Chapter
4
Caustic Corrosion
Locations
Generally, caustic corrosion is confined to (1) water-cooled tubes in re-
gions of high heat flux, (2) slanted or horizontal tubes, (3) locations be-
neath heavy deposits, and (4) heat-transfer regions at or adjacent either to
backing rings at welds or to other devices that disrupt flow.
General Description
The terms caustic gouging and ductile gouging refer to the corrosive inter-
action of sufficiently concentrated sodium hydroxide with a metal to pro-
duce distinct hemispherical or elliptical depressions. The depressions may
be filled with dense corrosion products that sometimes contain sparkling
crystals of magnetite. At times, a crust of hard deposits and corrosion
products containing magnetite crystals will surround and/or overlie the
attacked region. The affected metal surface generally has a smooth, rolling
contour.
The susceptibility of steel to attack by sodium hydroxide is based on the
amphoteric nature of iron oxides; that is, oxides of iron are corroded by
both low-pH and high-pH environments (Fig. 4.1). High-pH substances,
such as sodium hydroxide, will dissolve magnetite:
4NaOH H- Fe3O4 -> 2NaFeO2 + Na2FeO2 + 2H2O
Figure 4.1 Attack on steel at 31O0C (59O0F) by water of
varying degrees of acidity and alkalinity. (Curve by Partridge
and Hall, based on data of Berl and van Taack. Courtesy of
Herbert H. Uhlig, The Corrosion Handbook, John Wiley and
Sons, New York, 1948.)
When magnetite is removed, the sodium hydroxide may react directly with
the iron:
Fe + 2NaOH -* Na2FeO2 + H2
Critical Factors
Two critical factors contribute to caustic corrosion. The first is the avail-
ability of sodium hydroxide or of alkaline-producing salts (i.e., salts whose
solution in water may produce base). Sodium hydroxide is often intention-
ally added to boiler water at noncorrosive levels. It may also be introduced
unintentionally if chemical from a caustically regenerated demineralizer is
inadvertently released into makeup water. Alkaline-producing salts may
also contaminate the condensate by in-leakage through condensers, or
from process streams. Poorly controlled or malfunctioning chemical-feed
equipment may also cause excessive alkalinity.
The second contributing factor is the mechanism of concentration. Be-
cause sodium hydroxide and alkaline-producing salts are rarely present at
corrosive levels in the bulk environment, a means of concentrating them
must be present. Three basic concentration mechanisms exist:
1. Departure from nucleate boiling (DNB). The term nucleate boiling refers
to a condition in which discrete bubbles of steam nucleate at points on a
metal surface. Normally, as these steam bubbles form, minute concentra-
tions of boiler-water solids will develop at the metal surface, usually at the
interface of the bubble and the water. As the bubble separates from the
metal surface, the water will redissolve soluble solids such as sodium hy-
droxide (Fig. 1.1).
At the onset of DNB, the rate of bubble formation exceeds the rinsing
rate. Under these conditions, sodium hydroxide, as well as other dissolved
solids or suspended solids, will begin to concentrate (Fig. 1.3 and Fig. 4.2).
The presence of concentrated sodium hydroxide and other concentrated
corrosives will compromise the thin film of magnetic iron oxide, causing
metal loss.
Under the conditions of fully developed DNB, a stable film or blanket of
steam will form. Corrosives then concentrate at the edges of this blanket,
causing metal loss at the perimeter. The metal at the interior of the blanket
is left relatively intact.
2. Deposition. A similar situation occurs when deposits shield the metal
from the bulk water. Steam that forms under these thermally insulating
deposits escapes and leaves behind a corrosive residue that can deeply
gouge the metal surface (Fig. 4.3).
3. Evaporation at a waterline. Where a waterline exists, corrosives may
concentrate by evaporation, resulting in gouging along the waterline. In
horizontal or slanted tubes, a pair of parallel longitudinal trenches may
form (Fig. 4.4). If the tube is nearly full, the parallel trenches will coalesce
into a single longitudinal gouge along the top of the tube (Fig. 4.5). In
Figure 4.2 Sodium hydroxide con-
tent attainable in concentrating
film of boiler water. (Based on
data from International Critical
Tables, 3:370(1928). Courtesy of
Herbert H. Uhlig, The Corrosion
Handbook, John Wiley and Sons,
New York, 1948.)
Figure 4.3 Deep caustic gouging beneath insulating internal deposits. (Courtesy of National
Association of Corrosion Engineers.)
vertically oriented tubes, corrosive concentration at a waterline will yield a
circumferential gouge.
Identification
If affected surfaces are accessible, caustic corrosion can be identified by
simple visual examination. If not, nondestructive testing techniques such
as ultrasonic testing may be required. Steam studies using a hydrogen
analyzer may also be used to identify caustic corrosion.
Elimination
When the availability of sodium hydroxide or alkaline-producing salts and
the mechanism of concentration exist simultaneously, they govern suscep-
Figure 4.4 Caustic gouging along a longitudinal waterline. (Courtesy of National Association
of Corrosion Engineers.)
Figure 4.5 Caustic gouging resulting from evaporation at a waterline
riding along the crown of the tube. (Courtesy of National Association
of Corrosion Engineers.)
tibility to caustic corrosion. The following remedies may eliminate corro-
sion that depends on the availability of sodium hydroxide or alkaline-pro-
ducing salts:
• Reduce the amount of available free sodium hydroxide. This is the under-
lying concept that serves as the basis for coordinated phosphate programs
implemented in high-pressure boilers.
• Prevent inadvertent release of caustic regeneration chemicals from makeup-
water demineralizers.
• Prevent in-leakage of alkaline-producing salts into condensers. Because of
the powerful concentration mechanisms that may operate in a boiler, in-
leakage of only a few parts per million of contaminant may be sufficient to
cause localized corrosion.
• Prevent contamination of steam and condensate by process streams.
Although these remedies may eliminate corrosion that depends on the
availability of sodium hydroxide or alkaline-producing salts, preventing
localized concentration is the most effective means of avoiding caustic
corrosion; it is also the most difficult to achieve. The methods for prevent-
ing localized concentration include:
• Prevent DNB. This usually requires the elimination of hot spots,
achieved by controlling the boiler's operating parameters. Hot spots are
caused by excessive overfiring or underfiring, misadjusted burners, change
of fuel, gas channeling, excessive blowdown, etc.
• Prevent excessive water-side deposition. Tube sampling on a periodic
basis (usually annually) may be performed to measure the relative thick-
ness and amount of deposit buildup on tubes. Tube-sampling practices are
outlined in ASTM D887-82. Consult boiler manufacturers' recommenda-
tions for acid cleaning.
• Prevent the creation of waterlines in tubes. 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. In this situation, water velocity in the
boiler tubes is reduced to a fraction of its full-load value. If velocity be-
comes low enough, steam/water stratification occurs, creating stable or
metastable waterlines.
Cautions
It is very difficult to distinguish localized attack by high-pH substances
from localized attack by low-pH substances simply by visual examination.
A formal metallographic examination may be required. Evaluating the
types of concentrateable corrosives that may be contaminating boiler
water will aid in the determination.
Because corrosion products may fill the depressions caused by caustic
corrosion, the extent and depth of the affected area, and even the existence
of a corrosion site, may be overlooked. Probing a suspect area with a hard,
pointed instrument may aid in the determination, but because the corro-
sion products are often very hard, a corrosion site may remain undetected.
The presence of sparkling crystalline magnetite does not necessarily indi-
cate that caustic corrosion has occurred.
Related Problems
See also Chap. 1, "Water-Formed and Steam-Formed Deposits," and
Chap. 6, "Low-pH Corrosion during Service."
CASE HISTORY 4.1
Industry:
Utility
Specimen Location:
Back wall of power boiler
Specimen Orientation:
Vertical
Years in Service:
6
Water-Treatment Program:
Coordinated phosphate
Drum Pressure:
1500 psi (10.3 MPa)
Tube Specifications:
23A in. (7.0 cm) outer diameter
Numerous caustic attacks on the back wall of a cyclone-fired boiler (Fig. 4.6)
were all observed within a month. This type of attack had occurred once
previously. This boiler was acid-cleaned every 18 to 24 months. The gouge
was noted 1 year after the last acid cleaning.
Visual examinations disclosed hard layers of black, crystalline corrosion
products covering the attack site. Measurement revealed a 42% reduction in
tube-wall thickness. Microstructural examinations disclosed moderate
overheating in the gouged region. Evidence revealed that DNB, rather than
deposits, was responsible for caustic concentration in this case. Overfiring
during start-up and low flow rates of the feedwater were suspected.
Figure 4.6 Region of caustic gouging along internal surface.
CASE HISTORY 4.2
Industry:
Utility
Specimen Location:
Camera port, waterwall
Specimen Orientation:
Vertical and slanted, S-shaped
Years in Service:
25
Water-Treatment Program:
Coordinated phosphate
Drum Pressure:
2000 psi (13.8 MPa)
Tube Specifications:
3 in. (7.6 cm) outer diameter
Fuel:
Ground coal
Visual examinations disclosed a thickened patch of hard corrosion products
adjacent to one bend (Fig. 4.7). Perforation of the wall had not occurred, but
transverse cross sections cut through the site revealed substantial metal loss
(Fig. 4.8).
The gouging was caused by sodium hydroxide that concentrated to
corrosive levels along the site of a stable steam blanket, or perhaps at an
isolated site of thermally insulating deposits. Previous failures of this type
had not occurred in this region of the boiler. The boiler had been cleaned 4
years previously with a chelant and was in peaking service. Closer control
over the water-treatment program might help prevent this type of problem in
the future.
Figure 4.7 Patch of hard iron oxides on internal surface.
Figure 4.8 Cratered region beneath patch of iron oxides.
CASE HISTORY 4.3
Industry:
Utility
Specimen Location:
Bottom slag-screen tube
Specimen Orientation:
15 ° slope
Water-Treatment Program:
Coordinated phosphate
Drum Pressure:
2200 psi (15.2 MPa)
Tube Specifications:
3 in. (7.6 cm) outer diameter
A growing number of small leaks were occurring in lower slag-screen tubes of
this boiler. One of the leaking tubes was removed for examination.
Figure 4.9 illustrates the appearance of the internal surface in the area of
leakage. A small perforation of this rifled tube was observed in the center of
a large, elliptical area of metal loss (Fig. 4.10). This area has a smooth,
rolling metal-surface contour covered with a thick, irregular mound of
coarsely stratified iron oxides. The rest of the internal surface had suffered
no metal loss.
Since deposits were not present and evidence of a waterline was not
observed, it can be assumed that concentration of the caustic material was
caused by highly localized nonnucleate boiling (DNB). The rifling of the
internal surface is designed to induce swirling of the water to prevent
nonnucleate boiling and steam/water phase stratification. It is surprising,
therefore, to find severe caustic gouging in this tube design.
However, this boiler was idle on weekends. It is possible that highly localized
nonnucleate boiling may have occurred during start-up, before normal
boiler-water circulation was fully established.
Figure 4.9 Thick, irregular mound of hard iron oxides covering perforation.
Figure 4.10 Perforation at bottom of crater.
CASE HISTORY 4.4
Industry:
Chemical process industry, ammonia plant
Boiler Type:
Heat-recovery boiler
Specimen Location:
Bottom of U-tube bundle
Specimen Orientation:
Horizontal on bottom, curving to vertical
Years in Service:
8
Water-Treatment Program:
Coordinated phosphate
Drum Pressure:
1500 psi (10.3 MPa)
Tube Specifications:
3A in. (1.9 cm) outer diameter
Heat Source:
Reformer gas
A massive longitudinal perforation was evident on this tube section. The
failure resulted from wall thinning on the internal surface. Metal loss was
localized to the top of the tube and the area of metal loss was approximately
19 in. (48.3 cm) long (Fig. 4.11).
Examination revealed a distinct groove along the top of the internal
surface; this groove diminished gradually in both depth and width as the tube
assumed a vertical orientation. Vertical surfaces were not corroded. The
groove surface was very smooth, slightly rolling in contour, and covered with
a uniform coating of black iron oxide. The surface at the perimeter of the
groove was marked by a dense population of deep, hemispherical pits that
Figure 4.11 Groove approximately 14 in. (35.6 cm) from perforation.
existed in a distinct band along the sides and at each end of the groove (Fig.
4.12).
The grooving resulted from the concentration of sodium hydroxide due to
steam accumulation and channeling along the upper horizontal and slanted
regions of the internal surfaces. In addition to the important contribution of
the tube's horizontal and slanted orientation, the accumulation of steam
resulted from a condition of either excessive heat input in this region or
impaired coolant flow through these tubes. This problem might be corrected
by reducing heat input in this part of the boiler or by increasing water
velocity through these tubes.
These types of failures have also been prevented through the use of rifled
tubes. Closer control of the chemical program for the boiler water also might
eliminate this type of failure.
Figure 4.12 Band of hemispherical pits adjacent to groove. (Magnification: 6.5X.)
Figure 4.13 Grooved window section cut from inner bend.
CASE HISTORY 4.5
Industry:
Sugar
Specimen Location:
Top of riser tube near entrance into steam drum
Specimen Orientation:
Slanted
Water-Treatment Program:
Polymer
Drum Pressure:
450 psi (3.1 MPa)
Tube Specifications:
3 in. (7.6 cm) outer diameter
Fuel:
No. 6 fuel oil
Visual examinations revealed a longitudinal groove on the internal surface
along the top of the tube (Fig. 4.13). Perforation had not occurred, but as
much as 60% of the tube wall had been corroded.
The entire top side of the internal surface exhibited shallow metal loss in
a distinct band (Fig. 4.14), which narrowed and ended in a "spear point"
near the end of the tube in the steam drum. Sparkling black crystals of
magnetite were present in and around the groove. A total of six adjacent
tubes had been similarly affected.
Steam channeling along the top of the slanted section led to concentration
of sodium hydroxide. Steam channeling may indicate localized or general
excessive heat input. If appropriate alteration of operating parameters does
not eliminate the problem, the use of rifled tubes may be effective.
The affected boiler is operated continuously during 100 to 150 days of
campaign operation twice per year. The boiler is not operated during the
intervening period.
Figure 4.14 Internal surface showing wasted metal.