How can nitric acid from ozone generators cause harm on equipment?

Technical Note No. 049

Corrosive Nitric Acid from Ozone Generators Harms Equipment

Date: 16 December 2019, updated 1 March 2023

Authors: Peter Andersen, updated by Hayden Aubermann 

Issue 
It is in the user’s best interest to avoid the potential for corrosive contamination of Ozone 
Monitors or other equipment downstream of ozone generators. In the case of 2B Technologies 
instruments, this especially applies to our low concentration ozone models such as the Model 
106-L and Model 106-M. Choosing dry oxygen as the feed gas for ozone generators (rather 
than air) avoids the production of corrosive contamination. (Alternatively, 2B Technologies has 
developed a corrosion-resistant modification for the Model 106-M for customers who use 
ozone generators where the feed gas is air instead of oxygen. Please contact us to discuss 
this modification in greater detail.)

Overview
Selection of the feed gas for corona discharge ozone generators can have a large impact on 
the lifetime of your equipment. If air is used as the feed gas without a dryer, nitric acid is 
formed in the ozone generator. Nitric acid is a very strong acid that can quickly corrode 
equipment. The design of equipment for use with ozone (a strong oxidizer) does not ensure 
that the equipment will also be resistant to corrosion by nitric acid (a strong acid). Any 
component or gas handling equipment connected to the outlet of a corona discharge ozone 
generator may be damaged if using non-dried air as the feed gas. In addition, nitric acid can 
react with contaminates in the air feed gas and produce a chemical residue in the tubing and 
equipment downstream of the generator.

The best way to avoid producing nitric acid in corona discharge generators and to prevent 
residual contamination in your equipment is to use dry oxygen as the feed gas. At 2B 
Technologies, we only use oxygen feed gas with our corona discharge generators to prevent 
production of contamination in our equipment.

Technical Description
Xylem, an international leader in ozone generation, has published a technical paper that 
discusses the effects of feed gas quality:
“Importance of Feed Gas Quality for Ozone Generation,” N. Brueggemann, R. Fiekens, 
M. Salvermoser, and L. Zhou, Ozone News 46, 16-20 (2018).
http://www.ioa-ea3g.org/fileadmin/documents/Vol_46_No_1.pdf

Excerpts from this publication:
“Nitrogen can be dissociated by energetic discharge electron impact. If oxygen and ozone 
molecules are present, the atomic nitrogen is oxidized to nitrous oxides, finally yielding N2O 
and Nitrogen in its highest oxidation state, N2O5. Intermediate reactions are shown in 
equations. Nitrous oxide (N2O) is inert, escaping with the off gas. Dinitrogen pentoxide 
(N2O5) forms a white solid that adheres to inside surfaces of the ozone reactor.

” If humidity is added, N2O5 is converted into nitric acid, forms a sticky brown film on the 
dielectric tubes (10). 

N2O5 + 2H2O → 2HNO3 [10]

A second reaction is mentioned in literature. In a fast radical reaction nitrogen dioxide and a 
hydroxyl radical form nitric acid.

NO2 + OH → HNO3 [11]

In the worst case, the discharge gap gets clogged almost completely. Furthermore, the 
ozone generators inside surfaces will be attacked by the acid, leading to vessel corrosion. 
Excessive production of nitric acid, acting as an electrolyte, may lead to electrical arcing 
inside the ozone vessel, causing fuses designed to protect the electrodes, to blow. In the 
worst-case scenario, even dielectric failure could result. Therefore nitric acid formation is 
another strong reason why moisture should never be allowed to enter an ozone generator.”

Feed Gas Warnings from Ozone Generator Manufacturers
Formation of corrosive acids in corona discharge generators is an issue with how generators 
are operated. Several manufacturers of corona discharge ozone generators have warnings 
about this issue and proper selection of the feed gas. Below are examples of warning by 
manufacturers of ozone generators about selecting the proper feed gas to avoid corrosion of 
equipment.

Suez
https://www.suezwaterhandbook.com/processes-and-technologies/oxidationdisinfection/oxidation-and-disinfection-using-ozone/ozone-generation
"The ozone generator can be fed with air or oxygen; however, all species present will be 
affected by electronic activation. Therefore, ozone generation will be accompanied by several 
simultaneous secondary reactions. The most important of these reactions are the formation of 
nitrogen oxides, mainly N2O5 and N2O, created by the conversion of a small proportion of the 
nitrogen (up to 500 ppmV)."
"Therefore, it is extremely important for the carrier gas to be clean and dry. The presence of 
water vapor will have a direct effect on ozone generation performance and will also produce

Suez
https://www.suezwaterhandbook.com/processes-and-technologies/oxidation-disinfection/oxidation-and-disinfection-using-ozone/ozone-generation
"The ozone generator can be fed with air or oxygen; however, all species present will be 
affected by electronic activation. Therefore, ozone generation will be accompanied by several 
simultaneous secondary reactions. The most important of these reactions are the formation of 
nitrogen oxides, mainly N2O5 and N2O, created by the conversion of a small proportion of the 
nitrogen (up to 500 ppmV)."

"Therefore, it is extremely important for the carrier gas to be clean and dry. The presence of 
water vapor will have a direct effect on ozone generation performance and will also produce nitric acid from the nitrogen contained in the air. Nitric acid and water are deposited on the 
surface of the dielectric, making it a conductor. We recommend using a gas that has a dew 
point of less than. 65 °C at 1 bar absolute, i.e. air with a water vapor content of less than 5 
ppmV. Air humidity is reduced either through compression and cooling, or through desiccation 
over an adsorbent material."

Absolute Ozone
http://www.absoluteozone.com/glossary-of-terms.html
"While variations of the "hot spark" coronal discharge method of ozone production exist, 
including medical grade and industrial grade ozone generators, these units usually work by 
means of a corona discharge tube.[42] They are typically very cost-effective and do not require 
an oxygen source other than the ambient air. However, they also produce nitrogen oxides as a 
by-product. Use of an air dryer can reduce or eliminate nitric acid formation by removing water vapor and increase ozone production. Use of an oxygen concentrator can further increase the ozone production and further reduce the risk of nitric acid formation by removing not only the water vapor, but also the bulk of the nitrogen."

Lenntech
https://www.lenntech.com/library/ozone/generation/ozone-generation.htm
"Before the feed gas enters the ozone generator, air dryers should dry the air. Ambient air 
contains moisture, which reacts with ozone. This leads to a reduction of the ozone yield per 
kWh. An additional problem of high humidity is that undesired reactions occur in the corona 
unit. When increased amounts of water vapor are present, larger quantities of nitrogen oxides 
are formed when sparks discharge occurs. Nitrogen oxide can form nitric acid, which can 
cause corrosion."

Spartan Environmental Technologies
https://www.spartanwatertreatment.com/how-Ozone-is-made.html
"It is of utmost importance that a dry process gas is applied to the corona discharge. Limiting 
nitric acid formation is also important in order to protect the generators and to increase the 
efficiency of the generation process."

ClearWater Tech
https://www.cwtozone.com/products/air-preparation-systems/
"The air preparation system lowers the dew point of the feed gas. Moist feed gas (air) will 
cause nitric acid to form inside the generator which decreases ozone production and if not 
removed, causes corrosion and eventual failure of the generator’s internal components."

Ozone Solutions
http://www.ozoneapplications.com/info/cd_vs_uv.htm                                                            "Critical to CD ozone systems is proper air preparation. The gas feeding the ozone generator 
must be very dry (minimum -80 degrees F), because the presence of moisture affects ozone 
production and leads to the formation of nitric acid. Nitric acid is very corrosive to critical 
internal parts of a CD ozone generator, which can cause premature failure and will significantly increase the frequency of maintenance."