Step Corrosion Coupons Process
Corrosion coupons are pre-weighed and measured metal strips which are mounted in a special pipe system called a coupon rack. They are used to estimate the rate of metal corrosion by comparing the initial weight with the weight following 60, 90 or 120 days of exposure to the water in the system.
Corrosion coupons are available in a wide variety of materials to assess corrosion in all types of systems.
CORROSION COUPONS PREPARATION:
- Prepare Corrosion coupons for corrosion testing
- Coupon should be new
- Do not re-used coupons after exposure and analysis
- Chose the method preparation of the metal
- Grinding operation must controlled to avoid high surface operation that could change the microstructure coupon.
- Etch / stamp a permanent serial number of a coupon.
- Machine or polish the edge of coupon to remove cold-worked meta
1. Open the envelope and take out coupon carefully. Avoid contamination of coupon caused by contact with other materials, oil or chemicals.
2. Record the following information:
- Corrosion coupons serial number
- Installation date
- Name of system
- Location of the coupon in the system (including fluid or vapor phase), name of the platform, sketches the position of the coupon in the pipeline (put all data on the Envelope)
- See the typical corrosion coupon report from Envelope & Appendix A
3. Record the physical condition of pipeline such as:
- Dimension of pipeline
- Fluid / water temperature
- Flow rate
Number of :
- Platform Number of
- Pipeline Dimension of
- Pipeline Type of
- Fluid Pressure Density Viscosity Flow Rate
DURING REMOVAL CORROSION COUPONS:
1. Remove the coupon out of the system carefully.
2. Record the following items:
- Corrosion coupons serial number
- Removal date
- Observation of any erosion or mechanical damage
- Appearance of scale
- Appearance of corrosion product
- The coupon should be photograph immediately after removal, particularly if appearance of the corrosion product or scale is important.
3. Record any changes of fluid flow rate during exposure time:
- Any other pertinent data such as shut in time
- Changes in velocity and inhibitor treatment
- Type or materials for corrosion inhibitor
- Dosing rate and dosing time of corrosion inhibitor
- Position injection chemical in the pipeline should also be recorded
4. Put the coupon into original envelope carefully and seal it (Avoid contamination!)
- Send the coupon immediately to laboratory for analysis.
- Do not remove any scale or corrosion product in the field.
LABORATORY PROCEDURE FOR CLEANING AND WEIGHING CORROSION COUPONS AFTER EXPOSURE
- Open the envelope and take out the coupon carefully.
- Coupon don’t contact with other material or chemicals.
- Record serial number
- Take the photograph before and after cleaning
- Weight the coupon with accuracy 0.1 mg
- Visually examine the coupon and record observations
- Qualitative analysis of scale or foreign material
PROCESS CLEANING CORROSION COUPONS BEFORE ANALYSIS & WEIGHT LOSS CALCULATION
- Immerse the coupon in a suitable hydrocarbon solvent: Xylene / Toluene, to remove oil, oil-wet material and paraffin.
- Handle solvent under a ventilated hood.
- Dry in a gentle dry air stream
- Weight coupon to within 0.1 mg
FOR QUALITATIVE ANALYSIS
Preparation sample for qualitative analysis. Analysis for know the qualitative parameter such as to monitor present or absent of this substance:
- Iron Oxyde
- Ferric Sulfate
FOR QUANTITATIVE ANALYSIS
Preparation sample for quantitative analysis used acid – soluble deposits is desired and analyzed
Parameter for qualitative analysis:
1. Sulfate, SO4 % Wt
2. Chloride, Cl % Wt
3. Carbonate, CO3 % Wt
4. Iron, Fe2O3 % Wt
PREPARING PROCEDURE AFTER CLEANING
1. Coupon that are not coated with hard scale clean by blasting with glass beads.
2. Mass loss during blast cleaning should be determined.
3. After cleaning immerse the coupon in a saturated solution sodium bicarbonate for 1 minute to neutralize the acid.
4. Rinse with distillated water to removed neutralizer.
5. Rinse the coupon immediately in Xylene or Toluene and dry in steam of dry air. Visually examine coupon and record observation.
6. Pre weighed blank that was not exposed to the corrodent to the cleaning process to ensure that mass loss from cleaning is not significant.
7. Place the coupon into the original envelope carefully, avoid contamination seal it, and send to laboratory.
PROCEDURE FOR CALCULATION OF CORROSION RATE
Calculation of Average Corrosion Rate (CR).
1. Determine mass loss of Corrosion coupons and divide mass loss by product of metal density, total expose surface area, the exposure time to obtain the average rate of corrosion. The Following equation may be used to determine the average corrosion rate depending on the units desired.
Calculate of average corrosion rate, expressed as a uniform rate of thickness loss per unit time in mil per year (mpy) with the following Equation (1):
CR = average corrosion rate, mil per year (mpy)
W = mass loss, grams (g)
A = initial exposed surface area of coupon, square inches (in.2)
T = exposure time, days (d)
D = density of coupon metal, gram per cubic centimeter (g/cm3)
1 mm/y = 39.4 mpy
1 m/y = 0.0394 mpy (m = micrometer)
1 mpy = 0.0254 mm/y
1 mpy = 0.001 in./y (inches/year)
1 mil = 0.001 in.
LOCATION COUPON IN THE SYSTEM
Coupon should be located in the corrosion most occurred. Corrosion and design engineer should collaborate to ensure that sufficient access fitting for corrosion Monitoring are included in the design. Corrosion failure record can identify corrosion area. Ultrasonic and radiographic metal thickness measurement can be made to locate areas where corrosion has occurs. Coupon can function in the liquid or vapor phase a system.
Location for coupon:
(1). Dead fluid areas
(2). High velocity fluid stream and impingement points
(3). Down stream from point of possible oxygen entry: such as tanks, pumps vapor
recovery units, and water makeup line in gas sweetening system.
(4). Location where water is likely to collect in sour systems such as: suction
Scrubber or compressors, separators, water drain lines from dehydrators, and
Low spots in wet gas lines.
(5). Amine and glycol streams that contain sour gas
(6). Vapor section in glycol regenerators
(7). Area where a liquid/vapor interface occurs
In line handling wet gas, water can accumulate at changes in the line elevation. Coupon in such systems must be located where they will be water wet to correlate with corroding area. Coupon located in vapor phase could indicate only slight corrosion when water wet areas are corroding severely.
Corrosion subsurface well equipment can be monitored by installing cleaned and weighted tubing subs, or pup joint (600 mm [2 ft] long) can be installed in the sucker rod subs near the bottom, middle or top of the well. The used of coupons in the sucker rod in the sting acts as a coupon.
Corrosion wellhead fitting on high velocity flowing well that produces organic acids, carbon dioxide, and water may be very severe. Corrosion coupon should be located both upstream and downstream from chokes to evaluate the effects of changes in velocity, temperature and phases.
Coupon located in flow lines of wells may be affected by paraffin accumulation. Coupon should be located in a section of the line that is free of paraffin deposits. Coupon located in surface line from wells may not provide accurate information on down hole corrosion rate.
Flat coupon should be oriented in the system so that one edge faces the fluid flow. Replacement coupon should have the same orientation as previous coupons. Record should indicate the exact location of coupon in a line or vessel (i.e. top, middle, or bottom).
Corrosion in pipelines with small quantities of water is often monitored with test nipples; corrosion coupon must be carefully placed to ensure that they are subjected to line’s corrosive conditions. Coupons should be installed in both liquid and vapor phases.
In horizontal multiphase flow, phases can sometimes be stranded. Care must be taken to ensure the coupon is exposed to the corrosive phase(s). For example, in wet gas system, flush disk type coupon can be placed in annular flow sections of the pipe to ensure contact with the water phase.
Exposure time must be considered when interpreting corrosion coupon data. Short-term exposure (15 to 45 days) provides quick answer but may give higher corrosion
Rate than long-term exposures. Aggravating conditions, such as bacterial fouling, may take time to develop on the coupon. Short exposure times may be advantageous when evaluating inhibitor effectiveness. Longer exposures (60 to 90 days) are often required to detect and define pitting attack.
Multiple coupon holders can be used so that both the short and long-term effect can be evaluated. Because exposure time affects test results, exposure periods should be as consistent as practical. Tolerance of 7% allows a variation of 2 days on 30 days exposure. This is satisfactory for most applications.
When coupon are used to evaluate and monitor corrosion inhibitor treatment, new coupons should be installed just prior to treatment, new coupons should be installed just prior to treatment. This is particularly important when there is a long period between treatments (as inhibitor squeeze, tubing displacement, and infrequent batch treatment of gas wells).
Electronic corrosion and inhibitor film monitoring instruments include electrical resistance measuring instruments, polarization instrument, galvanic probes, and electrolytic and vacuum-type hydrogen probes. All of these instruments are useful in detecting short-term upsets that may not be detected by coupons, which measure average corrosion rates. Some of the polarization and galvanic probes have removable metal element that can be weight before and after exposure.
RECORDING DATA ON CORROSION COUPON REPORT
The typical corrosion coupon report form in Appendix a shows the type of information that should be reported in Corrosion Monitoring Program. A separate form should be used for each coupon. Similar coupon report forms are available from commercial laboratories and inhibitor suppliers. Complete records of coupon testing are very important in evaluating corrosion mitigation programs.
INTERPRETATION COUPON DATA
Data from corrosion coupons and other monitoring instruments seldom correlate exactly with the rate of corrosion observed in the system. Factors that can contribute to the lack of correlation include coupon location and multiphase flow characteristics.
Coupons installed in single – phase system, such as:
Water injection line correlate with corrosion rate on system in stratified multiphase systems; attack may be confined to the part of the coupon exposed to the corrosive phase. Coupon provides valuable information long-term exposures.
Intermittent conditions such as periodic entry of oxygen into a water system or into water gas system usually cannot be characterized by standard corrosion coupons with any degree of accuracy.
Coupon data reflect only the average rate of corrosion during the test period.
Coupon can be useful in providing back up for other types of corrosion monitors. Coupon data should be correlated also with the corrosion failure frequency in the system being studied. Continuous monitoring is essential so that changes in the corrosion rate in a system can be detected as soon as possible after they occur.
Qualitative guidelines for interpretation of measured corrosion rate and pitting rates are given in Table 2. The average corrosion and pitting rates shown in Table 2. Are intended for use only as guides. Coupons installed in dynamic system may indicate a higher rate of corrosion than is actually occurring on the interior wall. Also, coupons are initially clean and free from protective film that may be providing considerable protection to the pipe wall.
The rate of corrosion of a coupon may be much greater during the first days than after an exposure of one month. After the coupon has been exposed to the environment, protective films such as oil, carbonates, iron oxides, and sulfides may begin only after an “incubation period”.
Under deposit corrosion usually becomes severe only after the coupon made of a corrosion resistant metal may be exposed long enough for deposit form. A coupon made of corrosion – resistant metal may be exposed adjacent to the coupon under test to assess the effects of mechanical erosion