INTRODUCTION
The most significant contributors to boiler waterside corrosion are dissolved oxygen, acid, or caustic in the water and high temperature If any of these are uncontrolled, severe pitting, gouging, and embrittling of the tube metal can occur and lead to failure. A good understanding of the mechanisms and control of these factors is extremely important.
WHY SOME CORROSION IN THE BOILER IS NECESSARY
Water will rapidly corrode mild steel and as the temperature increases the reaction accelerates. The following reaction is typical of iron corrosion in a boiler:
3Fe + 4H20 > Fe2O4 + 4H2
Iron + steam > magnetite + hydrogen gas
The magnetite produced is black iron oxide. Under normal conditions, this is the typical product of corrosion. However, it is also this reaction that inhibits excessive corrosion in steam boilers. In a new or cleaned boiler, the initial corrosion process produces this magnetite film as a tenacious layer at the steel surface. Consequently, the corrosion reaction is self-inhibiting. This magnetite layer grows to an approximate thickness of 0.0004 to 0.001 inches, at which point any further corrosion process ceases. Periodic weakening or damaging of this protective layer does occur and proper internal boiler water treatment can repair this layer The normal corrosion in a clean boiler system progresses at approximately 1 mm per year. The appropriate pH level for maintenance of the magnetite layer is approximately 8.5 to 12.7. Most systems operate at a pH level of 10.5 to 11.5.
OTHER CAUSES OF CORROSION
Dissolved Oxygen: In previous discussions concerning mechanical and chemical deaeration, removal of dissolved oxygen was considered essential. When dissolved oxygen enters the steam boiler corrosion manifests itself in the form of severe deep pits almost exclusively at the water level in the boiler. If an oxygen attack has occurred, it is readily identifiable during inspection.
pH Variation (acidic or caustic attack): Previously, a pH of 10.5 to 11.5 was identified as ideal for boiler operation. Variation from the levels that are considered optimum for maintenance of the magnetite layer can cause general corrosion. Each of these will be discussed below.
Acidic Attack: If boiler water pH has dropped significantly below 8.5, a phenomenon known as thinning can occur. The normal manifestation of acidic attack is etching. In areas of high flow, the surfaces are smooth. In addition, any stressed area would be a principal area of attack.
Caustic Attack: Caustic attack or, as it is more commonly known, caustic corrosion, is often encountered in phosphate treated boilers in which deposits occur in high heat transfer areas. Boiler water can permeate the porous deposit. When it is coupled with significant heat flux, concentration of the boiler water occurs. Caustic soda (NaOH) is the only normal boiler water constituent that has high solubility and does not crystallize under these circumstances. The caustic concentration can get as high as 10,000 to 100,000 ppm. Localized attack due to extremely high pH (12.9+) will occur as will the formation of caustic-ferritic compounds through the dissolving of the protective magnetite film. Caustic corrosion (typically in the form of gouging) continues until the deposit is removed or the caustic concentration is reduced to normal.
Steamside Tracking: This form of corrosive attack generally occurs in lower temperature areas of the boiler. A series of factors may permit stratified flow of steam and water in a given tube. When the stratification occurs, the velocity of the steam-water mixture is not sufficient to maintain turbulent flow as the steam-water mixture passes through the tubes. The affected tubes normally reveal gouging at the steam-water line and thinning under any deposits that have accumulated. Usually, these systems contain a certain amount of caustic, which aggravates the rate of attack. Steamside tracking or blanketing is a direct corrosive attack, similar to the acid or caustic attack.
Stress Attack: Metallurgical examination is required to identify causes of stress attack. On occasion, intergranular or transgranular attacks can be seen on tube specimens. This type of attack can be a function of system condition or boiler water chemistry. It generally occurs in higher pressure systems.
CONCLUSIONS
Proper corrosion control can be achieved with the following good practices:
1. Proper boiler chemistry and operating pressures and conditions
2. Close scrutiny and control of boiler water chemistry
3. Frequent testing of boiler water chemistry
4. Thorough inspection of waterside areas during shutdown.
This has been a brief discussion of waterside corrosion in boilers. We at
Walter Louis Fluid Technologies are ready to answer any questions you may
have about any facet of your boiler or cooling tower operation. We can be
reached at 800-747-2019 or at sales@walterlouis.com.