The foundation of all fasteners (excluding pins and retaining rings) lies in their threads. Without threads, mating is impossible (except for special fasteners like rivets). Threads are regular helical protrusions and recesses machined on metal surfaces. Two complementary threads form a complete locking mechanism through rotational engagement. In China, common thread standards include metric (GB) and imperial (UNC/UNF). Imperial external threads are classified as 1A/2A/3A, while internal threads are 1B/2B/3B. Metric threads use precision grades (e.g., 6g for external, 6H for internal threads). Higher thread grades indicate better mating precision, regardless of the standard.
Grade 12.9 high-strength screws are prone to corrosion during packaging, storage, or transportation. Even if initially compliant, screws may develop surface rust during transit, leading to returns or losses. Corrosion primarily occurs due to exposure to atmospheric moisture and carbon dioxide, which react with the metal surface. Effective preventive measures are essential.
Electroplating is a common solution. Beyond aesthetics, its primary purpose is corrosion resistance. Mature electroplating processes include zinc plating (electrogalvanizing or hot-dip galvanizing) and nickel plating, selectable based on requirements.
However, Grade 12.9 screws typically avoid electroplating due to hydrogen embrittlement risks. Hydrogen absorption during plating can cause sudden, unpredictable fractures under stress-even after prolonged use. Instead, these screws undergo a specialized black oxide treatment. Unlike conventional blackening (e.g., hot-dip blackening), this process involves high-temperature oxidation under strictly controlled conditions, resulting in a glossy, oil-resistant black finish. This treatment provides excellent corrosion resistance, ensuring long-term durability in normal environments without affecting functionality.
