OCT COMPANY, INC.

Electrochemistry of Corrosion

 

The majority of corrosion will involve oxidation of metals accompanied by an equivalent reduction reaction that will consume electrons associated with corrosion reaction. Corrosion reactions are commonly referred to individually as being "half-cell" reactions, but reduction and oxidation are interrelated, therefore the electrical current of the anodes, whereas oxidation is prevalent, and the cathodes, whereas reduction is predominate, has to be equal in order for the electrical charge to be conserved in the overall system.

 

The reduction reaction usually controlls corrosion rate because of low concentrations of reducible species in most environments compared with the high concentration (essentially 100%) of the metal. 

 

Nearly all oilfield corrosion problems are due to the presence of water. Water can be an efficient solvent for most chemicals, and the majority of oilfield corrosion will occur when the surface of the metal is wetted by water phases containing dissolved chemicals that increase the conductivity of the water.

 

Corrosion cell

Anode:

Anode is a portion of the metal surface that is corroded and disolved into solution. When metal dissolves, it loses electrons resulting in positively charged ions (Fe++ for example) which is called oxidation. The positively charged iron ion goes into solution while the electrons are left in the metal. 

Fe  -->  F++  +  2e

 

Cathode:

Electrons generated in the oxidation reaction at Anode travel within the metal to the cathode portion of the metal surface and are consumed in reduction reactions, such as

2H+  +  2e  -->   H2

O2  +  4H+  +  4e  -->  2H2O

O2  +  2H2O  +  4e  -->  4OH-

 

Electrolyte:

Water, also called electrolyte, at the metal surface completes the electrical circuit. It receives iron ion generated at Anode and provides necessary ions to support reactions at cathode. 

 

The combination of anode, cathode and electrolyte is called a corrosion cell. 

 

Pitting / Localized Corrosion

Pitting is a form of localized corrosion which produces deep spots or pits. Corrosion rate at the pitted area is many times higher than the average corrosion rate over entire surface of the metal. The pitted area can be penetrated within a short time making pitting much more damaging than general corrosion. Pitting corrosion often happens at points where the passive layer is weekened or damaged. 

 

When under-deposit corrosion (another type of localized corrosion) occurs, anodic and cathodic areas are created by the environmental differences between the area covered by the deposit and exposed area. Mineral scale deposition, growth of bacteria and formation of biofilms are the main causes of oilfield under-deposit corrosion.

 

pH Effect 

The pH of the environment has a major effect on corrosivity. At low pHs, bare metal is exposed to the environment, and acid reduction on the surface controls corrosion rates. For intermediate pHs, a partially protective film of iron oxide reduces the corrosion rate and the diffusion of oxygen to cathodic locations on the metal surface controls. As the pH increases to even higher values, the surface becomes covered with mineral scales and corrosion is reduced.

 

Passivity

Most metals form oxide films in most corrosive environments. These passive films can be protective and retard corrosion. But they can also lead to fairly deep localized corrosion when the protective films are removed or defective. Carbon steel, the most common oilfield metal, seldom forms adequately protective passive films, and other means of corrosion are often necessary.

 

Corrosion Inhibitors

 

Although removal of oxygen with scavengers and pH adjustment would remarkably reduce corrosion rates, these approaches may not be practical in many oilfield environments. The use of corrosion inhibitors is often necessary. Another major advantage of corrosion inhibitors is that they can be modified on the fly without disrupting productions. 

 

Most oilfield corrosion inhibitors are film-forming organic chemicals. They work by adsorpted at the metal / water or metal / oil interface or by precipitating a passive layer of corrosion product at the interface to slow down the corrosion reactions. The most persistent films are formed by using a product that has the appropriate solubility or dispersibility in the oil and brine that allows the inhibitor to be carried to the metal surface. In general, water based corrosion inhibitors should be applied continuouly because films formed by water based inhibitors could be easily removed by produced fluids.

 

Commercial corrosion inhibitor packages often contain inorganic oxygen and H2S scavengers and oxigizing agents to inhance performance. 

 

Combination Corrosion Inhibitors

To avoid chemical incompatibility or to use a single injection point or pump, combination corrosion inhibitors are often formulated by combining corrosion inhibitors with scale inhibitors, oxygen scavengers or surfactants. 

 

Althought they could be the most acceptable method of treatment in specific cases, these specialized products are typically not as cost-effective as their individual counterparts when used separately. It is usually necessary to use two to three times more than would normally be used of a single purpose product to achieve the same performance in the system. 

 

Corrosion Inhibitor Formulation and Selection

 

Corrosion inhibitor chemistries can be formulated to fit for various applications. 

 

Product Stability Variables

• Thermal stability – pour point, hot/cold centrifuge, mud-bomb test

• Brine solubility and dispersibility – bottle testing

• Emulsion tendency 

• Mobility for gas lift application – NVR rack “gunking” tests

• High temperature capillary loop tests 

• High pressure viscometer, particle size analyzer, FTIR, and materials compatibility for umbilical products
   

Product Performance Evaluation

• Rotating Cylinder Electrode (RCE)

  • Efficient screening tool

  • Simulates mildly turbulent flow conditions 
        -  16,000 Reynolds # (6.5 Pa wall shear stress)

  • Most applicable to sweet environments, low flow velocities and temperatures below 190 F / 88C

  • Generates LPR and weight loss data

• Reaction Kettles

  • Similar to RCE test setup

  • Non-turbulent flow conditions

  • Most applicable to sweet or sour conditions at low acid gas partial pressures, laminar flow, and temperatures below 190 F / 88C

  • Generates LPR and weight loss data

• HT/HP Autoclaves

  • Used for extremely corrosive environments

  • Limits of 2,000 psi at 340 C 

  • Best suited for simulating bottom hole temperatures, high acid gas partial pressures, longer test durations

  • Non-turbulent flow conditions

  • Generates LPR and weight loss data

  • Pitting studies – removal and examination of coupons

• Jet Impingement

  • Generates very turbulent flow conditions         
        -  50-800 Pa wall shear stress

  • Best suited for simulating high velocity streams, sweet or sour environments, temperatures and pressures up to 300 F and 300 psig.  

  • Single test can evaluate multiple shear stresses and inhibitor concentrations 

  • The most severe test for inhibitor film adherence

  • Generates LPR data only

• HT/HP Rotating Cage

  • Generates very turbulent flow conditions         
        -  50-400 Pa wall shear stress

  • Best suited for simulating high velocity streams, sweet or sour environments, temperatures and pressures up to 650F and 5,500 psig.  

  • Generates coupon/weight loss data only

  • Visual analysis for pitting attack separates this technique from the jet impingement technique

• Pitting Analysis

  • Reflected light microscope 

 

Corrosion Rates

 

Corrosion rates can be measured in a number of ways:

• Depth of penetration

  • Most widely used expression of corrosion rate. It can be calculated in mm/yr (millimeters per year) or mpy (mils, i.e. thousandths of an inch, per year). The US Standard unit, mpy, is commonly used worldwide.

  • Loss of wall thickness is often used to determine remaining equipment life or safe operating pressures of the equipment. 

• Weight loss

  • Can be very misleading because most corrosion is localized and the average penetration rate calculated from weight loss of exposure samples couldn't indicate the true condition of corroded equipment. 

• Electrical current caused by corrosion

  • Corrosion rate can be calculated from electrical current associated with anodic dissolution of a metal using Faraday's law. 

• Time to failure

  • The most common concern of operators. 

 

Treatment Evaluation and Monitoring

 

There are numerous techniques can be used to evaluate a corrosion control program's performance. The most popular are:

• Weight loss coupons

  • Results can be affected by scaling

• Pony rod coupons

• Linear polarization (LPR)

  • Instantaneous and direct reading of corrosion rates

  • Used in water system with oil < 3%

  • Results can be affected by scaling

• Resistance measurements

  • Can be used in vapor or overhead systems

  • Susceptible to plugging from scaling or corrosion product

• Potentiodynamic polarization

• Iron counts

• Pulling record monitoring