For a more specific answer to your question, additional information is required (e.g. temperature, concentration, what other constituents are present, etc.). However, a quick look at Schweitzer's Corrosion Resistance Tables (5th Edition) may give you the answer you need. Schweitzer says 304/304L stainless steel is rated G (< 20 mpy penetration) for carbonic acid up to 210° F and 316/316L stainless steel is rated G for carbonic acid up to 350° F. Again, according to Schweitzer, brass is rated G for carbonic acid up to 210° F but Nickel 200/210 is rated G for carbonic acid only to 80° F. Perhaps brass will be better than nickel-plated brass. Another possibility is to use a plastic. Virtually all common plastics are listed as "Recommended" by Schweitzer albeit with different maximum temperature limits.