Caustic corrosion tests in the experimental boiler
Only a few caustic corrosion experiments under heat flux conditions in the experimental boiler were done. It is obvious that under dryout conditions and water conditioning with a non volatile agent like the caustics NaOH, LiOH, Na3PO4 severe corrosion can be expected. We did some experiments to get an idea how fast.
One of them was the 10the experiment. Conditions Dryout at a steam fraction of 70%, heatflux 300KW.h/m2, dosing NaOH up to a pH value of 10. The experiment would take 10 days, but after 3 days on friday evening I ordered as young researcher to finish the experiment. When we cut the testtube on monday we were surprised and glad finishing the test. Tht tube wall was corroded for half and at longer exposure times the testtube should be bursted certainly end destroyed the furnace certainly. Deliverance of a new oven would have delayed the research program at least half a year.
Caustic corrosion tests in autoclaves
Because experiments under static conditions (in autoclaves) are much cheaper and you can invent some specific conditions easy we did many projects in autoclaves. To be mentioned:
- small 50 ml autoclaves
- 500 ml autoclaves for electrochemical measurements
- 500 ml autoclave for steam blanketing experiments
- 2000 ml autoclaves for refreshing controlled water conditions
- 2000 ml autoclaves for refreshing controlled water conditions and constant strain tests
An example is the test sery on various steel on corrosion rate in caustics. Conclusions of these resarches are:
- There is a critical hydroxide concentration above which the corrosion rate increases rapidly. The critical concentration of LiOH amounts to about 0.5 molar and that of NaOH. 2-3 molar. KOH shows a critical concentration of 1.5 molar.
- The critical concentration for alkaline corrosion was only determined for a few heats of the 5 steels tested. It is not clear whether this critical concentration is also controlled by the chemical composition of the steel within the broad range of the various steel specifcations.
- The critical alkaline concentration at 250°C does not differ from that at 310°C. The corrosion rate in highly concentrated (4 and 10 molar) LiOH is five times higher than that in NaOH.
- Under the isothermal autoclave test conditions in 4 mol NaOH a so-called Potter and Mann oxide layer formed, leading to a parabolic corrosion rate. In 4 mol LiOH, however, a porous columnar oxide structure was formed which results in linear corrosion.
- The corrosion rate in 10 mol NaOH for carbon steels varied considerably for the 83 different heats. Carbon steel showed the highest corrosion rates. Steels 15Mo3, 14Mn4 and 13CrMo44 had about the same corrosion rates but were only slightly better than carbon steel. Steel l0CrMo910 had the lowest corrosion rate.
In the case of a fouled evaporator and the consequent danger of steam blanketing, NaOH and KOH are considered to be better acid-chloride corrosion inhibitors than LiOH. LiOH will give a high alkaline corrosion risk because of the low critical hydroxide concentration.
See for more information the following papers:
2, 6, 7, 8, 11, 13, 20, 23, 32 and 61