CORROSION 2016

Abstract

The Hanford site contains approximately 55 million gallons (208 million liters) of radioactive and chemically hazardous wastes that are stored in 177 underground storage tanks, of which 150 are single-shell tanks (SSTs) and the remaining are double-shell tanks (DSTs). Traditionally, the cyclic potentiodynamic polarization (CPP) technique (ASTM G61) has been used in the DST integrity program to evaluate the susceptibility of different tank steels to localized corrosion in specific waste chemistries. However, a number of waste chemistry conditions have been encountered where the CPP technique has produced an uncertain or undefined protection potential (E_prot) related to pitting corrosion. The ASTM G192 standard or Tsujikawa-Hisamatsu Electrochemical (THE) technique was originally developed to address similar problems in assessing the crevice corrosion susceptibility of corrosion resistant alloys. The ASTM G192 method basically consists of the sequential combination of three standard techniques (potentiodynamic, galvanostatic, and potentiostatic steps) to allow for a slower and more controlled method of applying anodic polarization to the specimen. This paper investigates the use of the ASTM G192 method to evaluate the pitting behavior of tank steels in various simulated waste chemistries. The protection potentials determined using the G192 method are compared to the CPP technique to highlight the cases where CPP tests could be overly conservative in estimating the protection potential related to pitting.

Introduction

The Hanford site contains approximately 55 million gallons (208 million liters) of radioactive and chemically hazardous wastes arising from weapons production beginning with World War II and continuing through the Cold War. The wastes are stored in 177 carbon steel underground storage tanks, of which 150 are single-shell tanks (SSTs) and the remaining are double-shell tanks (DSTs). The U.S. Department of Energy, Office of River Protection is responsible for retrieving the tank wastes, treating them in order to encapsulate them in glass logs, and then permanently closing the tanks and associated facilities. Current plans call for transferring the wastes from the SSTs into the DSTs over the next 25 years, retrieving wastes from the DSTs and vitrifying them, and closing all tanks by approximately 2048.¹ Such a time line places a great emphasis on maintaining the integrity of both types of tanks. The waste compositions in the storage tanks are grouped according to their main constituents such as nitrite/nitrate-based and carbonate-based chemistries. Most of the wastes are highly alkaline in nature; typically with pH values between 12 and 14. Under alkaline conditions, carbon steels will tend to be passive and undergo relatively slow, uniform corrosion. However, carbon steels can become susceptible to localized corrosion (e.g., pitting) and stress corrosion cracking (SCC) in the presence of certain aggressive constituents, such as chloride and nitrate, even in these passive conditions. ² Cyclic potentiodynamic polarization (CPP) tests have traditionally been used to determine the pitting susceptibility of tanks steels to the various waste chemistries that exist at the Hanford site. However, a number of waste chemistry conditions have been encountered where the CPP technique has produced ambiguous or undefined protection potentials (E_prot) related to pitting corrosion. The difficulty in accurately determining E_prot using the CPP technique is related to the interpretation of the reverse scan of the polarization curve or the hysteresis loops that are attributed to pitting or crevice corrosion repassivation.

Abstract

A cyclic-temperature environment is recognized as a severe condition for coatings. In oil and gas plant facilities a dehydrator is a typical kind of equipment operating in the most severe cyclic-temperature environment ranging from ambient to 300 deg-C. For a dehydrator, three coatings (i.e., “a heat resistant silicone liquid coating”, “an inorganic copolymer or coatings with an inert multipolymeric matrix liquid coating” or “a thermal sprayed aluminum coating”) are typically applied.

However, no experimental data comparing the performance of these coatings, i.e., heat cycle stability and corrosion resistance, under standard test conditions have been reported. Furthermore, no standard test method has been established to evaluate the coating performance in a cyclic-temperature environment ranging from ambient temperature to 300 deg-C.

This paper summarizes an overview of coatings for a cyclic-temperature environment first, and proposes a simple test method which includes a heat cycle between 21 to 300 deg-C simulating the dehydrator operation and exposure to salt spray. The performances of the three coatings were evaluated using this method. Finally, based on the test results and on information from recent process plant construction projects, factors to be considered for coating selection in the cyclic-temperature environment are discussed.

Introduction

Coatings are one of the most economical methods to prevent the external corrosion of equipment and piping in a harsh environment. Coatings are usually selected based on the operating temperature. However, for a cyclic-temperature environment, which is recognized as making equipment and piping highly susceptible to deterioration and subsequent corrosion, special attention is needed and robust, corrosion resistant coatings should be applied.

In this connection, newly attempted and developed coatings are being used for such cyclic-temperature environments. For example, the use of Thermal Sprayed Aluminum (TSA) coatings has increased greatly over the use of conventional heat resistant silicone liquid coating in recent years, and the developed liquid high temperature coatings are also currently available and are starting to be applied.

However, there have been no reports which compare the performance of such coatings under standard test conditions in a cyclic-temperature environment, and no standard test methods for such conditions have been proposed.

Abstract

In this work the corrosion rate of UNS⁽¹⁾ K12000 (DIN ⁽²⁾ 17100 ST 52) low-alloy structural steel in monoethanolamine (MEA) solution was studied. The corrosion testing was mainly performed on a rotating disc electrode, and electrochemical impedance was used to estimate the corrosion rate. The parameters studied were rotation rate, influence of oxygen and degradation of the MEA. The results shows a significant increase in the corrosion rate when oxygen was introduced to the system, especially when the rotation rate was high. The corrosion also increased when the MEA solution was degraded.

Introduction

During the last decades CO₂ emissions have gained a lot of attention due to their role in global warming. Aqueous alkanolamines can be used for CO₂ capture from the flue gas of a power plant. The alkanolamine selectively absorbs the CO₂ in an absorber column at low temperature and is then sent to a stripper. In the stripper, the CO₂-rich alkanolamine solution is heated to release almost pure CO₂. The lean alkanolamine solution is then recycled back to the absorber. The idea is further to transport the pure CO₂ into sequestration underground. Among the best suited alkanolamines to be used in a large scale process is MEA.

Corrosion in process equipment in alkanolamine CO₂ separation plants can be a serious problem. ¹ The anode reaction in the system is dissolution of iron metal. Oxygen is present in flue gas, and the cathode reaction can be both oxygen reduction and hydrogen evolution. ²

Corrosion in amines can increase by degradation of amines and formation of heat-stable salts; in particular, oxalic acid accelerates the corrosion. ^3-5 The thermal energy requirement for CO₂ separation decreases significantly with increasing MEA concentration, but higher MEA concentration is also expected to have a pronounced corrosive effect. ⁶ Stress corrosion cracking or hydrogen embrittlement can be a dangerous problem in MEA plants. In 1984 an amine absorber pressure vessel containing MEA ruptured because of hydrogen induced cracking, and 17 lives were lost.^{1, 7, 8}

Abstract

Corrosion tests were carried out to determine the effect of iron content on localised corrosion and stress corrosion cracking (SCC) resistance of Ni-Cr-Mo alloy weld overlays (i.e. Alloy 625) in H2S environments. In addition, the influence of iron content on the fatigue crack growth rate (FCGR) of the weld overlay in both air and a sour environment was investigated. Weld overlays with a range of iron contents (5–36%), were examined. These weld overlays were manufactured using gas metal arc/metal inert gas (GMA/MIG) welding and gas tungsten arc/tungsten inert gas (GTA/TIG) welding techniques and the required iron level in the weld overlay was achieved by changing the welding parameters. SCC tests were conducted in 25%w/v sodium chloride (NaCl) solution containing H₂ S and CO₂ (pH₂ S=14 bara, pCO₂ =28 bara) at 177°C. FCGR tests were conducted in air and in 25%w/v NaCl solution saturated with an H₂ S/CO₂ gas mixture (pH₂ S=0.4 bara) at ambient temperature and pressure.

Introduction

Corrosion-resistant weld overlays are often used to improve the service life of components made with an otherwise corrosion-prone material, such as carbon or low alloy steel. Ni-Cr-Mo welding consumables, such as Alloy 625 (ERNiCrMo-3), are commonly used for applications in seawater and sour environments. It has been known that the corrosion resistance of the weld overlay can be influenced by elemental segregation during solidification and by dilution from the carbon/low alloy steel substrate. The influence of the welding processes (e.g. MIG, TIG) on dilution has been studied by Gittos and Gooch. ^1-2 However, there is lack of data to demonstrate the effect of dilution level in the overlay on localised corrosion resistance (Kumar and Lee, ³ Chubb and Billingham⁴) and corrosion fatigue performance. To ensure the enhanced corrosion resistance offered by the corrosion-resistant overlay, current conservative approaches include the restriction of iron level in the overlay (e.g. 5-10%) and the number of the weld layers (e.g. 2-3 layers to achieve a thickness ≥3 mm). For example, DNV⁽¹⁾ -OS-F101:2007⁵ defines a limit for the iron content of overlays of less than 10%.

Abstract

Vapor Phase Corrosion Inhibitors (VCIs) are used for safe and cost-effective protection of a wide range of metal articles. One large market includes packaging materials for storage and transportation of metal parts. Plastic packaging films can be readily impregnated with VCIs to provide corrosion protection, in addition to the basic physical barrier (against water, dirt, vapors) afforded by the plastic. Generally, VCI containing plastic films are recyclable. Likewise, they can be made from recycled plastics. However, when manufacturing with commercially available recycle streams, use of the recycled plastic is often limited by contamination and extent of polymer degradation.

This paper will discuss the benefits of using in-house recycling lines; including improved environmental profile, better quality, and cost saving. The results are supported by data and experience with in-house recycling lines at two production facilities.

Introduction

Vapor Phase Corrosion Inhibitors (VCIs) are a well-known and highly versatile range of products for the prevention of corrosion.¹ VCIs can be delivered to the target metal in a variety of ways. One common product is plastic packaging. ² Plastic VCI films are a versatile and highly effective article for protection of items from corrosion. They are generally made from polyethylene, which is readily available, cost effective, and usually recyclable. ³ Production of VCI films usually results in the production of at least some “Scrap” film. This may be film of variable size produced during production start-up, or film that does not meet specifications. Scrap can be disposed as trash, but is preferably recycled. The usual mode of recycling is to reprocess it (melt processing) into pellets which can be re-used in production of new film. ³ It is often referred to as “Repro”. Reprocessing can be done in-house with dedicated machines or the scrap can be sent to external facilities that specialize in recycling. The quality of Repro can vary considerably with the quality/purity of the scrap and the conditions used for reprocessing (particularly temperature and shear). ⁴ In this paper, we report on studies varying the source and quantity of Repro and the effects on product quality. Results and commercial implications are discussed.

Abstract

We present a coordinated experimental and mathematical modeling effort to develop a three-stage model for determining the spatial and temporal potential, current, ionic species, and damage profiles for alloy 625 crevice corrosion applications in seawater solutions. In this effort stage one is defined as oxygen depletion inside the crevice, stage two the development of a critical crevice solution, and stage three long-term aggressive dissolution. In stage one, deoxygenation allows separation of the anodic and cathodic sites. In stage two, the critical crevice solution forms at the crevice tip then diffuses toward the crevice mouth. During this stage only minimal damage occurs. In stage three, once the critical crevice solution reaches a critical distance from the crevice mouth equivalent to IR* rapid propagation begins. We show that with appropriate experimental input data, and knowledge from the solution of the species dependent system, a damage evolution well-mixed model provides comparable results.

Introduction

Alloy 625 (UNS N06625), is a nickel-chromium-molybdenum alloy containing niobium which is widely used in the aerospace, chemical, petrochemical, marine service and nuclear industries for corrosion and heat resistance applications. The excellent corrosion resistance of this alloy has been attributed to the combined effect of its Cr (20-23 wt %) and Mo (8-10 wt %) content. Its Niobium content (Nb ~4wt%) has also been reported to contribute to corrosion resistance^1–8.

Nickel alloy 625 generally has an excellent corrosion resistance when exposed in highly oxidizing and reducing environments due to the formation of a protective passive film/oxide layer on the alloy surface⁹. The oxide layer is extremely stable and keeps uniform corrosion rates sufficiently low. However, nickel alloys, such as alloy 625, have been found to be susceptible to crevice corrosion when exposed to natural and chlorinated seawater^6,10,11. This type of damage occurs when narrow gaps develop resulting in a localized solution chemistry that leads to the breakdown of the passive film and the onset of rapid dissolution within the crevice. Metal surfaces shielded by gaskets, washers, bolt heads, lap joints, O-rings, or natural deposits are typical places for this type of corrosion to occur. To describe the mechanism of crevice corrosion several models have been proposed. The Oldfield and Sutton12 model explains the crevice corrosion process conceptually and mathematically. The model describes four fundamental steps leading to crevice corrosion. Initially there is deoxygenation within the crevice, increasing pH and Cl- of the crevice solution, crevice corrosion initiation due to breakdown of the passive film, and finally propagation of attack. Oldfield and Sutton define a CCS as a solution of pH and Cl- concentration sufficient to produce an anodic current of at least 10 µA/cm². They determined that for alloy 625 the critical crevice solution required a pH in a range of -0.25 to 0.50 and 6M Cl^{- 13}.

Abstract

Waste-to-Energy (WTE) and biomass-fired power generation systems are gaining in popularity worldwide as a means of cleanly disposing of waste and generating needed power in the same operation. Due to the variety of components in the feedstock, conditions vary greatly in the combustion sections of these systems. Municipal solid waste (MSW) often contains significant quantities of plastics which introduce halogens and halides into the environment when burned. Their presence greatly increases the corrosion rate of iron-base materials, thus, necessitating the use of higher nickel alloys. However many of the feed constituents can be rich in sulfur and phosphorus which are very detrimental to the performance of nickel-base alloys. Thus the makeup of the feed to be burned largely dictates the optimum materials for use in the boiler. The performance of several iron- and nickel-base materials are evaluated with respect to the environment and conclusions are drawn as to the optimum products for various components of different power generation systems.

Introduction

The use of waste-derived fuels as alternatives to the more conventional fossil fuels, coal and natural gas, is gaining in popularity due to economic and environmental advantages. The consumption of renewable energy in the U.S. accounted for 9.4% of total energy production in 2011; 4.5% was from biomass.⁽¹⁾ For comparison in 2012 11.8% of total energy production in Europe came from renewables (the goal for the EU is 20% by 2020); 7.7% was from biomass and renewable wastes. However these fuels can introduce aggressive conditions into the boiler which must be addressed in evaluating the use of these fuels. In addition bio-waste can also be used as feedstock for conversion to useful chemical compounds such a liquid fuels. By use of Fischer-Tropsch technology biowaste feedstock is converted to useful material. Here again however conditions can be very aggressive necessitating the use of corrosion-resistant alloys.

Abstract

For decades pipelines have been operated in remote and environmentally sensitive areas as well as within populated locations. Proper maintenance of pipelines can prevent internal corrosion to a remarkable degree. The methods employed are primarily mechanical cleaning (pigging) and chemical treatment (corrosion inhibitors, biocides), often used in combination. Corrosion issues arise in areas of the pipeline typically under localized areas containing sediments that tend to be an agglomeration of solids, waxes and water. The resulting corrosion defects can then become ideal locations for sediment and water to continue to gather and create deep, dirt filled localized pitting that cannot be protected through chemical treatment without the aid of mechanical cleaning (pigging).

In an effort to increase the knowledge of the cleaning efficiency of typical pig designs at removing sludge and debris from pre-existing corrosion pits, a novel test setup and method has been devised.

A recirculating flow loop was constructed with the capabilities of launching a 102 mm (4”) diameter cleaning pig using either crude oil or water as the pumped fluid. During the test, a pig would be passed through a test apparatus which housed flush mounted coupons with variously sized pits, packed with manufactured sediment (sludge). Following the pigging operation, the coupons were removed and analyzed via laser scanning techniques to measure sludge volume removal and maximum depth of cleaning. The pigs’ cleaning abilities were compared based on both metrics and information was gathered based on the profile of the sludge’s surfaces post pigging, as well as images of the pigs with adhered sludge.

Introduction

For decades pipelines have been operated in remote and environmentally sensitive areas as well as within populated locations. Alberta has over 415,000 km of buried pipelines that distribute energy, water and other products throughout the province. ¹ This large network of pipelines is a critical part of Alberta’s infrastructure. In 2011, pipeline operators reported 717 spill incidences according to the Energy Resources Conservation Board (ERCB). ² Of the incidences reported in 2011, 16 % were due to internal corrosion. ²

Abstract

There are aggressive wells in Canada, Germany, and the Middle East that have high temperature, high H₂S and CO₂ content, and which produce several tons of elemental sulfur each day. These wells must be produced with co-injection of sulfur solvents to prevent the plugging of the well bore. As one might expect these wells can also be highly corrosive and chemical products must also include corrosion inhibitors to extend the lifespan of the wells. Qualification of sulfur solvents with corrosion resistant properties is the key to mitigating the risk of these assets. The qualification process can be challenging since laboratory testing under so harsh conditions requires addition of liquid H₂S and liquid CO₂ to the autoclaves at room temperature. This process requires equation-of-state calculations to model the contents of the autoclave at room temperature to achieve at-temperature conditions.

This paper addresses one such qualification where the field conditions were predicted to be extremely sour with 35% of H₂S, 9.5% of CO₂, and a total pressure of 3400 psi at bottom hole. Temperatures were expected to exceed 300°F. The wells are also expected to produce significant amounts of elemental sulfur (>100 lb/MMScf). A combination sulfur solvent with corrosion inhibitor product was developed specifically for this sour gas field. The sulfur uptake, boiling point, emulsion tendency and the corrosion inhibition performance of the new product were evaluated in the laboratory.

Introduction

Many operating companies are reluctant to produce sour gas fields because of the high cost of treating the gas to remove H₂S and other contaminants to meet the gas specification for sale. However, as sweet fields are being depleted, more and more sour oil and gas wells are being developed. At high H₂S concentration and high temperature, H₂S may dissociate and form elemental sulfur. ¹

Abstract

The mitigation of corrosion in carbon steel pipelines due to the addition of corrosion inhibitors has traditionally been described by using adsorption isotherms. Consequently, current models of corrosion mitigation by inhibitors are based on the use of adsorption isotherms to predict surface coverage, inhibitor efficiency and ultimately the corrosion rate as a function of inhibitor concentration. However, a coverage does not properly describe the underlying electrochemical mechanisms, nor can it predict the resulting change in corrosion potential. The goal of this research is to analyze and explain how the underlying electrochemical reactions are affected by the presence of adsorbed corrosion inhibitor and the shift in corrosion potential that occurs. Two CO₂ corrosion inhibitors are studied here: tail oil fatty acid / diehylenetriamine imidazoline and quaternary alkyl benzyl dimethyl ammonium chloride. A mechanistic model was developed based on electrochemical kinetics and by using a mitigation factor, θ, which accounts for the overall retardation in the anodic and cathodic reactions. It was found that the retardation of the electrochemical reactions affected by these inhibitor can be modeled by using a single parameter: surface coverage factor.

Introduction

The oil and gas industry has long considered the use of corrosion inhibitors as an effective and affordable method to mitigate corrosion.¹ Most literature characterize inhibitor effectiveness through the use of a surface coverage factor. ^2–4 A corrosion inhibitor is a chemical substance that significantly reduces the corrosion rate in certain environments when it is added in small concentrations². Much of the knowledge about corrosion inhibition for specific environments has been obtained by simulating the operating field conditions in the laboratory and measuring the inhibition efficiency of different corrosion inhibitors^2–4. Engineers have used inhibition efficiency to develop mathematical models based upon adsorption isotherms by assuming that the inhibitors cover the metal surface as a uniform thin film and this coverage is directly proportional to the inhibition efficiency^3,5–8 This approach to describing and modeling inhibitor effectiveness is helpful, but there is a need to develop a more mechanistic approach to modeling that requires a more detailed understanding of the adsorption and electrochemical mechanisms underlying corrosion inhibition.