303 & 304 Stainless compatibility 1

303 stainless has much higher carbon than 304, this may cause an electrochemical cell to form since the higher carbon content in the 303 will cause it to be less noble.

Nick

The main difference is the sulfur content: 303 has a minimum of 0.15 wt% S to aid machining, while 304 has a maximum of 0.03 wt% S. In both cases, the solubility of the austenitic metal matrix is exceeded, and the S is present primarily as MnS inclusions. The pitting potential of the bulk alloys is about equivalent although the 303 will have roughly 5 times as many localized surface discontinuities. Hence, it is a very good idea to passivate 303 in order to remove surface MnS.

NickE is correct in that 303 contains 0.15 wt% C vs. 0.08 wt% C in 304. However, 303 typically has an extra 1 wt% Cr to compensate (average 19 % Cr vs. av. 18% Cr in 304).

Some producers add 0.6% Mo (optional) to 303 to improve resistance to pitting.
All things considered, and especially if you passivate to remove surface Mn, there should not be a galvanic problem between 303 and 304.

The worst problems I have seen with threaded connections involving 303, 304, 304L, 316 and 316L and combinations thereof involve galling (cold welding). This can occur under thread loadings as small as 1000 psi. Advise using an anti-galling thread lubricant – many have been recommended in this forum. For more information on galling between various SS combinations (self and dissimilar), request Carpenter Technologies' SS Manual. Galling information is in the chapter on Fabrication.
Hope this helps,
Ken

Thanks Ken & Nick. I think that should answer my questions for now. I will add to this post if there is anything else I am looking for.

Chad

Chad,

The simple answer is no. They are basically the same alloy with the exception of sulfur. The 303 can also be very low carbon. I have seen some at a .015 carbin level.

One other thing to realize, 303 will crack, especially in small diameters due to the sulfide stringers whereas 304 will not crack.