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Corrosion inhibitor for ammonia absorption systems
Dr. rer. nat. Franziska Krahl
An alternative to chromium(VI) compounds
Project management: Dr. Franziska Krahl, Dr. Steffen Feja
In contrast to electrically driven compression chillers or compression heat pumps, ammonia absorption systems (chillers and heat pumps) are driven by thermal energy. If the heat energy is supplied by solar thermal energy, geothermal energy or waste heat these systems can be operated without fossil fuels. Refrigerant and absorbent, ammonia and water, are naturally occurring compounds with no contribution to global warming (GWP = 0, ODP = 0).
Ammonia absorption systems are made of carbon steel for cost reasons. One disadvantage of ammonia as a working substance is the formation of highly corrosive solutions with water. Corrosion occurs increasingly at temperatures above 90 °C, which would result in long-term failure of the systems. Beside using corrosion-inhibiting substances (so-called corrosion inhibitors), expensive stainless steel can be used as an alternative. Experience has shown, however, that despite the reduced corrosion of stainless steel, corrosion inhibitors have to be added for the long-term and trouble-free operation of the systems. Until today, chromium(VI) compounds, mainly sodium chromate or sodium dichromate, have been used for this purpose. Other known inhibitors offer significantly lower corrosion protection and are not used in the case of carbon steel for temperatures above 90 °C.
However, the use of Cr(VI) compounds has been severely restricted within the EU since 2017 due to their carcinogenic and toxic properties. Authorisation must be applied for the usage of Cr(VI) compounds. The additional regulatory and administrative effort required is considerable. Overall, however, the associated and ongoing continued use of a toxic, carcinogenic and environmentally harmful class of substances is significantly more problematic. Especially for the innovative segment of absorption systems, this shortcoming reduces the environmental friendliness of the technology.
The aim of the project is to identify a corrosion inhibitor for industrial ammonia absorption systems providing temperatures down to -60 °C, to replace chromium(VI) compounds. The inhibitor should fulfil the following criteria:
- Corrosion protection up to and including 180 °C (input), also for carbon steel
- Prevention of the formation of non-condensable gases
- Corrosion protection in the alkaline pH range
- Corrosion protection in the absence of oxygen
- Long-term stability and non-volatility
- Same corrosion protection level as chromium(VI) compounds
By means of an innovative matrix rapid test, numerous chemicals and substance combinations will first be evaluated with regard to their application potential as corrosion inhibitors for stainless steel and carbon steel in ammonia solutions. Subsequently, promising candidates will be further investigated in laboratory tests by simulating the specific corrosion conditions.
The effectiveness of possible Cr(VI)-free inhibitors will also to be tested in real absorption circuits (demonstrators) and in field tests. As a manufacturer or operator of ammonia absorption systems please feel welcome to contact us.