If the magnetite layer is broken down by corrosive action, high temperature hydrogen atoms diffuse into the metal, combine with the carbon and form methane. Large CH-3 molecules causes internal stress and cracking along crystal boundaries and sharp sided pits or cracks in tubes appear. Generally confined to internal surfaces of water carrying tubes that are actively corroding. Usually occurs in regions of high heat flux, beneath heavy deposits, in slanted and horizontal tubes and in heat regions at or adjacent to backing rings at welds or near devices that disrupt flow. Uncommon in boilers with a working pressure of less than 70 bar. A typical sequence would be:
1) NaOH removes the magnetite
2) Free hydrogen is formed ( hydrogen in its atomic rather than diatomic state) by either the reaction of water with the iron reforming the magnetite or by NaOH reacting with the iron.
3) This free hydrogen can diffuse into the steel where it combines at the grain boundaries to form molecular hydrogen or reacts with the iron carbide to form methane.
4) As neither molecular hydrogen or methane can diffuse through the steel the gasses build up, increasing pressure and leading to failure at the grain boundaries.
6) These microcracks accumulate reducing tensile stress and leading to a thick-walled failure. Sections may be blown out.
7) This form of damage may also occur in regions of low pH.
8) For boilers operating above 70 bar, where high pH corrosion has occurred the possibility of hydrogen damage should be considered.
Ultra-high pressure boilers are susceptible to a form of attack which damages the internal structure of the metal causing it to become brittle. This type of attack is called hydrogen damage.
Hydrogen damage occurs when there is a very rapid corrosion reaction in ultra-high pressure boilers. Hydrogen atoms formed at a cathode are small enough to enter the boiler metal. When they have entered the metal tubing the hydrogen atoms react with the carbon which is normally present. The reaction product of carbon and hydrogen is methane. Methane is a very large gas molecule and it causes internal pressure to form within the metal of steel tubing. These high methane pressures cause grains of steel to separate and eventually cause cracks in the metal tubing.
C+4H —> CH4
Hydrogen damaged boiler tubing has been most frequently found after incidents of acid corrosion resulting from seawater condenser leakage and improper pH control of the boiler water. Hydrogen damage has been observed to occur in a matter of hours when acid corrosion is taking place.
One of the most serious aspects of hydrogen is that affected tubing cannot be detected by any method other than removing tubes for metallurgical examination. It is, of course, impossible to inspect all of the tubes in a boiler by this technique and as a result, boilers are often left with badly damaged and unreliable tubing after repairs of failed tubes have been made. The remaining tubes often fail a few at a time over an extended period.