Top Ten Ways to Control Corrosion in Process Plants
By Michael J. Humphries, Ph.D.
Corrosion in industrial facilities in general, and process plants in
particular, is a costly degradation mechanism. It could also have
safety, environmental, and equipment reliability consequences should it
result in a loss in pressure containment. Listed below are the top
ten ways to control corrosion in process plants.
- Corrosion of Buried Piping or Pipelines
Apply impressed current cathodic protection and polarize the system to
–850 Mv. If there is evidence of microbiological activity, polarize to
–950 Mv. For new equipment, apply a coating to reduce CP system demand
and backfill with graded fill to prevent coating damage by rocks.
- Corrosion of Underwater Pipelines
Apply cathodic protection using impressed current or sacrificial
anodes. Polarize to –850 Mv. For new facilities apply a protection
system incorporating a heavy extruded jacket over a liquid applied
coating. Joint protection is critical, and should employ a heat shrunk
sleeve after the joint is prepared and coated.
- Corrosion by Organic Sulfur Compounds in Hydrocarbon
If carbon steel has proven inadequate, upgrade to 5 Cr or higher
alloys. Carbon steel clad with stainless steel is a good alternative
in many applications. Do not use solid 13 Cr or Duplex stainless
steels above 600ºF because they may embrittle. Be aware that low
silicon carbon steel components sulfide faster than higher silicon
- Corrosion by Naphthenic Acids
For the most economical approach, inject a Naphthenic Acid corrosion
inhibitor in conjunction with limited alloy upgrading to Type 317
stainless steel of circuits that cannot be protected by the inhibitor.
Alloy upgrading alone is rarely cost effective, and only practical if
there is a longer term supply of high TAN crude.
- Corrosion by H2S/Hydrogen
Selectively upgrade circuits to stainless steel. The severity of
attack depends on the H2S content of the stream and the
liquid/vapor ratio. For many applications, Type 304/304L is adequate.
Various API, NACE, and other industry documents can provide guidance
on alloy selection (e.g., API RP 571, NACE MR 0103, etc.).
- Corrosion at Pipe Supports
Piping experiences increased external corrosion where it sits on a
support. Water and corrosives can become trapped in the crevice
between the pipe and the support, and movement between pipe and
support dislodges scale which would retard corrosion. Corrosion at
this location is difficult to monitor and measure. It can be
controlled by attaching a shaped shoe to the outside of the pipe at
- Corrosion in Recirculating Cooling Water Systems
Corrosion is controlled by application of a biocide and a chemical
treatment to control scaling and corrosion. When hydrocarbon leaks
into the system, the biocide is consumed by the hydrocarbon and
organic deposits form, leading to localized corrosion. A first step in
controlling corrosion is to break the “Vicious Cycle” initiated by
- Corrosion in Boilers
Provided feedwater treatment is correctly maintained, online corrosion
is rarely a problem. Many corrosion problems in boilers occur when the
unit is offline. Boiler layup procedures are critical and should
include oxygen removal and exclusion, and chemicals to raise pH.
Alternatively, the boiler may be dry stored, with water completely
removed. The fireside should be maintained warm to prevent water and
- Corrosion Under Insulation
Field experience has shown many cases of Corrosion Under Insulation
(CUI) on carbon steel equipment operating between 25 and 350ºF (Ref.
NACE SP0198-2010) when water penetrates the insulation. Salts carried
in with the water, or derived from the insulation, form a corrosive
environment at the pipe surface. Corrosion is controlled by applying a
coating such as an epoxy or epoxy phenolic to the pipe and by ensuring
that the weatherproofing over the insulation excludes water.
- Corrosion in Crude Column Overheads
Corrosion in the overhead of atmospheric crude units is caused by
condensation of acids or desublimation of ammonium chloride. It is
usual to add inhibitors to control acid corrosion. Control of
corrosion under ammonium chloride deposits has become a more critical
problem due to increased ammonia levels caused by ammonia recycle
within the refinery. Corrosion under Ammonium Chloride deposits is
best controlled by a continuous water wash in the overhead system.
Water introduced to the overhead must be removed in the downstream