| The main difference between 304 and 316 stainless | | | | can significantly impact the corrosion rate. Halides |
| steel pipe is that 316 contains 2%-3% molybdenum | | | | generally increase corrosion while aeration or the |
| and 304 has no molybdenum. The "moly" is added to | | | | presence of oxidizing agents usually increases the |
| improve the corrosion resistance to chlorides (like sea | | | | corrosion rate of non-ferrous materials and reduces |
| water). So, while 316 stainless steel pipe is generally | | | | the corrosion rates of stainless steel pipe alloys. I |
| considered more corrosion resistant than 304, | | | | strongly recommend laboratory corrosion studies be |
| depending on the nature of the corrosive media the | | | | run on your specific stream as a part of your |
| corrosion rates of 304 and 316 could be similar. | | | | material of construction selection process. |
| Generally, SS 304/304L is assumed to be | | | | I have a large storage tank of 93% sulfuric acid. I am |
| corrosion-resistant material. But when we come to | | | | having excessive corrosion of the top of the 4-inch |
| Piping Specs with SS 304/304L MOC, its value is | | | | carbon steel outlet pipe. I am thinking of replacing the |
| taken as 0.063 inches -- same for SS316/316L . | | | | outlet pipe with Schedule 120 carbon steel pipe. Is |
| Please explain. | | | | there any more resistant material, insert, or coating |
| We have a plant that produces fertilizer. In one | | | | you could recommend for increased life? |
| section we mix 40% phosphoric acid and 98% | | | | Carbon steels are only acceptable for 93% sulfuric |
| sulfuric acid together in a ratio of 75-80 : 25-20 | | | | acid when fluid velocity is low (< 3 ft/sec). For |
| phosphoric acid:sulfuric acid. After that the mixture is | | | | 4-inch diameter piping or less with velocities up to 5.9 |
| cooled to a temperature of 80 degrees C. What is | | | | ft/sec, 316L stainless steel pipe is a good choice. For |
| the best material of construction for transfer piping? | | | | velocities higher than 5.9 ft/sec, Alloy 20Cb-3 (UNS |
| The existing material, 304 stainless steel pipe, shows | | | | N08020) has been used successfully. For additional |
| problems at the welds. | | | | information, consult NACE Recommended Practice |
| From a materials of construction perspective, this is a | | | | RP-0391 "Materials for the Handling and Storage of |
| tricky mixture to deal with, especially at 80 degrees | | | | Commercial Concentrated (90 to 100%) Sulfuric Acid |
| C and higher. Phosphoric acid is less corrosive than | | | | at Ambient Temperatures." |
| sulfuric acid. Pure phosphoric acid has no oxidizing | | | | In your question, you said you are experiencing |
| power but commercial phosphoric acid contains | | | | problems with 304 stainless steel pipe at the welds. If |
| impurities such as fluorides and chlorides that can | | | | this is the case, you might consider moving to 304L |
| significantly increase its corrosivity. The corrosivity of | | | | stainless steel pipe. Low carbon versions of austenitic |
| sulfuric acid depends on many factors including | | | | Stainless steel seamless pipe like 304L are designed |
| temperature, concentration, the presence of oxidizing | | | | to eliminate problems associated with carbide |
| or reducing impurities, velocity effects, and solids in | | | | precipitation and chromium depletion at welds. If 304L |
| suspension. | | | | doesn't work, try gradually moving up to a higher |
| It is usually not wise to select materials of | | | | alloy. Possible candidate materials in order of generally |
| construction for sulfuric acid handling equipment | | | | increasing corrosion resistance are: 316L stainless steel |
| based only on published corrosion data since corrosion | | | | pipe, 20-type alloys like 20Cb-3, higher chromium |
| by sulfuric acid is a complex phenomenon. Small | | | | Fe-Ni-Mo alloys like Alloy 31, and nickel-base |
| differences in impurities, velocity, or concentration | | | | molybdenum-chromium alloys like C-276. |