A36 is a carbon steel alloy that is generally only made for special orders. According to Bushwick Metals, it usually has a minimum yield of 36 kilo-pounds per square inch (KSI), a measure of stress capacity, and a minimum tensile strength of 58 KSI.
A588 has a mixture of copper and steel, and is part of the high-strength, low-alloy class of steels. It has a high capacity for corrosion resistance and is lighter than other grades while maintaining a minimum yield of 50 KSI.
Advantages of A36
The primary advantage of A36 over A588 is that it is harder and, over time, more durable in environments with a minimum of moisture. Its cost is higher, and its added weight and tendencies toward corrosion make many builders choose A588, although A36 is the preferred ASTM standard for M, S, C, MC and L shapes and sections.
Advantages of A588
A588 can resist corrosion between two and four times as effectively as carbon steel alloys that do not have copper. Also, because it is lighter than A36, in situations where lighter steels will save money, A588 is the better choice.