This isn’t perfect but should be good enough from a distance.
It mixes two separate mixtures of diffuse and glossy, mixed using the same factor but in reverse to each other.
There should be an element of anisotropy (which isn’t in this shader) but it doesn’t get applied in the real world in the same way as in brushed metal. Galvanisation uses crystallisation and the direction of the anisotropy depends on the positioning of the crystals or “cells” in a voronoi texture.
Here is the .blend file.
galvanised_metal_shader.blend (646 KB)