I can only give you some general guidelines as you have not provided important information like acid concentration, temperature, whether chlorides or fluorides are present, etc. 316L stainless steel is the most commonly used material of construction for phosphoric acid service. Pumps and valve MOCs used are cast type 316L (CF3M). For acids contaminated with chlorides or fluorides, pumps and valves must be made of a higher alloy material of construction such as CD4M-Cu or Alloy C-276. Nuts and bolts used to connect flanged pipe can be made of carbon steel but they will corrode if there are leaks at the flanges. If this is a problem, 316L stainless steel nuts and bolts can be used.