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