AN INTRODUCTION TO HYDROGEN DAMAGE AN INTRODUCTION TO HYDROGEN DAMAGE
General Description
Hydrogen damage may occur where corrosion reactions result in the production of atomic hydrogen. Damage may result from a high-pH corrosion reaction or from a low-pH corrosion reaction. Damage resulting from a high-pH corrosion reaction is simply caustic corrosion
This form of deterioration is a direct result of electrochemical corrosion reactions in which hydrogen in the atomic form is liberated.* It is typically confined to internal surfaces of water-carrying tubes that are actively corroding.
Generally, hydrogen damage is confined to water-cooled tubes. Damage usually occurs in regions of high heat flux; beneath heavy deposits; in slanted or horizontal tubes; and in heat-transfer regions at or adjacent to backing rings at welds, or near other devices that disrupt flow. Experience
has shown that hydrogen damage rarely occurs in boilers operating below 1000 psi (6.9 MPa).
Concentrated sodium hydroxide dissolves the magnetic iron oxide according
to the following reaction:
If atomic hydrogen is liberated, it is capable of diffusing into the steel. Some of this diffused atomic hydrogen will combine at grain boundaries or inclusions in the metal to produce molecular hydrogen, or will react with iron carbides in the metal to produce methane.
Since neither molecular hydrogen nor methane is capable of diffusing through the steel, these gases accumulate, primarily at grain boundaries.
Eventually, gas pressures will cause separation of the metal at its grain boundaries, producing discontinuous intergranular microcracks (Fig. 1). As microcracks accumulate, tube strength diminishes until stresses imposed by boiler pressure exceed the tensile strength of the remaining,
intact metal. At this point a thick-walled, longitudinal burst may occur (Fig. 2). Depending on the extent of hydrogen damage, a large, rectangular section of the wall frequently will be blown out, producing a gaping hole (Fig. 3).
Hydrogen damage may also result from a low-pH corrosion reaction in an operating boiler.
Atomic hydrogen may be liberated during corrosion resulting from local low-pH conditions. Atomic hydrogen is capable of diffusing into the metal and reacting to form molecular hydrogen or methane, as described above. Hydrogen damage resulting from exposure to low-pH conditions is mechanistically and physically identical to that resulting from high-pH conditions. The difference is merely the source of the atomic hydrogen.
Critical Factors
The critical factors governing hydrogen damage resulting from high-pH corrosion
The critical factors governing hydrogen damage resulting from low-pH corrosion
Identification
It is generally not possible to visually identify hydrogen damage prior to failure. In boilers operating at more than 1000 psi (6.9 MPa), areas that have sustained either high-pH or low-pH corrosion should be considered suspect.
Generally, hydrogen damage is difficult to detect by nondestructive means, although sophisticated ultrasonic techniques have been developed to reveal hydrogen-damaged metal. Ultrasonic thickness checks may disclose corroded areas that should be considered suspect.
Gouging and hydrogen damage resulting from low-pH conditions may be distinguished from damage resulting from high-pH conditions by a consideration of the boiler-water chemistry and the chemistry of the probable sources of contamination. For example, a common source of contamination of boiler water is condenser in-leakage. The source of the cooling water determines whether the in-leakage is acid-producing or base-producing.
Fresh water from lakes and rivers usually provides dissolved solids that hydrolyze in the boiler-water environment to form a high-pH substance, such as sodium hydroxide. In contrast, seawater and water from recirculating cooling-water systems incorporating cooling towers may contain dissolved solids that hydrolyze to form acidic solutions.
Elimination
Two critical factors govern susceptibility to hydrogen damage. These are the availability of high- or low-pH substances, and a mechanism of concentration. Both must be present simultaneously for hydrogen damage to occur.
To eliminate the availability of high- or low-pH substances, the following steps should be taken:
1- Reduce the amount of available free sodium hydroxide. This can be done in the case of hydrogen damage caused by high pH.
2- Prevent inadvertent release of regeneration chemicals from makeup-water demineralizers.
3- Prevent condenser in-leakage. Because of the powerful concentration mechanisms that may operate in a boiler, in-leakage of only a few parts per million of contaminants may be sufficient to cause localized corrosion and hydrogen damage.
4- Prevent contamination of steam and condensate by process streams. Preventing localized concentration of corrosive substances is the most effective means of avoiding hydrogen damage. It is also the most difficult to achieve.
5 - Preventing departure from nucleate boiling (DNB), excessive water-side deposition, and the creation of waterlines in tubes may help prevent localized concentration of corrosive substances.
Prevent departure from nucleate boiling. Preventing DNB usually requires the elimination of hot spots, which is accomplished by controlling the boiler's operating parameters. Hot spots may be caused by excessive overfiring or underfiring, misadjusted burners, change of fuel, gas channeling,
and excessive blow down.
6- Prevent excessive water-side depositions. To prevent excessive water-side deposition, tube sampling on a periodic basis (usually annually) may be performed to measure relative thickness and amount of deposit buildup on tubes. Tube-sampling practices are outlined in ASTM D887-82. Consult boiler manufacturers' recommendations for acid cleaning.
7- Prevent waterline formation. Slanted and horizontal tubes are especially susceptible to the formation of waterlines. Boiler operation at excessively
low water levels or excessive blowdown rates may create waterlines. Waterlines may also be created by excessive load reduction when pressure remains constant. When load is reduced and pressure remains constant, water velocity in boiler tubes is reduced to a fraction of its full-load value. If
it becomes low enough, steam/water stratification occurs and creates stable or metastable waterlines.
Cautions
Hydrogen damage typically produces thick-walled ruptures. Other failure mechanisms producing thick-walled ruptures include stress-corrosion cracking, corrosion fatigue, stress rupture, and, in some rare cases, severe overheating. It may be difficult to visually distinguish ruptures caused by
hydrogen damage from other ruptures, although certain features may serve as an aid
AN INTRODUCTION TO HYDROGEN DAMAGE
General Description
Hydrogen damage may occur where corrosion reactions result in the production of atomic hydrogen. Damage may result from a high-pH corrosion reaction or from a low-pH corrosion reaction. Damage resulting from a high-pH corrosion reaction is simply caustic corrosion
This form of deterioration is a direct result of electrochemical corrosion reactions in which hydrogen in the atomic form is liberated.* It is typically confined to internal surfaces of water-carrying tubes that are actively corroding.
Generally, hydrogen damage is confined to water-cooled tubes. Damage usually occurs in regions of high heat flux; beneath heavy deposits; in slanted or horizontal tubes; and in heat-transfer regions at or adjacent to backing rings at welds, or near other devices that disrupt flow. Experience
has shown that hydrogen damage rarely occurs in boilers operating below 1000 psi (6.9 MPa).
Concentrated sodium hydroxide dissolves the magnetic iron oxide according
to the following reaction:
4NaOH + Fe₃O₄ ↦ 2NaFeO₂ + Na₂FeO₂ + 2H₂O
With the protective covering destroyed, water is then able to react directly with iron to evolve atomic hydrogen:
3Fe + 4H2O↦Fe₃O₄ + 8H ↑
The sodium hydroxide itself may also react with the iron to produce hydrogen:
Fe + 2NaOH↦ Na₂FeO₂+ 2H ↑
If atomic hydrogen is liberated, it is capable of diffusing into the steel. Some of this diffused atomic hydrogen will combine at grain boundaries or inclusions in the metal to produce molecular hydrogen, or will react with iron carbides in the metal to produce methane.
Fe₃C + 4H -» CH₄ + 3Fe
Since neither molecular hydrogen nor methane is capable of diffusing through the steel, these gases accumulate, primarily at grain boundaries.
Eventually, gas pressures will cause separation of the metal at its grain boundaries, producing discontinuous intergranular microcracks (Fig. 1). As microcracks accumulate, tube strength diminishes until stresses imposed by boiler pressure exceed the tensile strength of the remaining,
intact metal. At this point a thick-walled, longitudinal burst may occur (Fig. 2). Depending on the extent of hydrogen damage, a large, rectangular section of the wall frequently will be blown out, producing a gaping hole (Fig. 3).
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| FIG 1 |
 |
| FIG 2 |
 |
| FIG 3 |
Hydrogen damage may also result from a low-pH corrosion reaction in an operating boiler.
Atomic hydrogen may be liberated during corrosion resulting from local low-pH conditions. Atomic hydrogen is capable of diffusing into the metal and reacting to form molecular hydrogen or methane, as described above. Hydrogen damage resulting from exposure to low-pH conditions is mechanistically and physically identical to that resulting from high-pH conditions. The difference is merely the source of the atomic hydrogen.
Critical Factors
The critical factors governing hydrogen damage resulting from high-pH corrosion
The critical factors governing hydrogen damage resulting from low-pH corrosion
Identification
It is generally not possible to visually identify hydrogen damage prior to failure. In boilers operating at more than 1000 psi (6.9 MPa), areas that have sustained either high-pH or low-pH corrosion should be considered suspect.
Generally, hydrogen damage is difficult to detect by nondestructive means, although sophisticated ultrasonic techniques have been developed to reveal hydrogen-damaged metal. Ultrasonic thickness checks may disclose corroded areas that should be considered suspect.
Gouging and hydrogen damage resulting from low-pH conditions may be distinguished from damage resulting from high-pH conditions by a consideration of the boiler-water chemistry and the chemistry of the probable sources of contamination. For example, a common source of contamination of boiler water is condenser in-leakage. The source of the cooling water determines whether the in-leakage is acid-producing or base-producing.
Fresh water from lakes and rivers usually provides dissolved solids that hydrolyze in the boiler-water environment to form a high-pH substance, such as sodium hydroxide. In contrast, seawater and water from recirculating cooling-water systems incorporating cooling towers may contain dissolved solids that hydrolyze to form acidic solutions.
Elimination
Two critical factors govern susceptibility to hydrogen damage. These are the availability of high- or low-pH substances, and a mechanism of concentration. Both must be present simultaneously for hydrogen damage to occur.
To eliminate the availability of high- or low-pH substances, the following steps should be taken:
1- Reduce the amount of available free sodium hydroxide. This can be done in the case of hydrogen damage caused by high pH.
2- Prevent inadvertent release of regeneration chemicals from makeup-water demineralizers.
3- Prevent condenser in-leakage. Because of the powerful concentration mechanisms that may operate in a boiler, in-leakage of only a few parts per million of contaminants may be sufficient to cause localized corrosion and hydrogen damage.
4- Prevent contamination of steam and condensate by process streams. Preventing localized concentration of corrosive substances is the most effective means of avoiding hydrogen damage. It is also the most difficult to achieve.
5 - Preventing departure from nucleate boiling (DNB), excessive water-side deposition, and the creation of waterlines in tubes may help prevent localized concentration of corrosive substances.
Prevent departure from nucleate boiling. Preventing DNB usually requires the elimination of hot spots, which is accomplished by controlling the boiler's operating parameters. Hot spots may be caused by excessive overfiring or underfiring, misadjusted burners, change of fuel, gas channeling,
and excessive blow down.
6- Prevent excessive water-side depositions. To prevent excessive water-side deposition, tube sampling on a periodic basis (usually annually) may be performed to measure relative thickness and amount of deposit buildup on tubes. Tube-sampling practices are outlined in ASTM D887-82. Consult boiler manufacturers' recommendations for acid cleaning.
7- Prevent waterline formation. Slanted and horizontal tubes are especially susceptible to the formation of waterlines. Boiler operation at excessively
low water levels or excessive blowdown rates may create waterlines. Waterlines may also be created by excessive load reduction when pressure remains constant. When load is reduced and pressure remains constant, water velocity in boiler tubes is reduced to a fraction of its full-load value. If
it becomes low enough, steam/water stratification occurs and creates stable or metastable waterlines.
Cautions
Hydrogen damage typically produces thick-walled ruptures. Other failure mechanisms producing thick-walled ruptures include stress-corrosion cracking, corrosion fatigue, stress rupture, and, in some rare cases, severe overheating. It may be difficult to visually distinguish ruptures caused by
hydrogen damage from other ruptures, although certain features may serve as an aid
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