Article by Dr. Elayaperumal Kailasanathan, Corrosion and Metallurgical Consultant, India
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material was tested and confirmed. The tubes had a wall thickness of
2.11 mm and frequently leaked during operation (steam on the shell side
and organic acid on the tube side) after only four months of service.
The designed service conditions were as follows: The organic acid, which
was preheated in an upstream preheater, entered the first pass tubes of
the vaporizer at 70°C, and was vaporized in the heat exchanger. The
vapor left the heat exchanger at 185°C and at a pressure of 4.0 kg/cm2g
and entered a further downstream super-heater.
super heater. Leakage only occurred in the intermediate vaporizer.
The acid, bought from outside sources, was of 99.8 % purity. The
levels of impurities, such as lower-molecular-weight organic acid,
sulfuric acid, chlorides, and moisture in the as-received acid (which
usually lead to organic acid corrosion), were found to be in traces
insufficient to induce corrosion. From a purity perspective, therefore,
the organic acid used was not corrosive to 316Lstainless steel. The
evaporator was a 4-pass straight-tube heat exchanger having 120 tubes in
total. The leakages occurred only in the second and third pass tubes.
The leaking tubes were inspected closely on the inside surface
(tube-side, acid side) after being split longitudinally.
Fig. 2. The stereo microscopic
view of the leaked area on the
inside surface of a leaking tube. (8x).
- The tubes removed from the first and fourth passes – about 30 tubes per pass, showed a good, smooth, corrosion-free surface.
- The tubes removed from second and third passes had a rough
inside surface. The roughness was not uniform in all the 360 degree
positions of the tubes. There were longitudinal streaks of roughness
along the length of the tube, separated by a few degrees in the angular
direction. On close examination, using a magnifying lens, it was found
that the streaks contained a series of circular and elliptical cavities
along the length of the tubes. These were mainly elliptical, with a long
axis parallel to the length of the tubes. See Fig. 1.
inside surface aligned in a longitudinal stretch along the length of the
tube. This is not uniform corrosion by an organic acid. The
longitudinal stretching of the cavities is nearer to the outlet end and
farther from the inlet.
of the wall of the tube of the inside surface and examined under an
optical microscope at a magnification of 100x. See Fig. 3.
Fig. 3. An optical microscopic view of an affected spot. Longitudinal section (100 x.)
This photograph clearly shows the attack on the inside surface – a cavitation
with a clear elliptical attack and a long axis parallel to the tube length.
The following are inferences from the above site information and sample examinations:
- There is no metallurgical quality problem in the 316L stainless steel items used for the entire process-wetting parts.
- The acid used is not corrosive to 316L stainless steel at 70°C
in liquid form at the operating pressure of 4.2 kg/ cm2 g (Pass No.1). - At temperatures approaching 185°C and an operating pressure of
4.0 kg/cm2 g, the acid vapor is also not corrosive to 316Lstainless
steel (Pass No.4). - Tube leakage occurred at passes 2 and 3 only, where the phase change from liquid-to-vapor occurred. (Passes 2 and 3)
- This leakage is not due to any uniform chemical corrosion, but
to mechanical cavitation assisted by velocity-induced erosion of
high-intensity vapor bubbles. These bubbles formed during evaporation at
points of high-temperature gradient where they burst on the metal
surface (cavitation) assisted by localized high velocity moving the
vapor bubbles. In turn they made elliptical cavities on the inside
surface, elongated in the flow direction. This cavitation-erosion
process ate away the metal over a period of time resulting in leakage.
This is also not erosion–corrosion.
The remedial measures suggested are as follows:
- Reduce the overall velocity of the fluid passing through the
thirty tubes of each pass to the extent that the production rate allows. - Upgrade the metallurgy of the tubes to duplex stainless steel
2205 (UNS S31803) or 2507 (UNS32750). These are austenitic+ferritic
duplex stainless steels with mechanical yield strengths double that of
316Lstainless steels. Minimum values are 220 and 450 N/mm2 for 316L and
2205, respectively. At the same time, their corrosion resistance to
organic acids is far better than that of 316Lstainless steels. As their
mechanical strength is higher, the duplex stainless steel tubes can be
thinner, thereby also reducing velocities.
Fig. 4. A visual photograph of the
inside surface of the nozzle pipe
surface near the leaked hole.
Fig. 5. A stereo microscopic view
of the inside corroded surface (10x).
Uniform roughness with shiny
surface can be seen.
Nozzle pipe
Its size was 40 NB Sch 40. It contained a temperature-sensing device, which also conformed to 316L stainless steel. This showed leakage within a few months. An average thickness of about 3.70 mm was completely eaten away resulting in leaky holes (see Fig. 4).
One can see uniform corrosion on the inside surface, unlike the cavitation marks seen on the tubes. This led to the thinning of the wall, resulting in leakage. Uniform corrosion can also be clearly seen under a stereo microscope at a low magnification of 10x (see Fig. 5).
The service condition of the pipe to which the nozzle was attached was as follows: The organic acid vapor was at a temperature of 185°C and at a pressure of 4.0 kg/cm2g. Under these conditions the pipe sections did not show any corrosion. Only the nozzle pipe had corroded uniformly. Plant personnel stated that while the process pipe sections were thermally insulated, the nozzle pipe had not been insulated. Hence the organic acid vapor, which was in contact with the inside surface of the nozzle pipe, had condensed into liquid acid at a high pressure and corroded away the wall of the pipe from the inside leading to leakage. The organic acid liquid (not vapor), which was under pressure at an elevated temperatures was corrosive, although it was not corrosive above the boiling point at atmospheric pressure, where it was in vapor form.
Thus once the organic acid vapor condensed to liquid acid at 185°C (elevated temperature) and at 4.0 kg/cm2g (elevated pressure), it became very corrosive. From earlier data on the remnant nozzle thickness and over a service period of eighteen months, the corrosion rate was calculated to be 1.85 mm/year, which was an intolerably high corrosion rate.
The final diagnosis was that the nozzle pipe leaked due to excessive corrosion by the liquid organic acid which condensed at high temperatures and high pressures from the vapor on the stainless steel surface due to a lack of thermal insulation on the corresponding outer portions of the nozzle pipe sections.
The remedial measure is to insulate the entire hot portion exposed to the organic acid vapor, thereby avoiding condensation.
On comparing both cases it can be clearly seen that what has happened in the vaporizer tube is not chemical corrosion, but mechanical cavitation-erosion for which duplex stainless steel is much better than 316Lstainless steel.
ABOUT THE AUTHOR
Dr Perumal can be reached at keperumal@gmail.com
