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Different Types of Corrosion
- Recognition, Mechanisms, Prevention and Prediction

Dew Point Corrosion

Recognition of Dew Point Corrosion

What is Dew Point Corrosion? Dew point corrosion refers to corrosion resulting from condensation of acid gases such as sulfur dioxide, sulfur trioxide, hydrogen chloride, and others within the combustion products. At temperatures below dew points, these gases and the water vapor in the flue gas will condense to form sulfurous acid, sulfuric acid and hydrochloric acid which can lead to severe corrosion of carbon steels, low alloy steels, stainless steels, and other alloys.

How to identify Dew Point Corrosion? Dew Point Corrosion occurs ONLY when the metal surface temperature falls below the dew point temperature of a flue gas. Dew Point corrosion typically results in excessive metal loss, leading to perforation or rupture.

What structures are affected by Dew Point Corrosion? All fired process heaters and boilers that burn fuels containing sulfur have the potential for sulfuric acid dew-point corrosion in the economizer sections and in the stacks. The stack is the most vulnerable unit in the gas-handling system because of poorly controlled operation of the furnace and other components of the gas train. Heat-Recovery Steam Generators (HRSG’s) that have 300 Series SS feedwater heaters may suffer chloride-induced stress corrosion cracking from the gas side (OD) when the temperature of the inlet water is below the dewpoint of hydrochloric acid. 300 Series SS feedwater heaters in HRSG’s are potentially at risk if the atmosphere of the combustion turbine includes chlorine.

Mechanisms of Dew Point Corrosion

What causes dew point corrosion? The concentration of contaminants (sulfur and chlorides) in the fuel and the operating temperature of flue gas metal surfaces determine the likelihood and severity of dew point corrosion. Since all fuels contain some amount of sulfur, sulfuric and sulfurous acid dewpoint corrosion can occur if the metal temperatures are below the dewpoint. The dewpoint of sulfuric acid depends on the concentration of sulfur trioxide in the flue gas, but is typically about 280°F (138°C). Similarly, the dewpoint of hydrochloric acid depends on the concentration of hydrogen chloride. It is typically about 130°F (54°C). Software tools such as DewPoint-Compass can be used to predict the risk of dew point corrosion under the prevailing operating conditions or to aid materials selection.

Prevention and Prediction of Dew Point Corrosion

How to prevent dew point corrosion? Dew point corrosion can be prevented through:

• Maintain the metallic surfaces above the dew point temperatures of flue gases.

• Materials selection and design modifications (e.g. use of CRA liner in stacks) with a software tool such as DewPoint-Compass.

• For HRSG’s, avoid the use of 300 Series SS in the feedwater heaters if the environment is likely to contain chlorides.

• Similar damage occurs in oil-fired boilers when the units are water-washed to remove ash if the final rinse does not neutralize the acid salts. Sodium carbonate should be added to the final rinse as a basic solution to neutralize the acidic ash constituents.

How to predict dew point corrosion? Dew point corrosion can be predicted using software such as "DewPoint-Compass: Prediction of Dew Point Temperatures and The Risk of Dew Point Corrosion". Click the link for details on the software.

For more details on Dew Point Corrosion

Where can I learn more about dew point corrosion? More details on dew point corrosion are included in the following corrosion courses which you can take as in-house training courses, course-on-demand, online courses or distance learning courses:

A Basic Course in Corrosion and Its Prevention (5-day module)
Corrosion, Metallurgy, Failure Analysis and Prevention (5 days)
Methods and Materials for Corrosion Control and Prevention (5 days)

API 583 Corrosion Under Insulation and Fireproofing -Detection, Mitigation and Prevention (5 days)

API 584 Integrity Operating Windows (5 days)

Corrosion Control by Materials Selection and Design (5 days)

If you require corrosion expert witness or corrosion consulting service on dew point corrosion, our NACE certified Corrosion Specialist is able to help. Contact us for a quote.

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