A carriage bolt, also known s a coach bolt or as a 'cup square hex bolt', has a domed head, and the shank is topped by a short square section under the head. The square section grips into the part being fixed (typically wood), preventing the bolt from turning when the nut is tightened. A rib neck carriage bolt has several longitudinal ribs instead of the square section, to grip into a metal part being fixed.
Galvanised coatings prevent oxidation of the protected metal to create a longer lasting finish. It does this by forming a barrier and by acting as a sacrificial anode if this barrier is damaged. Galvanising is a plating that (in contrast to iron oxide) does not trigger a breakdown of the substrates surface integrity as it is formed. The galvanising, if left intact, 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.