A4 Stainless Steel (ISO 3506 group A4 (316 / 1.4401), commonly supplied A4-80) is specified for marine, coastal and chemical applications. When extra-thick flat washers are specified in this grade, the dimensions remain per Trade series (heavy-duty thick plain washers); the material determines corrosion behavior (superior, including marine and chloride environments) and the service temperature range (approx. -200 °C to +400 °C, subject to the property class and the joint). Both are covered below with the size table.
Why Choose A4 Stainless Steel for Extra-Thick Flat Washers
Specified for marine, coastal and chemical applications. For extra-thick flat washers, this matters when superior, including marine and chloride environments is the primary requirement.
Mechanical & Physical Properties
| Grade designation | ISO 3506 group A4 (316 / 1.4401), commonly supplied A4-80 |
|---|---|
| Magnetic response | Largely non-magnetic |
| Service temperature | approx. -200 °C to +400 °C, subject to the property class and the joint |
| Corrosion characteristic | Superior, including marine and chloride environments |
Galvanic & Galling Considerations
For stainless extra-thick flat washers the compatibility question is galvanic rather than galling: joined to aluminum or galvanized steel in wet service, stainless drives corrosion of the less-noble metal. Isolate with a coating, sleeve or non-conductive washer where the joint sees moisture. Note also that stainless is softer than hardened carbon steel, so where the part carries a wear or shear duty, check the load against the grade before substituting.
Comparable Grades
For Extra-Thick Flat Washers, the alternative stainless grade is A2 (304). The practical dividing line is chlorides: A2 (304) is fine until salt is involved, and A4 (316) exists for the cases where it is — coastal, marine and road-salt service. Neither reaches the strength of a plated class 8.8 or 10.9 fastener.
Using Stainless Fasteners in Aluminum
The galvanic couple here runs the wrong way: aluminum is the less noble metal, so it corrodes and the fastener does not. What governs the rate is the ratio of areas — a large anode and a small cathode is a tolerable combination, the reverse concentrates the attack into thin material around the hole. An isolating washer plus sealant breaks the circuit; where that is impractical, match the fastener to the substrate.
Sizes of Extra-Thick Flat Washers Available in A4 Stainless Steel
| Bolt size | Material | Relative thickness | Purpose | Finish options |
|---|---|---|---|---|
| M10 | carbon steel or stainless steel | greater than the standard plain washer | reduce local deformation under heavy clamping | zinc plated or plain |
| M12 | carbon steel or stainless steel | greater than the standard plain washer | reduce local deformation under heavy clamping | zinc plated or plain |
| M16 | carbon steel or stainless steel | greater than the standard plain washer | increase bearing area on soft members | zinc plated or plain |
| M20 | carbon steel or stainless steel | greater than the standard plain washer | increase bearing area on soft members | zinc plated or plain |
