Is corrosion in compost facilities due to high or low pH? Its an important question if we are considering appropriate coatings for metals.
Corrosion in compost facilities is common, and has been addressed in a number of excellent publications more than 10 years ago (for example: https://www.biocycle.net/2006/09/20/building-longevity-into-composting-buildings/, http://compostingcouncil.org/wp/wp-content/uploads/2014/02/7-BuildingsCorrosionControl.pdf
Volatile compounds produced at compost facilities can also be highly corrosive, such as reduced sulphur compounds and ammonia. Research has not been entirely clear on what specifically causes the corrosion, whether its high or low pH. A 2009 study in Alberta indicated that low pH composting environments were more corrosive than high pH environments (http://www.compost.org/English/PDF/conf2009/B4%20Research%20Matters/L%20Zhou,%20U%20Alberta,%20Quantifying%20Metal%20Contaminants.pdf)
We were recently put to the test. Was the corrosion in the metal near the ceiling already after one year of operation due to high or low pH? In composting environments where ammonia is emitted, theory suggests that it may be a high pH environment associated with high moisture and ammonia that may cause the corrosion. Physics and chemistry tell us the following:
- Ammonia is lighter than air and moves upward
- Warm, moist air moves upward
- Most reduced sulphur compounds are heavier than air and will tend to move downward
- Ammonia concentrations of 50 ppm in air will result in a condensate pH of just over 10.
Does air quality testing confirm this? We tested this using a simple apparatus for capturing condensate using a bucket and two ice packs as per below:
Depending on the environment, it takes about 2 hours to collect the condensate. We measured ammonia concentration, relative humidity and temperature in the air. In our particular study, where the ammonia concentration was up to 50 ppm and the relative humidity 100%, the pH of the condensate was 9.5 to 10.
We concluded that corrosion near the ceilings of compost facilities are most likely due to high pH resulting from high ammonia in the warm moist air that rises to the ceilings. This information cannot necessarily be extrapolated to metals within the composting environment where other compounds such as reduced sulphur compounds may also exhibit a strong corrosive effect. We have seen how a low pH environment in food waste compost facilities results in rapid corrosion of metal.