In the last article, we introduced the materials used in heat exchangers and condenser pipes in power plants. Today, we will focus on the difference between the common failure mechanism of copper alloy and stainless steel tubing used in power plants.
Possible erosion of copper alloys tubing:
Steam side erosion
Ammonia grooves and stress corrosion cracks are the most common damage to the steam-side copper alloys. Ammonia Grooves – Deaerating additives such as hydrazine can create ammonia grooves. The combination of Ammonia and condensate along the support plate generate the downstream groove.
Stress Corrosion Crack (SCC)
Both admiralty brass and aluminum brass are sensitive to stress corrosion crack induced by ammonia. Stress corrosion cracks are rapidly formed in pipes and ammonia with high residual stress. It is very common to damage condenser pipes caused by ammonia grooves and stress corrosion cracks.
Cooling waterside erosion
Erosion corrosion – when the water flow rate is high, the water will wash away the protective oxide layer on the copper alloy, causing erosion-corrosion. For navy and aluminum brasses, this occurs when the water velocity is greater than 1.8 m/s, even though the overall velocity of the water is lower, but the local area eddy currents can cause this phenomenon. The general place where this erosion occurs is at the water inlet end. Pipe blockage – such as a blockage caused by a bulge in the pipe formed by a jig – swirls that form around it can cause pipe holes within a few days.
H2S and sulfuric acid
H2S and sulfuric acid destroy the protective oxide layer and prevent it from regenerating. Most H2S and sulfuric acid come from decaying plants, sulphuric acid-reducing bacteria (MIC), or wastewater. Typically, this failure begins to occur in 90-10 copper-nickel pipelines six months after the conversion of the existing cooling water source from clean water to treated wastewater. General corrosion and copper transfer The oxide layer on the copper tube is porous, allowing the diffusion of copper ions into the water. When water conditions are non-corrosive, copper dissolves slowly, and copper pipes with a service life of 25 years are not uncommon. However, the transfer of copper can still have an impact elsewhere. For example, when replacing a typical 300MW condenser made of navy brass pipe, the original pipe will weigh 50% less than the original 400,000 pounds. This means that 200,000 pounds of the copper alloy have been dissolved. The copper either goes into the steam or into the cooling water. When copper is plated on boiler pipes, it causes catastrophic liquid metal embrittlement.
Possible erosion of stainless steel tubing
Steam side
All stainless steels including the commercial grades (TP 304, TP316 and their derivatives) are resistant to most boiler chemicals including all hydrazine derivatives. One mechanism that causes early damage at higher temperatures is chloride stress corrosion cracking (SCC), which occurs in feedwater heaters. Steel containing 8%Ni (TP 304) is sensitive to stress corrosion crack. More damage occurs when generating equipment is switched from baseload to cycle mode. Chloride concentrates in alternately wet and dry areas, mainly in the cooling area after overheating.
The cooling waterside
Pitting and crevice corrosion — TP304 and TP316 are sensitive to pitting, crevice corrosion and MICs associated with crevice corrosion. TP304 and TP316 should not be considered if the chloride content in cooling water exceeds 150ppm and 500ppm, respectively. If treated wastewater is used as a cooling water source, it is recommended to use titanium tubes instead of TP304 and TP316.
