Lincoln Outershield 71M works just fine with 100% CO2. We had used it for 15 years with CO2 and some occasional use in the satellite locations using 75/25. I have run 15 to 20 PQR tests with it using CO2 (every 3 years - AWS D1.5).
You can see CO2 is listed on the data sheet for 71M - as well as argon/CO2 mixes. The last Lincoln annual C of C that I have for 71M was run by them with 100% CO2.
http://www.mylincolnelectric.com/Catalog/consumabledatasheet.aspx?p=5743Edit : added 2nd link
http://www.mylincolnelectric.com/LEExtranet/certs/pdf_lib/US14160_20080717.pdfJohn's direction to galvanizeit.org will lead to a lot of information on the topic. One thing to note is that is is not only the silicon content that affects the zinc layer - there are other elements responsible as well. In fact different heats of steel can lead to differences in appearance due to variations between the heats.