Why are some hex nuts blue in color?
It is often asked why some high strength hex nuts that are supplied with high strength bolts appear to be blue or some other color. This is normally the case with galvanized, high strength, heavy hex nuts and the reason for this is because of the wax lubricant applied to the nuts. According to the ASTM A563 specification, “Hot-dip and mechanically deposited zinc-coated Grade DH nuts shall be provided with an additional lubricant which shall be clean to the touch”. The dye color is so that the presence of the lubricant is obvious and recognizable. Supplementary Requirement S2 under ASTM A563 specifies that the lubricant shall have a contrasting color to make its presence obvious, however it is not required unless specified. Since the A563 specification only requires an additional lubricant for galvanized DH nuts, Supplementary Requirement S1 can be specified, which requires that the nuts shall be provided with an additional lubricant regardless of the finish.
When lubricated A563DH nuts are supplied, they will typically possess a contrasting color, most often blue, regardless of whether or not the supplementary requirement, S2, has been specified. This makes the lubrication easy to recognize and alleviates any potential confusion. Another thing to keep in mind is that ASTM A194 2H nuts may also come with a dyed lubricated coating. The reason for this is because the ASTM A563 specification allows for A194 2H nuts as an acceptable substitute for A563DH.