Fasteners are most commonly protected from corrosion by plating or coating. In addition to corrosion protection, these surface finishes provide lubricity during the drilling and tapping of threaded fasteners which can help to reduce the tapping torque required to install them. The major types of finishes are as follows:
Phosphate coating.
This black or gray appearing finish is most commonly used on drywall screws. Phosphate is a
porous coating, which is usually applied in combination with oil. It is the lowest cost of all fastener finishes and as such, offers only a minimal barrier to corrosion. It is suitable for indoor applications only, where there is minimal chance of exposure to moisture.
Mechanical zinc plating.
Powdered zinc is applied to the fastener surface by tumbling the fasteners with water, a chemical catalyst, and glass beads which pound the zinc onto the fastener surface. The thickness of the zinc can range from 0.2 to 3 mils (1 mil = .001 inch = 25.4um where um is microns) with increasing thickness of zinc improving the corrosion resistance. The zinc coating remains relatively porous, resulting in a coating with good galvanic protection, but a lesser degree of barrier protection. A chromate sealer can be applied over the zinc for increased barrier protection. This application method cannot uniformly apply zinc to the root diameter of threaded or knurled parts, and thus can provide maximum protection only on smooth fasteners.
Electro-zinc plating.
This most common fastener finish offers good galvanic and barrier protection. The steel fasteners are first cleaned in acid to insure satisfactory adhesion of the zinc. The fasteners are then introduced to a solution of zinc and an electrical current is passed through the fasteners.
The fasteners, acting as a cathode, attract zinc from the solution which builds up on the surface of the fasteners. The length of time the fasteners are left in the solution determines the coating thickness which can range from 3 to 25 um. The most common zinc thickness on fasteners is called commercial grade, which is 3.5 to 5 um. Clear or yellow chromate is applied as a sealer over the zinc.
There is often confusion concerning the words “galvanized” and “plated” when referring to fastener finish. For fasteners, the term “plated” generally means a zinc coating less than 1 mil thick, and “galvanized “ refers to a zinc coating greater than 1 mil thick and generally implies application by hot dipping. Table 2 shows the relative thickness of zinc coating on typical building components.
Acid cleaning and the electroplating process cause hydrogen to form on the surface of the steel. Once hydrogen is formed, it is difficult to remove and can lead to hydrogen embrittlement cracking. Of greatest concern is that hydrogen embrittlement failures are typically delayed failures, occurring months or years after the fastener was installed. The failure occurs because hydrogen atoms migrate through the steel to the point of highest stress. If too much hydrogen is in the part, the hydrogen will build up and eventually cause the steel to crack. The higher the yield strength of the steel and the greater the hardness achieved after heat treatment, the more likely hydrogen embrittlement will occur. High-strength bolts, powder-actuated drive pins, and pneumatically driven pins are examples of fasteners with these characteristics.
Hydrogen embrittlement can be minimized by baking the parts after plating. Quality control tests can then be run on the fasteners to insure the possibility of a hydrogen embrittlement failure is reduced.