Zinc coatings prevent oxidation of the protected metal by forming a barrier and by acting as a sacrificial anode if this barrier is damaged. Zinc oxide is a fine white dust that (in contrast to iron oxide) does not cause a breakdown of the substrate's surface integrity as it is formed. Indeed the zinc oxide, if undisturbed, can act as a barrier to further oxidation, in a way similar to the protection afforded to aluminum and stainless steels by their oxide layers. The majority of hardware parts are zinc plated, rather than cadmium plated..inc-flake coatings are sacrificial coatings which contain zinc and/or aluminium elements that oxidize sacrificially to ensure the substrate of to which they are applied remains corrosion free. These coating systems were designed for automotive and industrial parts that require greater protection than what standard paint and other coating systems can supply.
Zinc-flake coatings are used to protect non-flexing or rarely-flexing parts like brake rotors, drums and callipers; brake lines and hose clamps; fuel tanks and filler pipes; mufflers; the underside of hoods; clips, fasteners, springs and stampings; seat belt components and door lock components…and pretty much anything else that comes in contact with the elements and can survive a 450-degree curing cycle. These coatings can also prevent galvanic corrosion as a result of dissimilar metals coming in contact with each other. This is an extremely beneficial attribute in today’s age of light-weighting vehicles where aluminium components are more prevalent.
The purpose of the specification is to provide requirements for a thin film zinc plating to provide corrosion protection. This coating is primarily used on fasteners, but is also appropriate for other small parts made from ferrous material.
IS 1573 : Specification for electroplated coatings of zinc on iron and steel
ASTM F2329 / F2329M – 15 : Standard Specification for Zinc Coatingto Fasteners
Zinc plating is a relatively complex process that requires a high level of expertise. Specialized equipment and machinery is also required, including a rectifier, plating station, ancillary tank for proper dissolution of the zinc anode and a reservoir. Here’s an overview of how a typical zinc plating process works:
a) Preparing the plating solution
a) Acid Zone
a) Alkaline Zinc
a) Zinc plating procedure
a) Electrical current process
a) Post treatment procedure
• Preparing and cleaning the surface — It’s essential to thoroughly clean the surface of the substrate prior to plating. Any debris or contaminants remaining on the surface will prevent proper adhesion of the zinc coating. An alkaline detergent solution is typically used for surface cleaning, which is followed by the application of an acid treatment to remove surface rust. This latter procedure is referred to as pickling.
• Preparing the plating solution — Zinc plating requires the immersion of the substrate into a specially formulated electrolyte solution, which is referred to as the plating bath. The bath consists of the zinc metal ionic solution and various chemicals that facilitate plating processes. They also help produce the desired chemical and physical properties of the finished product. Specific types of zinc electrolyte solutions include.
• Acid zinc — This is a widely used plating technology known for its high efficiency, fast deposition and superior covering power. However, acid zinc also provides poor throwing power and thickness distribution.
• Alkaline zinc — This offers less plating efficiency than acid zinc and a slower electrode position rate, but it provides better thickness distribution and ductility
• Choosing the appropriate zinc plating procedure — After solution preparation, the parts are ready for plating. The chosen method could involve rack plating, where larger parts are affixed to metal racks which are placed inside the tank containing the plating bath. The parts remain stationary during plating. Barrel plating is normally used for smaller parts — instead of a plating tank, the parts are placed inside a barrel and rotated, which provides a more uniform finish.
• Introducing the electrical current — Electroplating is also known as electro deposition because an electrical current is used to deposit metal ions onto the surface of the substrate. In the case of zinc plating, the substrate serves as the cathode. A DC current originating at the anode is introduced into the bath and flows to the substrate. The zinc ions are then deposited onto the surface. The current flows from the cathode back to the anode to complete the circuit.
• Post-treatment procedure — Upon completion of the electro deposition process, the parts are ready for post-treatment. This normally involves rinsing the parts in water to remove any remaining contaminants and plating bath remnants. In cases of heavy contamination, the parts may need to be rinsed several times. The final step is to thoroughly dry the zinc-plated parts. In situations where additional corrosion protection is required, the application of passives and sealers can be included in the post-treatment process.
During acid cleaning and in the electroplating process, atomic hydrogen produced can diffuse into the steel and embrittle the structure of fastener. The electroplated coating traps the hydrogen inside the fasteners, which can migrate to areas of high stress and cause small micro cracks & ultimately lead to brittle failure, unless they are baked soon after plating to drive the hydrogen out. High strength fasteners are particularly prone to hydrogen embrittlement
because greater the strength (or hardness) of the alloy fasteners, the greater the susceptibility to hydrogen damage failure. At KE the post-plating de-embrittlement baking is carried out on Fully Automatic Electroplating line with synchronized Auto loading system & hydrogen De-Embrittlement furnace & Passivation line. Thus no time is wasted in - between the processes, allowing for highest quality plating.
Photographs of the sample parts, before inspection must be included. The photograph or image Caption must clearly include the part reference, measured values and exposure conditions.
The technical staff member and the laboratory manager must sign and certify the results presented in the report at the end of the document