There are over 4,000 manufacturers of screws, nuts, and bolts in the United States and out of all the products they make, metric hex nuts are the most commonly used. A standard six-sided nut, metric hex nuts are available in a few different materials and classes.
Stainless steel 18-8 Hex Nut
This is the most common stainless steel nut for general use in routine hardware scenarios.
Zinc plated class 8.8 steel Hex Nut
This nut is made of a tempered, medium carbon steel. Generally, class 8.8 is a hardened steel, but perhaps not as strong as class 10.9. It’s most commonly used with automotive applications. Class 8.8 is Zinc plated, which promotes moderate resistance to corrosion, and retains some similarities to grade 5.
Zinc plated class 10.9 steel Hex Nut
This nut is made from medium carbon steel that’s tempered. Class 10.9 is definitely stronger than class 8.8. This type of nut is most commonly associated with automotive applications that require a high degree of strength. Class 10.9 is also plated with zinc for resistance to moderate corrosion and retains some similarities to grade 8.
Chrome plated class 8.8 steel Hex Nut
This hex nut is also made from medium-tempered carbon steel and is hardened, but it’s definitely not as strong as class 10.9. Most often used in automotive applications, class 8.8 is similar to grade 5. The chrome plating gives off a mirror-like, polished finish.
Naturally, this just scratches the surface. Metric hex nuts are also available in each of the above listed classes with nylon inserts to resist loosening. Hex jam nuts, which are a thinner version of the hex nut, and usually only available in stainless steel. JIS hex nuts have a smaller outside diameter to meet some outdated specifications and also are usually only available in stainless steel. Additionally, heavy hex nuts, heavy hex bolts, hex flange bolts and hex head bolts all have their own, separate specifications and availability in different finishes.
References: www.bigbolts.com