What Is Caustic Gouging?
The metallurgy in steam boilers today is very different from the past. Metallurgists have learned a great deal of information from boiler tube failures. New stronger materials have provided more corrosion resistance than ever before, although even the best metallurgy cannot compensate for all the possible failures. In most cases, failures are caused by improper operation or repairs of the boiler.
High pressure boilers (>500 psig) have little room for error before boiler tube failure occurs. In the case of low to medium pressure steam boilers, many of the corrosion and tube failure are deposit related.
It is a common belief among boiler operators that a thin layer of mineral scale actually helps protect the boiler tubes from corrosion. Though the mineral coating on the tube may look smooth and protective, the steel beneath the coating is constantly suffering an attack of under deposit corrosion. The thin coating of mineral scale serves to increase the energy level input required to produce the same amount of steam.
What Causes Caustic Gouging?
Caustic gouging is caused by high heat transfer and poor circulation of boiler water at the high heat transfer location in the boiler. Any condition that promotes the formation of high concentrations of caustic levels can result in caustic gouging.
One type of caustic gouging that can occur in low pressure boilers is caused by departure from nucleate boiling or DNB. During nucleate boiling steam bubbles form at distinct points on the metal surface. As steam bubbles leave the surface, additional water washes the surface clean. However, as the steaming rate is further increased the bubbles form faster than the surface can be washed.
As a result the concentration of caustic at that particular surface increases and caustic gouging can result. As the steaming rate further increases, a stable steam ‘blanket’ develops and caustic gouging can develop along the edges of the blanket. This type of caustic gouging can often be seen in low pressure boilers that have improperly rolled tubes.
What Can I Do To Prevent Caustic Gouging?
The tube should be rolled evenly into the cleaned tube sheet. There should be no allowance for the tube to be stressed beyond the width of the tube sheet. If a tube is rolled improperly, the steel will stretch beyond the intended limits and cause an area of high stress.
The highly stressed tube area can promote DNB during high fire cycles causing the precipitation of caustic onto the metal tube. The high concentration of caustic will dissolve the protective magnetite layer on the steel tube and eventually corrode through the tube surface and cause tube failure.
It is best to use a certified boiler installer to provide the repair/replacement of tube in any boiler to prevent improper rolling of replacement tubes. Further, the installer can adjust the burner and flame controls to assure the proper operation to prevent overheating of the boiler surfaces. Overheating of a properly rolled tube can cause caustic gouging. Though caustic gouging is more common in a water tube boiler, fire tube boilers can sustain the same type of attack.
Controlling the amount of caustic in boiler water can also help control this type of corrosion. The amount of caustic in a boiler can be adjusted by controlling the sodium hydroxide feed rate or by limiting the concentration of carbonates in the boiler water.
Many boilers have a limiting factor of alkalinity caused by the amount of carbonate in the make-up water used by the boiler. Carbonates will break down in a boiler to form both caustic and carbon dioxide. Caustic is desirable in a steam boiler but excessive levels should be prevented.
Slow warm ups of boilers should be used to prevent local hots spots causing caustic gouging also. All boilers should be started on low fire to prevent eventual tube failure.