4. Corrosion Barriers/Preventions
Corrosion barriers aims at reducing the effect of conditions that
can lead to corrosion. This can be done by putting measures in
place such as;
• Material selection
• Application of chemicals such as oxygen scavengers and
corrosion inhibitors
• Pipeline design
• Avoiding the use of de-similar metals.
• Application of coating.
5. Material Selection
This is a step in the process of FEED design. The main goal is to
minimize cost while meeting product performance.
Material option
• Carbon Steel and low alloy steel
• Austenitic SS
• High-alloy austenitic SS
• Duplex SS
• Nickle base alloys
6. Application of Chemicals
Corrosion Inhibitors
A corrosion inhibitor is a chemical compound that, when added to a liquid or gas,
decreases the corrosion rate of a material, typically a metal or an alloy.
• At the design phase it is assumed that the inhibitor efficiency is sufficient to
reduce the residual corrosion rate to 0.1 mm/yr.
• In the case of high corrosivity fluid (i.e. CR > 5mm/yr) the company standard
considers that the inhibitor efficiency is not able to reduce the corrosion rate
to 0.1 mm/yr and a maximum inhibitor efficiency (E) 95% should be imposed
for design purpose and the residual corrosion rate will be calculated as CRinh =
(1-E)*CR.
CA= (CRinh*DL*CIA) +CR*DL*(1-CIA)
where:
CRinh is the residual corrosion rate equal to 0.1mm/yr or (1-E)*CR in case of high
corrosivity fluid.
DL is the design life.
CIA is the corrosion inhibitor availability. In the design phase this value is assumed
to be equal or greater than 90%.
7. Other Chemical Injected Includes;
Oxygen Scavengers. Added to enclosed packaging to
help remove or decrease the level of oxygen in the
package. They are used to help maintain product safety
and extend shelf life.
Biocide. Used to control the level of microbiological
organism within the system, which can lead to
Microbial Induced Corrosion.
8. Oxygen Barriers
• Deaerator. This is a mechanical device that is used for the removal of
oxygen. It’s intended use is for the reduction of oxygen concentration
down to less than 100ppb
• Chemical Scavenges. These are chemicals, usually with ammonium
bisulphite (ABS) used to reduce the oxygen concentration to less than
20ppb.
Microbial Control Systems
• Chlorination. As a minimum the seawater inlet is chlorinated usually
after the coarse filters
• Biocide. Similar to chlorination, biocide are applied to control microbial
activities. It is recommended that biocide are injected down stream of
the deaerator.
• Pigging. Regular pigging where possible, can reduce the effect of
microbial activity.
9. Erosion/Solid Control
To control solids and flowrate that can lead to erosion corrosion
within the pipeline system, several controls are put in place;
• Flowrate. This involves setting a minimum and maximum
flowrates to limit solids settling or eroding the pipe walls
• Coarse filter. This filters are installed in the inlet off the
seawater injection to prevent suspended solid and organic
particles large than 2 mm from entering the injection system
• Fine filter. Similar to coarse filter, the fine filter is used to
prevent residual solid particles size below 50 microns.
10. Galvanic Corrosion Control
This type of corrosion can be controlled or made harmless by;
• By avoiding the combination of larger area of old material
which have become more noble over long period of time, with
a small area of new material which are more active
• Use of more noble weld consumer than the base material
• Addition of corrosion inhibitors
• Insulating coupling points
• Application of metallic coating on the one of the materials.
11. Crevice Corrosion Control
This type of corrosion can be reduced by;
• Avoiding crevices when possible.
• Design of pipeline to avoid deposition.
• Application cathodic protection
Atmospheric Corrosion Control
Mainly caused by the present of oxygen and water on the
base metal surface. This type of corrosion is controlled by;
• Coating of the base material
• Cathodic protection.
12. ASSURANCE
This is a process or procedure performed to ensure that the barriers put in place
are effective against corrosion threat. As a minimum the monitoring should
include;
• Corrosion monitoring
• Oxygen monitoring
• Bisulphite residual measurement
• Chloride monitoring
• Total suspended solid monitoring
• Flowrate and fluid velocity
13. CORROSION MONITORING
Corrosion Coupons. Corrosion coupons are a minimum for corrosion
monitoring. Corrosion coupons should be remove not less than 90 days and
not more than 180 days after insertion, and all coupons should be exposed
for the same time.
Types
• Strip Coupon
• Flush Disc Coupon
Deterioration mechanism
14. Corrosion probes. Secondary options include electrical resistance
(ER) and linear polarisation resistance (LPR) probes.
Main function is to measure metal loss due to corrosion/erosion
Types
1. Electric Resistance(ER) Probe (Flush and Tubular ER Probe )
2. Linear Polarized Resistance (LPR) Probe
15. Electrical Resistance (ER) Probe
ER probe is a device that provides a basic measurement of metal loss, while the probe is
in-situ and permanently exposed to the process.
It measures the change in ohmic resistance of a corroding metal element expose to the
process stream. Wire Loop
ER Probe
Flush ER Probe
ER probes have all the advantages of coupons, plus:
• They are applicable to all working environments gases, liquids, solids,
particulate flows.
• Direct corrosion rates can be obtained.
• Probe remains installed in-line until operational life has been exhausted.
• They respond quickly to corrosion upsets and can be used to trigger an alarm.
ER probes are available in a variety of element geometries, metallurgies and
sensitivities and can be configured for flush mounting such that pigging operations
can take place without the necessity to remove probes. The range of sensitivities
allows the operator to select the most dynamic response consistent with process
requirements.
16. Corrosion rate calculation based on data
retrieved from a probe
AT40 probe, which has a life span of 10mils, is used in a process application. October,29th
2013, the probe had a reading of 72 divisions and on November 23rd 2013, the probe had
a reading of 95 divisions.The corrosion rate calculation for this period will be;
Corrosionrate(mpy)=0.365x(Readingfinal–Readinginitial)xProbespan
Timeindays
0.356x(95–72)x10mils
25days
=3.36mpy
1mpy=0.0254mm/yr
3.36mpy=0.0853mm/yr
Mechanism Corrosion Rate (mm/year)
Low Moderate High Severe
General Below 0.025 0.025 to 0.126 0.127 to 0.254 Above 0.254
17. Linear Polarized Probes
Linear polarization resistance monitoring is an electrochemical method of measuring
corrosion.
It measures the relationship between the electrochemical potential and current
generated between the electrodes in the process fluid which allows the calculation of
corrosion rate.
LPR Theory
R= V/I
Resistance measure if inversely proportional to corrosion rate
LPR probe with 20mV
applied between the
electrode
Electrode
Process fluid
• The advantage of the LPR technique is that the
measurement of corrosion rate is made
instantaneously.
• This is a more powerful tool than either coupons
or ER where the fundamental measurement is
metal loss and where some period of exposure is
required to determine corrosion rate.
• The disadvantage to the LPR technique is that it
can only be successfully performed in relatively
clean aqueous electrolytic environments. LPR will
not work in gases or water/oil emulsions where
fouling of the electrodes will prevent
measurements being made.
18. Oxygen Monitoring
This should be carried out by a manual colorimetric method no less than once
per shift, complemented by online monitoring by, e.g. an Orbisphere electrode.
The latter should be calibrated at intervals of not less than 30 days.
Orbisphere
19. Bacterial Monitoring
• Monitoring should be carried out for both planktonic and sessile
bacteria, for both sulphate reducing bacteria (SRB) and general
aerobic bacteria (GAB) also known as general heterotrophic
bacteria (GHB).
• Monitoring should take place at a consistent time ideally before
biocide treatment but certainly not during.
• Biocide residuals should be measured once per month where a
suitable analytical method is readily available.
20. Bisulphite residuals
• These should be measured at the same
time as the manual oxygen residuals.
• A suitable method is the Hach drop
count SU-5 kit or other equivalent.
Chlorine monitoring
This should be carried out by a manual colorimetric
method based on diphenylene diamine (DPD) at the
inlet and outlet of the deaerator tower. Chlorine residual
should be measured at the same time as oxygen
measurements.
Total Suspended Solids Monitoring
TSS monitoring should typically be carried out weekly,
but at not less than monthly frequency.
