Black Oxide vs Galvanizing for High-Strength bolts Comparison & Application Guide

Black oxide and galvanizing are the two most common surface treatments for grade 8.8, 10.9 and 12.9 high-strength bolts. Improper selection will cause rusting, torque deviation and thread galling. This article compares their performance and applicable scenarios.

Process Principle

Black oxide forms a thin protective film without changing thread size, ensuring high precision. Galvanizing forms a zinc protective layer with excellent anti-rust performance but increases surface thickness.

Performance Comparison

Galvanizing provides much better salt spray and moisture resistance, suitable for outdoor and humid conditions. Black oxide features stable friction coefficient and precise torque control, ideal for high-precision heavy-duty assembly.

Selection Suggestions by Grade

Grade 12.9 bolts prefer black oxide for precise torque control. Grade 10.9 bolts use black oxide for indoor equipment and galvanizing for outdoor steel structures. Grade 8.8 bolts are widely applicable with both treatments according to working environments.

Application Notes

Use black oxide for high-precision and high-torque connections. Avoid black oxide bolts in coastal and humid environments. Check zinc accumulation on galvanized threads before assembly to prevent slipping and galling.