ASTM F1941 Specification

• Standard electrodeposition process

• Barrel-Plating Process
The preparation and metallic coating of threaded fasteners is usually accomplished by the barrel-plating process. In this process, quantities of an item are placed within a containment vessel, called a barrel. The barrel is designed to move the group of items, together, through each of the process steps, allowing ready ingress and egress of processing solutions and rinses. As the barrel is moved through the process steps, it is also rotated such that the individual items are constantly cascading over one another. This can damage the external threads of fasteners. The effect of thread damage is worse on heavy fine threaded fasteners than on light course threaded fasteners. In some of the process steps, notably the electrocleaning and electroplating steps, an electric current is applied to the group of items. The cascading action randomly exposes the surface of each individual piece to the process electrodes while also maintaining electrical continuity between all of the parts. The local coating thickness on a part is a result of the electrical current density at that location. Therefore, the coating thickness on an individual screw or bolt tends to be greatest at the extremities (head and threaded tip). The extremities being the high current density areas receive the greatest coating thickness. In contrast, the center or recesses such as the bottom of the threads, which are the low current density areas, receive the lowest coating thickness. This phenomenon is accentuated with increasing length and decreasing diameter of the screw or bolt. The extremity-to-center coating thickness ratio increases with increasing length and decreasing diameter, but is also a function of process parameters such as plating solution chemistry and efficiency, anodic/cathodic efficiency, average current density and plating time.
• Rack-Plating Process
The preparation and metallic coating of threaded fasteners can be accomplished by the rack-plating process, particularly on large size fasteners where thread fit and/or damage is a concern, or for smaller size fasteners, when it is economically feasible. In this process, quantities of an item are placed on a support, called a rack. The rack is designed to move the group of items, together, through each of the process steps, allowing ready ingress and egress of processing solutions and rinses. In some of the process steps, notably the electrocleaning and electroplating steps, an electric current is applied to the group of items. The electrical continuity is maintained between the parts by the rack itself. The average current density is usually low enough such that the extremity-to-center coating thickness ratio is much lower than with barrel-plating. The external thread damage is also minimized in comparison to barrel-plating due to the absence of tumbling.