ZINC PLATING IS A WIDELY used coating for the sacrificial corrosion protection of steel. It is used throughout the range of industry, especially on stampings, fasteners and automotive parts. Zinc also provides an excellent base for painting and other coatings. The final product is aesthetically pleasing, typically very bright, and quite uniform in thickness Zinc is applied through the process of electroplating.
The steel parts get a cathodic, or negative charge applied to them which attracts zinc metal ions from the plating bath to the surface of the part. When these ions contact the steel surface and the negative charge, they are reduced to metallic zinc and electrochemically bonded to the steel surface creating the zinc deposit. Chemicals are added to the bath to modify the deposit to make it smooth, uniform and bright. Small parts that are not fragile can be barrel processed which saves cost while larger parts and parts with fragile critical features are rack plated to avoid damaging the parts in the barrel.
Zinc is almost always provided with a supplementary chromate or passivate which runs the range of color from blue bright or
clear, to iridescent, yellow, bronze, olive drab, and black. Most chromates today are trivalent (+3) due to the toxicity of hexavalent chromium (+6). Though the correct technical term for trivalent chromium coatings is “Chromite”, the term chromate has been around so long people use it interchangeably and simply refer to the coating as “Trivalent Chromate”.
The chromate is a conversion coating on top of the zinc that is sacrificial to the zinc plating. Corrosion resistance is good with the new generations of chromates, typically reaching 48 to 120 hours neutral salt spray to white corrosion. Yellow trivalent chromates perform equally well, however, unlike their hexavalent predecessors, trivalent yellows are made yellow with a dye.
These dyed systems usually result in very poor adhesion for subsequent processes such as paints, powder coats, adhesives, and other coatings.
Thick film or high-performance chromates can exceed 240 hours to white corrosion. Hexavalent chromates are still available but capacity is limited as most platers do not use or offer hexavalent chromate options for zinc due to environmental and health regulations coupled with mandates such as RoHS and ELV Directives. The appearance of red rust is a function of the zinc thickness once white corrosion occurs.
Zinc-plated and chromated parts may also have a topcoat seal applied. These seals, which are normally either silicate-based organic topcoats or lacquers, have two primary functions. They offer added corrosion resistance to the chromate and zinc system by providing a barrier to the chromate and, depending on the formulation, some protection to bare surfaces such
as deep holes and tube IDs.
The other benefit these seals can provide is a modification of torque tension properties to provide a consistent coefficient of friction. This is important in robotic/automated assembly where torque tension and torque retention can be critical properties in safety.
Substrate preparation is the most critical factor in achieving a top-notch finish and maximum corrosion resistance. Certain parts may require additional pretreatment outside the normal process. If parts are heavily covered in excess machining coolant or stamping oils, a degreasing operation is usually needed to remove the excess oils and to avoid contamination of the plating line baths. This can be done online or offline depending on the plating system set-up.
Heat-treated parts present certain risks that must be accounted for and require best practices to ensure the integrity and safety of the part when put into use. If a part is hardened and then machined, ground, cold-formed or cold straightened after being heat-treated, that part should be stress tenacious contaminants, relieved by baking prior to pretreatment and plating. Media blasting such as glass bead and shot peening as well as thread rolling impart a compressive stress on the part which actually improves the function of the part and therefore it would not require a stress relief bake prior to pretreatment.
Parts with scale may first require descaling before processing to promote good plating adhesion and an aesthetically pleasing finished part. This may include welded fabrications, heat-treated parts with heavy scale, hot rolled steel, and thermal deburred
parts. There are several options for descaling including alkaline descalers, acid descalers and pickles, and media blasting or tumbling.
Acid pickling should not be done on heat-treated parts as this process can impart a condition called hydrogen embrittlement, which can render the steel part very brittle due to the entrapment of hydrogen gas in the steel. Unfortunately, this defect is typically not discovered until failure occurs after the part is put into use and under load. Media blasting is the preferred method for scale removal on hardened steels. They are usually the most costly methods as well.
Most parts do not require the pretreatments described above and are typically processed through the plating line right out of the containers they were shipped in. There are two typical process flows in zinc plating:
- Soak cleaner – To remove soils, grease, oil, etc.
- Electro cleaner – To loosen and remove oxides and scales and other tenacious contaminants.
- Acid activation – To finish removal of any oxides and metallic contaminates and to activate the substrate for zinc.
- Zinc Plate – Parts are plated to thickness specified for the corrosion resistance required.
- Chromate/Passivate – To add the specified conversion coating to provide the finish and corrosion resistance required.
- Seal (if required) – To further add to the corrosion resistance and if required to impart a specific coefficent or friction to modify torque tension properties of plated part.
BARREL SYSTEM (most common with fasteners)
- Soak cleaner
- Acid activation
- Electro cleaner
- Weak acid – To neutralize the alkaline electro cleaner and keep parts active for plating.
- Zinc Plate
- Seal (if required)
Throughout the world, zinc is one of the most widely used corrosion inhibiting coatings. It has been electroplated onto parts since at least the start of the 19th century when electro galvanizing research was sparked by Alessandro Volta’s invention of the electrochemical battery.
The finish has endured due to the reliability of the process, the cost-effectiveness of the finish, and the availability of the process. It endures despite ever-growing regulatory pressures such as RoHS, REACH, ELV, and other laws and directives. In the 1980s, tightening regulations and the need for better corrosion protection and replacements for toxic processes such as cadmium and hexchrome chromates led to a new wave of research. This research led to the invention of hexavalent chrome-free passivates and zinc alloy coatings that improved on the zinc electroplate while providing environmentally friendly alternatives for many environmentally unfriendly processes.
Alloys like tin zinc provide enhanced corrosion protection and lubricity. Zinc nickel provides up to 10 times the galvanic protection on steel that zinc does while offering better resistance to other chemicals such as fertilizers for agriculture applications and chlorides for saltwater exposure.
When done properly and with the proper attention to alloy constituents and alloy condition, zinc electroplating is an easy choice for applications throughout virtually every industry. As a finisher, make sure you get all the information to properly pretreat, plate and post-treat the parts. As a manufacturer looking to get your parts plated, make sure you provide the
finisher with all the information needed to provide a process that utilizes best practice to yield the desired quality zinc plating finish.