Stainless Steel Grades: A2 vs A4

As is clearly evident from the vast multitude of applications that stainless steel is used for, it is one of the world's most versatile materials. One of the reasons for this exceptional versatility is that the alloy mixture used to create the metal can be varied in many ways by adding different elements, which results in a range of stainless steel grades. In this edition of the BS Stainless blog we look at two of the most common of these grades: A2 and A4.

To be classed as stainless steel (sometimes referred to as inox steel), a steel alloy must contain at least 10.5% chromium; it is this element that makes the stainless steel so strongly resistant to the damaging effects of corrosion. Carbon steel, which does not contain this minimum level of chromium, rusts easily in the presence of moisture and air. The rust (more correctly iron oxide) speeds up corrosion in this material. Chromium in stainless steel forms a layer of chromium oxide (known as the passive layer) on the surface, which helps to prevent corrosion.

A2 Stainless Steel
A2 stainless steel is commonly referred to as grade 304. Yet another name for it is 18/8 stainless and this hints at its chemical structure: A2 stainless steel contains 18% chromium and 8% nickel. One of the most popular grades, A2 is very durable provides a good level of resistance to oxidation and corrosion. As it is immune to the effects of all kinds of foods, organic chemicals, dyes and sterilising solutions, it is especially suitable in areas where hygiene is important, as well as for creating fasteners and fixings. The material is easy to fabricate and clean and can be supplied in a wide variety of appearances. 

A4 Stainless Steel
A4 stainless steel is also known as grade 316 or 18/10 stainless. Once again, this latter nomenclature refers to its chemical composition, which contains 18% chromium and 10% nickel. The crucial difference is that, in addition to a higher level of nickel, A4 stainless steel also contains 2% molybdenum. This element dramatically increases the corrosion resistance of the finished material, enabling it to withstand the effects of acids, iodides, bromides, solvents and industrial chemicals. 

The molybdenum content of A4 stainless steel also helps it to strongly resist attack from chlorides, such as those found in seawater. It is for this reason that A4 is sometime referred to as being of 'marine grade' and why it is so useful in this demanding environment.

Both of these grades of stainless steel are part of the austenitic family, sometimes collectively referred to as the 300 series. Austenitic stainless steels are mostly not magnetic though some magnetism may be induced by welding or a cold working process. 

BS Stainless supplies a comprehensive collection of stainless steel products in both of these common grades, as well as many others. Please browse our website for full details of our product portfolio and do not hesitate to contact a member of our expert technical team if you have any questions.