be as durable as a metal coating in an abrasive or impact environment. The cost factor must also be considered, so a zinc coating may be "good enough" in its intended environment, and the expense of a plastic coating is not necessary.
In terms of cost-effective solutions, as already noted, the total environment of the part must be analyzed. In addition, there are considerations such as upfront cost vs. life-cycle cost. For example, most mud rotors in the oil drilling industry are chrome plated. However, there is some movement to a carbide coating applied via a thermal spray process. Not the process but the material itself is considerably more expensive than the chrome, resulting in a total cost which is 2–3 times that of chrome plating. However, the part lasts for 4–5 times longer. Those users who value the lifetime cost of the part are looking at shifting to the carbide coating; those who find the issue of upfront cost to be more important, tend to stay with chrome plating.
Chrome plating also has a value in the repair cycle, because a part can be brought back to correct size tolerance if not worn more than the upper end of the thickness of chrome plating. Although somewhat expensive to apply this much chrome, it is still less expensive than replacing the whole part.
The electroplating industry has been active for hundreds of years, and is well known. Today, there is a movement away from some of the coatings, such as cadmium and chromium, because of the environmental problems associated with the processes. Manufacturers have learned to produce in a more environmentally responsible manner, but some stigma remains. There is a continual search for new coatings applied in other manners, but the plating which is still done is because it provides a cost-effective best solution. Other than the use of plating in the electronics industry, which needs the ability to provide a very thin coating, I do not see growth in electroplating. Where ultrathin coatings are required, I would expect to see nanocoatings come to the fore.
Corrosionpedia: Can you describe some of the different methods used in the application process?
Jane Debbrecht: Mass plating is used to coat a number of small parts, such as screws. It is generally used to coat with zinc, or galvanize. All of the small parts are placed in a basket, and plated in a tank en masse. It does not guarantee that every part of the surface is coated, and hence lends itself particularly to zinc plating. This is because of the way in which zinc performs, in that it is a sacrificial coating. As long as any is there, it will corrode preferentially to the underlying steel. It is thus quite inexpensive, but does the job.
Rack plating occurs when individual parts are placed in a rack and then into the plating tank. They are thus not touching each other, and will be completely plated. This requires more handling to set each part in its place on a rack, and thus will involve more cost. This is done when it is necessary to ensure the part is completely plated. An extension of this idea is when a single piece is plated, as occurs for large parts. It is very important to completely clean the part so no oil or contaminants prevent plating in any spot, and it may take eight hours or more in chrome plating to acquire the required thickness of coating. This obviously adds considerably more expense in handling, electricity costs and tank time.
Continuous plating is used to plate a coil of steel, usually either with zinc or tin for the food industry. As its name implies, the coil is unrolled through a series of tanks, between anodes, to plate the part in a continuous manner. This results in a thin layer of coating, and could not be used in the example above requiring eight hours of plating, because the length of the line of tanks would be prohibitively expensive.
Line plating refers to a more automated version of mass plating. The basket of parts must first be immersed in a cleaning tank, then moved to the plating tank, then to a rinse tank, for example. This creates the "process line" which the parts travel through.
Corrosionpedia: What metals are the most commonly used metals used to create effective electroplated corrosion prevention?
Jane Debbrecht: The most common metal coatings applied by electroplating for industrial use are zinc, nickel, chromium, copper and tin, and gold, silver and copper in the electronics industry where conductivity rather than corrosion is of importance. As with any coating, the choice is determined by the most cost-effective solution in a given environment.
Zinc plating, or galvanizing, is used extensively in the automotive industry and for outdoor metal structures. Zinc is more prone to corrosion (oxidation) than is the underlying steel, and so it is used as a sacrificial coating — as long as there is zinc present, it will corrode preferentially to the steel corroding, and in this manner prevents the corrosion of the underlying metal. This is quite different from the concept of applying a coating to provide a complete barrier to prevent the corrosive environment from reaching the substrate.
Tin is used most frequently in the food industry for corrosion protection, because it is non-toxic and also because it is quite ductile, allowing tin-plated steel to be formed into a can, for example, without damage to the tin layer. It is not strong enough, though, for harsher industrial environments.
Hard chrome plating results in a very hard coating, which is both corrosion resistant and more resistant to abrasive wear, and also provides lubricity. It is used extensively in the oil & gas industry. The wear resistance and lubricity make it desirable for the rod in hydraulic cylinders, and in airplane landing gear.
Nickel is particularly valuable for saltwater corrosion resistance. For this reason, it is often plated on a part first, followed by chrome plating, to provide a hard coating resistant in several environments.