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Galvanized vs Stainless Steel Bolts

Galvanized bolts are made from ordinary carbon steel bolts treated with an electroplated coating, while stainless steel bolts are made entirely of stainless steel material. We can choose  the right bolts according to following aspects:

Color: Stainless steel bolts, after cleaning or passivation, retain their natural metallic color, offering a sleek and textured appearance. In contrast, galvanized bolts come in various finishes such as black zinc, white zinc, or rainbow zinc, giving them corresponding black, white, or colorful appearances.

Corrosion Resistance: Galvanized bolts have a zinc coating that provides good anti-corrosion and rust-proofing properties. However, over time, this zinc layer may peel off, leading to rust and shortening the bolt’s lifespan. Stainless steel bolts, on the other hand, have inherent corrosion resistance due to their material, offering superior durability and longevity.

Strength and Application: Due to material differences, galvanized bolts generally have lower strength compared to stainless steel bolts. Stainless steel bolts are stronger and suitable for use in high-temperature environments. Therefore, in high-temperature and high-pressure applications, stainless steel bolts are a better choice.

Galvanized bolts are best suited for indoor, dry environments, while stainless steel bolts are more appropriate for humid, high-temperature, and high-pressure conditions. Galvanized bolts are typically used for low-load applications, whereas stainless steel bolts can handle high-strength load demands.

How to identify stainless steel bolts?

1.We can identify whether a bolt is made of carbon steel or stainless steel by using a magnet test or observing its color and gloss. If the bolts are attracted to a magnet, it is made of carbon steel. If it is not attracted to a magnet, it is made of stainless steel.

2.Carbon steel usually have a purple or blue color, with visible traces of iron oxide on the surface, and their gloss is less pronounced. Stainless steel bolts, have a more uniform color, typically gray or silver-white, with a higher surface gloss. Stainless steel does not rust during use, unlike carbon steel, which often shows noticeable surface oxidation.

To distinguish between 304 und 316 stainless steel bolts, the simplest methods include the following:

Magnet Test: 316 stainless steel is nearly non-magnetic, while 304 stainless steel has a little magnetic properties. A magnet can be used to test for material magnetism.

Testing Solution: Specialized stainless steel testing solutions can also be used. For example, Ni8 solution is used for 304 stainless steel, and Mo2 solution for 316 stainless steel. If the solution changes color, it indicates the presence of the corresponding elements.

What grade are stainless steel bolts?

The strength grades of stainless steel bolts are typically divided into three main levels: 8.8, 10.9, and 12.9, representing different ultimate tensile strength limits. Bolts with a grade of 8.8 have an ultimate tensile strength of 800 MPa, those with a grade of 10.9 have 1000 MPa, and those with a grade of 12.9 have 1200 MPa.

Additionally, the performance grades of stainless steel bolts range from 3.6 to 12.9, comprising 10 levels. The first number represents 1/100 of the material’s ultimate tensile strength, while the second number indicates 10 times the ratio of the material’s yield strength to its ultimate tensile strength.

Stainless steel bolts are commonly made from materials like SUS201, SUS304, and SUS316, which offer excellent rust resistance and acid resistance, making them suitable for various corrosive environments. Different materials exhibit variations in strength and corrosion resistance. For instance:

A2-70 stainless steel screws are suitable for applications requiring relatively high strength.

A4-70 screws, with superior corrosion resistance and strength, are ideal for use in corrosive environments.

Torque values for metric stainless steel bolts

As a common type of fastener, Stainless steel bolts are widely used across various industries. During installation, torque control is very important as it directly impacts the fastening performance and safety of the bolts. The recommended torque values for stainless steel bolts vary depending on their size and grade.

1. M5 Stainless Steel Bolts: A recommended torque of 2–4 N·m ensures proper fastening while avoiding thread damage from over-tightening.
2. M8 Stainless Steel Bolts: A torque range of 5–7 N·m is recommended, meeting the fastening requirements for most applications.
3. M16 Stainless Steel Bolts: For this kind of larger size bolts, the torque values depend on the bolt grade:
   • Grade 4.8: 104.6 N·m
   • Grade 5.8: 130.8 N·m
   • Grade 6.8: 156.9 N·m
   • Grade 8.8: 209.2 N·m
   • Grade 10.9: 294.2 N·m

When selecting the torque value, it’s essential to match it appropriately to the specific grade and application requirements of the bolt.