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Evaluation and Selection of Corrosion Inhibitors Chandran Udumbasseri, Technical consultant

cudumbasseri@yahoo.co.in.

Any method that can measure the corrosion rate is used to evaluate the efficiency of a corrosion inhibitor. Side effects such as environmental compatibility, emulsion formation, viscosity and pour point density are also studied for final selection of a corrosion inhibitor. So there are three steps

1. Laboratory evaluation 2. Compatibility evaluation including cost 3. Final field evaluation

Laboratory test methods The tests carried out in laboratory should be under the conditions that simulate field operation conditions. So product composition, temperature, pressure, mode of inhibitor addition and environmental conditions are to be studied during the screening stage. Laboratory tests include methodology that generate corrosion and measuring techniques that determine corrosion rate and inhibitor efficiency There are direct and indirect variables that influence the inhibitor performance. Composition, temperature and pressure are direct variables. Indirect variables are hydrodynamic parameters such as flow, mass transfer coefficient, wall shear stress, Reynolds number etc. Methodologies:

1. Wheel test: this test is performed by adding corrosive fluids and inhibitor to a 200ml bottle with a metal coupon (dried and weighed accurately). The bottle is purged before and after filling with corrosive gas and capped. The bottles are then secured to the circumference of a wheel and rotating it.. when high pressure and temperature are used for testing autoclaves are used in place of glass bottles. Corrosive gases such as CO2 and H2S are purged at different partial pressures. Wheel test is a good method for screening of inhibitors at preliminary stages. It discriminates between poor and good inhibitor. But it cannot differentiate the best from several good ones. Flow pattern in wheel cannot be measured.

2. Bubble test: this test also called stirred corrosion test (or kettle test) is a flexible method for monitoring corrosion rate and inhibitor performance. The equipments consists of beaker (1liter) with glass lid having inlets for working, counter and reference electrodes, gas inlet-outlet and thermometer. In this method both composition and temperature can be simulated to the field condition. Here the fluid movement is generated by the purged corrosive gas that creates bubble flow. But there are no theoretical equations that can describe the flow conditions in this test.

3. Static test: the equipment used for bubble test can also be used for the static test. This method is used to evaluate the inhibitor performance in the absence of flow. The rate is calculated by weight loss or by electrochemical methods

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4. Rotating Disc Electrode (RDE) method: the method is simple and can give required information quickly. It is inexpensive. It consists of a rotating unit driven by motor. Connections are made by brush contacts and measurements are made by electrochemical instruments. Fluid mechanics can be applied for RDE and current density calculated. Laminar flow and hydrodynamic conditions are correlated in RDE method making it applicable to pipe conditions. RDE can be used to study under flow conditions at high pressure and temperatures.

5. Rotating Cylinder Electrode (RCE): this system provides stable and reproducible flow in small volumes. It operates in the turbulent regime.RCE is similar in design to RDE. Low and high rotation speed can be controlled. In RCE the reaction rate is controlled by mass transfer. The concentration changes can be calculated even at turbulent flow provided ohmic polarization is constant. RCE has found a variety of applications in natural turbulent industrial processes

6. Rotating Cage Test: even though this method is a promising methodologies for evaluating inhibitors, it cannot calculate and correlate hydrodynamic conditions

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Coupons are supported between two Teflon discs mounted at a fixed distance on a stirring rod.

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During the rotation of central rod a vertex is formed which widens with speed of rotation and reached walls of the container. There are four zones:

1. Homogeneous zone that increase with speed – length and width 2. Side wall affected zone-as speed increases the width reaches the walls and collide 3. Turbulent zone: vertex length penetrates into the rotating cage and creates turbulent

flow 4. Top cover affected zone: the liquid level reaches the top cover and restricts the vortex

widening 7. Jet Impingement: this test can simulate reliably and repeatedly high turbulence

conditions at high temperature and pressure for gas, liquid and multiphase turbulent systems. It requires small volumes and can be easily controlled.

8. Humidity chambers: inhibitors used for vapor phase corrosion are tested in humidity chambers and temperature and relative humidity are measured.

Measuring techniques: There are two types of measurements; non-destructive and destructive. Destructive techniques alters the corrosion process during the measuring process (potentio- dynamic polarization) or if the material is physically removed from the environment (weight loss). Non destructive techniques include linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS) and electrochemical noise which can be used for repeated measurement

1. Weight loss: weight loss determination is a common method employed in corrosion rate calculation. The coupons are weighed before and after the experiments. Corrosion rate in wheel tests and rotating cage tests can be determined by weight loss method. Calculation:

Inhibitor efficiency is given by % inhibition (%P)

2. Electrochemical methods: Linear polarization resistance (LPR), electrochemical noise (both are non destructive), potentio-dynamic polarization (destructive) are electrochemical methods of measurement techniques.

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3. Solution Analysis: In this method the products of corrosion are analyzed if they are readily soluble in process liquid. Like metal ions that are soluble in process water can be analyzed and determined.

Evaluation of compatibility:

1. Cost: corrosion inhibitors are used to stop corrosion and thereby increase the life span of plant components and equipments. If a cheaper material can do this work, then operation cost can be reduced considerably by avoiding plant shutdown, maintenance activities, regular expensive inspections, etc.

2. Environmental issues: recently, there has been increasing concern about toxicity, biodegradability and bioaccumulation of the discharged inhibitors in living kingdom, both human and animal.

3. Quality Control: spectroscopic techniques such as C13 NMR and FTIR are used to ascertain the quality of the products

4. Emulsion formation: the effect of the corrosion inhibitor on the emulsion forming tendency of crude oil is ascertained to know inhibitors impact of forming stable emulsion which are difficult process problem when the emulsion need to be broken to get the dried crude oil

Field evaluation 1. Weight loss coupons: A pre-weighed metal coupon is exposed in to the system for a

fixed duration and retrieved. Equipments are available to place and remove coupons without shutting down the plant or reducing pressure, temperature or flow. Coupon measurement gives time averaged corrosion rate and not represent corrosion rate throughout the system.

2. LPR probes: the presence of conducting solutions such as NaCl solution is needed for LPR probe functioning. If there is no water in the system, then the probe cannot work.

3. Mass counts: historical metal count data are useful for determining corrosion trends. But collecting the data is difficult.

4. Pipeline integrity gauge: internal inspection of pipe lines with sensor fitted pigs can monitor corrosion inside the pipe and inhibitor treatment frequency can be scheduled, but quit expensive.

5. Field Signature method: this method measures wall thickness at monitored area. This is not measuring the efficiency of the inhibitor, but monitors corrosion, erosion and cracking.

6. Visual inspection: installing small video cameras help to visually understand the situation and take actions.

7. Hydrogen permeation: cathodic reaction is given below:

The atomic hydrogen formed inside the pipeline can diffuse to outside and combine to form molecular hydrogen. Monitoring this hydrogen on outside surface can provide a measurement technique of corrosion inside.

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CORROSION INHIBITOR Deep well oil and gas represent extremely corrosive system due to the presence of mineralized water, carbon dioxide, and hydrogen sulfide with hydrocarbon. Presence of elemental sulfur, polysulfide, organic acids, and oxygen from injected water, elevated temperature, pressure and microbiological activity intensifies the corrosion damage. Most of the corrosion inhibitors form protective film coating on carbon steel of the pipe line. Palmitic imidazoline, oleic imidazoline, its amides, quaternary ammonium salts (benzalkonium chloride), etc are used as inhibitors. They are nitrogen containing organic compounds. The nitrogen makes bonds with the metal surface. Nitrogen has one lone pair of electrons which is used for this bonding to metal surface and clings to the surface.

The long chain hydrocarbon part of the imidazoline (palmitic, oleic groups) just project out of the surface and its orientation is controlled by the pendant group (CH2CH2NH2 group). This long hydrocarbon chain does not allow water molecules and salt ions to come to the proximity of the metal surface and protect the surface from corrosion. Corrosion reaction: Fe + H2O + O ----- Fe2+ + 2(OH)- The nitrogen containing compound makes a protective film on the inner surface of the carbon steel pipe line and protect it from water molecule , ions, etc that are responsible for corrosion. The chemistry of crude emulsion is not changing from well up to the gas oil separator. When three phases (gas, oil and water) are separated in the separator completely, then there is no problem of corrosion.

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So a nitrogen compound which can make coating on the metal surface quickly and stable against the attacks of gas, oil and water will be a three phase corrosion inhibitor. Pendant group play role in locking the hydrocarbon tail in position. Reaction: Diethylene triamine + oleic acid NH (CH2CH2NH2)2 + CH3 (CH2)7CH=CH (CH2)7COOH In the reaction two molecules of water are eliminated to form ringed structure. Some Corrosion Inhibitor Formulations All the below formulations are diluted by 45% with heavy aromatic solvent for supply and performance evaluation Formulations

S/N Ingredients Formulations

1 2 3 4 5

1 Crude Tall Oil (fatty acid) 70.99 65.99 44.99 34.99 30.99

2 C18 Trimer Unsat Fatty acid 0.0 4.0 28.0 28.0 20.0

3 2[(2-Dimethyl amino) ethoxy] ethanol 27.0 10.0 6.0 5.0 3.0

4 Dimethyl amino ethanol 2.00 2.00 1.5 1.5 1.0

5 Antifoam 0.01 0.01 0.01 0.01 0.01

6 Alcohol solvent (IPA/IBA) - 18.0 19.5 25.5 20.0

7 Aromatic solvent - - - - 17.0

8 Oxylate Polymer - - - - 2.0

9 Alkylated sulfonic acid - - - 5.0 6.0

Method of preparation

1. Weigh 1Lt RB flask and add C18 trimer acid 2. Add crude tall oil 3. Fit a stirring device and mix the content 4. Add alkanol amines drop wise ( reaction is exothermic) 5. Cool the flask to maintain around 25-35oC 6. Continue stirring for another r 1 hr after addition of amines 7. When the temperature at ambient add diluting solvent (alcohol/aromatic solvent)

Note: in the case of F4 and F8, 8. Add sulfonic acid slowly (reaction is exothermic) 9. Maintain low temperature 10. Add diluting aromatic solvent followed by polymer.

Testing: comparative testing Wheel box method-coupon testing

1. Weigh test coupons to 4 decimals 2. Add the required brine and oil mixture to test bottles

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3. The coupons are placed in the test bottles 4. Purge with CO2 5. Dose the above reaction mixtures at different concentrations 6. Purge with CO2 and seal the bottles 7. Place the test bottles on the rotating machine 8. Maintain the specified temperature (NACE ID182) and run for the specified time 9. Cool to ambient, take out the coupons, clean with water and acetone and dry. 10. Weigh and calculate loss of material.

PMAC Rotating Wheel Corrosion Oven Wheel method is one of the oldest methods for evaluating performance of corrosion inhibitors. Some of those advantages are: Many samples can be cost effectively tested at the same time, normally in the region of 24

being a comfortable sample volume to handle.

Sour analysis can more easily be conducted as the test is carried out in sealed sample cells.

Long-term tests (days, weeks or months in some cases) can be conducted with minimal supervision.

Effect of Pitting, stress cracking and interface corrosion can be assessed far easier by coupon inspection after the exposure period.

Since the coupon is alternately immersed in the crude oil and then the brine due to the rotational effect of the wheel partitioning tendencies of the inhibitors tend to be more realistic and compare better with that of field results.

PRINCIPLE OF OPERATION: A weighed metal coupon (it is prefer-able to have a coupon with a large surface area to weight ratio rather than the reverse) is placed in a glass jar or cell and filled with the required ratio of deoxygenated (nitrogen / carbon dioxide saturated.) brine and crude, the jar is then allowed to overflow slightly to eliminate all gas space from the bottle.

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Rotating Cylinder Electrode Apparatus -Atmospheric & LP When a fluid over a surface labile to corrosion undergo corrosion over a time in that environmental condition.Fluid flowing past a corroding surface often affects the way in which a metal corrodes in an environment. PMAC systems’ RCE apparatus is designed to measure the corrosion rate under a variety of shear conditions, utilizing a variety of standard or non-standard rotating electrodes.

Complete with cylinder electrode assembly, sample and disc amber

The PMAC RCE Rotator with separate controller provides a highly reli-able, low cost capability for routine RCE testing. The motor controller unit comes with a digital display of speed and an accumulator (rev. total). This rotator offers a simple and economical solution for routine hydrodynamic voltammerty.

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The rotator features a solid-state con-trolled geared-system capable of rotat-ing an electrode at rates between 250 and 10,000 RPM from a low voltage dc motor, encapsulated in sealed unit. The electrode rotation rate is set using a convenient digital push button located on the front panel of the control box. The internal counter and display facility assures the user of test perform-ance, even with viscose fluids