The answer to this question depends somewhat on the maximum temperature. Assuming you will be at less than 50 degrees C, 316L stainless steel is an acceptable material of construction for 100% acetic acid and 37% formaldehyde.

If flanged piping is used, consider depositing a weld overlay of a higher alloy material (e.g., Type 20 alloy) on the 316L flanged face surfaces to prevent concentration cell corrosion. Welded pipe joints must have full penetration welds to prevent crevice corrosion at the abutting joints. 85% formic acid is a bit more tricky, especially since literature corrosion data for formic acid is quite variable.

For temperatures less than 50 degrees C and 90%+ formic acid, 316L stainless steel can be used for piping. For 30 to 90% formic acid at less than 50  degrees C, copper alloys or nickel based alloys (e.g., Hastelloy B or C276) give satisfactory performance in the absence of air or other oxidizing impurities (e.g., Fe+3). When oxidizing impurities are present, glass-lined or PVDF-lined pipe can be used. As before, if flanged alloy piping is used, crevice corrosion of gasket faces can be an issue so weld overlaying the faces with a higher alloy(e.g., Type 20 alloy) is recommended. For pharmaceutical applications, glass-lined or PVDF-lined pipe is often preferred to eliminate the possibility of metal ion contamination.