Electrochemical process during oxygen related general corrosion. Metal losing part is anode and the electron receiving end is cathode.
The corrosion cycle: from stable iron mineral to unstable but useful metals and to stable corrosion products via corrosion process[1].

Corrosion is a natural potential hazard associated with oil and gas production and transportation. This results from the fact that an aqueous phase is inevitably co-produced with the oil and/or gas. The inherent corrosivity of this aqueous phase is then heavily dependent on the levels of dissolved CO2 and H2S acidic gases which are also co produced. This has long been recognised[2] and has prompted extensive studies of the subject[3][4] leading to the development of predictive models or criteria which attempt to relate corrosion risk to acidic gas partial pressure. Fluid flow also plays a critical in the corrosion process.

In the oilfield, corrosion is typically managed by applying corrosion inhibitor or usage of corrosion resistant alloys.

CO2 corrosion

CO2 corrosion or "sweet corrosion" is due to the formation of carbonic acid when CO2 is dissolved in water. This type of corrosion the most prevalent form of attack associated with oil and gas production and transportation and its understanding, prediction and control are key requirements to sound facilities design and subsequent assurance of continuing integrity. The form of attack is often localised (Mesa attack)[5] which may result from flow induced and/or mechanical damage to a "protective" surface film - commonly iron carbonate - but can also be influenced by water chemistry, for example the presence of acetic acid, and steel composition.

As a general statement the inherent corrosivity of the aqueous phase will be dependent on CO2 partial pressure, temperature, salt content and pH. Condensed waters (associated with gas production) are in general more corrosive than formation waters as they do not contain any buffering or scale forming species.

It is also often observed, at least in laboratory tests, that water or brine acidified with CO2 to a given pH produces a more corrosive solution than acidifying to the same pH with mineral acid. This is generally attributed to the fact that because carbonic acid (H2CO3) is not fully dissociated in solution.

Rule of Thumb criteria for carbon and low alloy steels:

  • ppCO2 < 7 psi (0.5 bar) Corrosion Unlikely
  • 7 psi (0.5 bar) < ppCO2 < 30 psi (2 bar) Corrosion Possible
  • pCO2 > 30 psi (2 bar) Corrosion

H2S corrosion

H2S can and does cause metal loss attack which generally tends to be localised due to the tendency for iron sulphide films to be formed on a steel surface (pH dependent) which breakdown or spall locally leading to pitting. Whether this mechanism predominates over that of CO2 attack, which is usually inevitably present, will depend on circumstance and the relative levels of CO2 and H2S present. Thus for many cases CO2 corrosion dominates but the rate of attack is modified in the presence of H2S, i.e. general rates lower but risk of localised attack increased.


However, the presence of H2S is of primary concern because of the risk of introducing cracking and the possibility of subsequent catastrophic failure. Susceptibility is dependent on the steel type, hardness, partial pressure of H2S, pH and temperature. The type of cracking is generally dependent on the yield strength of the steel.

Galvanic corrosion

Galvanic corrosion occurs when two different metals have physical or electrical contact with each other and are immersed in a common electrolyte, or when the same metal is exposed to electrolyte with different concentrations. In a galvanic couple, the more active metal (the anode) corrodes at an accelerated rate and the more noble metal (the cathode) corrodes at a retarded (؟) rate. When immersed separately, each metal corrodes at its own rate. What type of metal(s) to use is readily determined by following the galvanic series. For example, zinc is often used as a sacrificial anode for steel structures. Galvanic corrosion is of major interest to the marine industry and also anywhere water (via impurities such as salt) contacts pipes or metal structures.

Galvanic corrosion of aluminum

In this photo, a 67 cm thick aluminum alloy plate is physically (and hence, electrically) connected to a 10-mm thick mild steel structural support. Galvanic corrosion occurred on the aluminium plate along the joint with the mild steel. Perforation of aluminum plate occurred within 2 years due to the large acceleration factor in galvanic corrosion.[6]

Factors such as relative size of anode, types of metal, and operating conditions (temperature, humidity, salinity, etc.) affect galvanic corrosion. The surface area ratio of the anode and cathode directly affects the corrosion rates of the materials. Galvanic corrosion is often utilized in sacrificial anodes.

Galvanic series

In a given environment (one standard medium is aerated, room-temperature seawater), one metal will be either more noble or more active than the next, based on how strongly its ions are bound to the surface. Two metals in electrical contact share the same electrons, so that the "tug-of-war" at each surface is analogous to competition for free electrons between the two materials. Using the electrolyte as a host for the flow of ions in the same direction; the active metal will take electrons from the noble one. The resulting mass flow or electrical current can be measured to establish a hierarchy of materials in the medium of interest. This hierarchy is called a galvanic series, and can be a very useful in predicting and understanding corrosion.

Corrosion removal

Often it is possible to chemically remove the products of corrosion to give a clean surface, but one that may exhibit artifacts of corrosion such as pitting. For example phosphoric acid in the form of naval jelly is often applied to ferrous tools or surfaces to remove rust.

Corrosion removal should not be confused with Electropolishing which removes some layers of the underlying metal to make a smooth surface. For example phosphoric acid (again) may be used to electropolish copper but it does this by removing copper, not the products of copper corrosion.

Resistance to corrosion

Some metals are more intrinsically resistant to corrosion than others, either due to the fundamental nature of the electrochemical processes involved or due to the details of how reaction products form. For some examples, see galvanic series.

There are various ways of protecting carbon steel from corrosion including painting, hot dip galvanizing, and combinations of these. [7]

If a more susceptible material is used, many techniques can be applied during an item's manufacture and use to protect its materials from damage.

Intrinsic chemistry

Gold nuggets do not naturally corrode, even on a geological time scale.

The materials most resistant to corrosion are those for which corrosion is thermodynamically unfavorable. Any corrosion products of gold or platinum tend to decompose spontaneously into pure metal, which is why these elements can be found in metallic form on Earth, and is a large part of their intrinsic value. More common "base" metals can only be protected by more temporary means.

Some metals have naturally slow reaction kinetics, even though their corrosion is thermodynamically favorable. These include such metals as zinc, magnesium, and cadmium. While corrosion of these metals is continuous and ongoing, it happens at an acceptably slow rate. An extreme example is graphite, which releases large amounts of energy upon oxidation, but has such slow kinetics that it is effectively immune to electrochemical corrosion under normal conditions.

Passivation

Passivation refers to the spontaneous formation of an ultra-thin film of corrosion products known as passive film, on the metal's surface that act as a barrier to further oxidation. The chemical composition and microstructure of a passive film are different from the underlying metal. Typical passive film thickness on aluminum, stainless steels and alloys is within 10 nanometers. The passive film is different from oxide layer or scale that are frequently formed at high temperatures and are in the micrometer thickness range. The passive film has the unique property of self-healing while the oxide layer or oxide scale does not. For example, when you scratch the surface of a stainless steel, the damaged passive film will be healed spontaneously by the instantaneous oxidation of chromium from the underlying metal. Passivation in natural environments such as air, water and soil at moderate pH is seen in such materials as aluminium, stainless steel, titanium, and silicon.

Passivation is primarily determined by metallurgical and environmental factors. The effect of pH is recorded using Pourbaix diagrams, but many other factors are influential. Some conditions that inhibit passivation include: high pH for aluminium and zinc, low pH or the presence of chloride ions for stainless steel, high temperature for titanium (in which case the oxide dissolves into the metal, rather than the electrolyte) and fluoride ions for silicon. On the other hand, sometimes unusual conditions can bring on passivation in materials that are normally unprotected, as the alkaline environment of concrete does for steel rebar. Exposure to a liquid metal such as mercury or hot solder can often circumvent passivation mechanisms.

Corrosion in passivated materials

Passivation is extremely useful in mitigating corrosion damage, however even a high-quality alloy will corrode if its ability to form a passivating film is hindered. Proper selection of the right grade of material for the specific environment is important for the long-lasting performance of this group of materials. If breakdown occurs in the passive film due to chemical or mechanical factors, the resulting major modes of corrosion may include pitting corrosion, crevice corrosion and stress corrosion cracking.

Pitting corrosion

The scheme of pitting corrosion

Certain conditions, such as low concentrations of oxygen or high concentrations of species such as chloride which complete as anions, can interfere with a given alloy's ability to re-form a passivating film. In the worst case, almost all of the surface will remain protected, but tiny local fluctuations will degrade the oxide film in a few critical points. Corrosion at these points will be greatly amplified, and can cause corrosion pits of several types, depending upon conditions. While the corrosion pits only nucleate under fairly extreme circumstances, they can continue to grow even when conditions return to normal, since the interior of a pit is naturally deprived of oxygen and locally the pH decreases to very low values and the corrosion rate increases due to an auto-catalytic process. In extreme cases, the sharp tips of extremely long and narrow corrosion pits can cause stress concentration to the point that otherwise tough alloys can shatter; a thin film pierced by an invisibly small hole can hide a thumb sized pit from view. These problems are especially dangerous because they are difficult to detect before a part or structure fails. Pitting remains among the most common and damaging forms of corrosion in passivated alloys[citation needed], but it can be prevented by control of the alloy's environment.

Weld decay and knifeline attack

normal microstructure
sensitized microstructure

Stainless steel can pose special corrosion challenges, since its passivating behavior relies on the presence of a major alloying component (Chromium, at least 11.5%). Due to the elevated temperatures of welding or during improper heat treatment, chromium carbides can form in the grain boundaries of stainless alloys. This chemical reaction robs the material of chromium in the zone near the grain boundary, making those areas much less resistant to corrosion. This creates a galvanic couple with the well-protected alloy nearby, which leads to weld decay (corrosion of the grain boundaries in the heat affected zones) in highly corrosive environments.

A stainless steel is said to be sensitized if chromium carbides are formed in the microstructure. A typical microstructure of a normalized type 304 stainless steel shows no signs of sensitization while a heavily sensitized steel shows the presence of grain boundary precipitates. The dark lines in the sensitized microstructure are networks of chromium carbides formed along the grain boundaries.[8]

Special alloys, either with low carbon content or with added carbon "getters" such as titanium and niobium (in types 321 and 347, respectively), can prevent this effect, but the latter require special heat treatment after welding to prevent the similar phenomenon of knifeline attack. As its name implies, corrosion is limited to a very narrow zone adjacent to the weld, often only a few micrometres across, making it even less noticeable.

Crevice corrosion

Crevice corrosion of type 316 stainless steel

Crevice corrosion is a localized form of corrosion occurring in confined spaces (crevices) to which the access of the working fluid from the environment is limited and a differential aeration cell is set up, leading to the active corrosion inside the crevices. Examples of crevices are gaps and contact areas between parts, under gaskets or seals, inside cracks and seams, spaces filled with deposits and under sludge piles.

This photo shows that corrosion occurred in the crevice between the tube and tube sheet (both made of type 316 stainless steel) of a heat exchanger in a sea water desalination plant.[9]

Crevice corrosion is influenced by the crevice type (metal-metal, metal-nonmetal), crevice geometry (size, surface finish), and metallurgical and environmental factors. The susceptibility to crevice corrosion can be evaluated with ASTM standard procedures. A critical crevice corrosion temperature (CCT) is commonly used to rank a material's resistance to crevice corrosion.

Microbial corrosion

Microbial corrosion, or commonly known as microbiologically influenced corrosion (MIC), is a corrosion caused or promoted by microorganisms, usually chemoautotrophs. It can apply to both metallic and non-metallic materials, in the presence or absence of oxygen. Sulfate-reducing bacteria are active in the absence of oxygen (anaerobic); they produce hydrogen sulfide, causing sulfide stress cracking. In the presence of oxygen (aerobic), some bacteria may directly oxidize iron to iron oxides and hydroxides, other bacteria oxidize sulfur and produce sulfuric acid causing biogenic sulfide corrosion. Concentration cells can form in the deposits of corrosion products, leading to localized corrosion.

Accelerated Low Water Corrosion (ALWC) is a particularly aggressive form of MIC that affects steel piles in seawater near the low water tide mark. It is characterised by an orange sludge, which smells of hydrogen sulphide when treated with acid. Corrosion rates can be very high and design corrosion allowances can soon be exceeded leading to premature failure of the steel pile.[10] Piles that have been coating and have cathodic protection installed at the time of construction are not susceptible to ALWC. For unprotected piles, sacrificial anodes can be installed local to the affected areas to inhibit the corrosion or a complete retrofitted sacrificial anode system can be installed. Affected areas can also be treated electrochemically by using an electrode to first produce chlorine to kill the bacteria, and then to produced a calcareous deposit, which will help shield the metal from further attack.

High temperature corrosion

High temperature corrosion is chemical deterioration of a material (typically a metal) under very high temperature conditions. This non-galvanic form of corrosion can occur when a metal is subjected to a high temperature atmosphere containing oxygen, sulfur or other compounds capable of oxidising (or assisting the oxidation of) the material concerned. For example, materials used in aerospace, power generation and even in car engines have to resist sustained periods at high temperature in which they may be exposed to an atmosphere containing potentially highly corrosive products of combustion.

The products of high temperature corrosion can potentially be turned to the advantage of the engineer. The formation of oxides on stainless steels, for example, can provide a protective layer preventing further atmospheric attack, allowing for a material to be used for sustained periods at both room and high temperature in hostile conditions. Such high temperature corrosion products in the form of compacted oxide layer glazes have also been shown to prevent or reduce wear during high temperature sliding contact of metallic (or metallic and ceramic) surfaces.

Methods of protection from corrosion

Surface treatments

Applied coatings

Galvanized surface

Plating, painting, and the application of enamel are the most common anti-corrosion treatments. They work by providing a barrier of corrosion-resistant material between the damaging environment and the structural material. Aside from cosmetic and manufacturing issues, there are tradeoffs in mechanical flexibility versus resistance to abrasion and high temperature. Platings usually fail only in small sections, and if the plating is more noble than the substrate (for example, chromium on steel), a galvanic couple will cause any exposed area to corrode much more rapidly than an unplated surface would. For this reason, it is often wise to plate with active metal such as zinc or cadmium. Painting either by roller or brush is more desirable for tight spaces; spray would be better for larger coating areas such as steel decks and waterfront applications. Flexible polyurethane coatings, like Durabak-M26 for example, can provide an anti-corrosive seal with a highly durable slip resistant membrane. Painted coatings are relatively easy to apply and have fast drying times although temperature and humidity may cause dry times to vary.

Reactive coatings

If the environment is controlled (especially in recirculating systems), corrosion inhibitors can often be added to it. These form an electrically insulating or chemically impermeable coating on exposed metal surfaces, to suppress electrochemical reactions. Such methods obviously make the system less sensitive to scratches or defects in the coating, since extra inhibitors can be made available wherever metal becomes exposed. Chemicals that inhibit corrosion include some of the salts in hard water (Roman water systems are famous for their mineral deposits), chromates, phosphates, polyaniline, other conducting polymers and a wide range of specially-designed chemicals that resemble surfactants (i.e. long-chain organic molecules with ionic end groups).

Anodization

This figure-8 descender is annodized with a yellow finish. Climbing equipment is available in a wide range of anodized colors.

Aluminium alloys often undergo a surface treatment. Electrochemical conditions in the bath are carefully adjusted so that uniform pores several nanometers wide appear in the metal's oxide film. These pores allow the oxide to grow much thicker than passivating conditions would allow. At the end of the treatment, the pores are allowed to seal, forming a harder-than-usual surface layer. If this coating is scratched, normal passivation processes take over to protect the damaged area.

Anodizing is very resilient to weathering and corrosion, so it is commonly used for building facades and other areas that the surface will come into regular contact with the elements. Whilst being resilient, it must be cleaned frequently. If left without cleaning, panel edge staining will naturally occur.

Biofilm coatings

A new form of protection has been developed by applying certain species of bacterial films to the surface of metals in highly corrosive environments. This process increases the corrosion resistance substantially. Alternatively, antimicrobial-producing biofilms can be used to inhibit mild steel corrosion from sulfate-reducing bacteria.[11]

Controlled permeability formwork

Controlled permeability formwork (CPF) is a method of preventing the corrosion of reinforcement by naturally enhancing the durability of the cover during concrete placement. CPF has been used in environments to combat the effects of carbonation, chlorides, frost and abrasion.

Cathodic protection

Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the cathode of an electrochemical cell. Cathodic protection systems are most commonly used to protect steel, water, and fuel pipelines and tanks; steel pier piles, ships, and offshore oil platforms.

Sacrificial anode protection

Sacrificial anode in the hull of a ship.

For effective CP, the potential of the steel surface is polarized (pushed) more negative until the metal surface has a uniform potential. With a uniform potential, the driving force for the corrosion reaction is halted. For galvanic CP systems, the anode material corrodes under the influence of the steel, and eventually it must be replaced. The polarization is caused by the current flow from the anode to the cathode, driven by the difference in electrochemical potential between the anode and the cathode.

Impressed current cathodic protection

For larger structures, galvanic anodes cannot economically deliver enough current to provide complete protection. Impressed Current Cathodic Protection (ICCP) systems use anodes connected to a DC power source (such as a cathodic protection rectifier). Anodes for ICCP systems are tubular and solid rod shapes of various specialized materials. These include high silicon cast iron, RUST, mixed metal oxide or platinum coated titanium or niobium coated rod and wires.

Anodic protection

Anodic protection impresses anodic current on the structure to be protected (opposite to the cathodic protection). It is appropriate for metals that exhibit passivity (e.g., stainless steel) and suitably small passive current over a wide range of potentials. It is used in aggressive environments, e.g., solutions of sulfuric acid.

Rate of Corrosion

A simple test for measuring corrosion is the weight loss method.The method involves exposing a clean weighed piece of the metal or alloy to the corrosive environment for a specified time followed by cleaning to remove corrosion products and weighing the piece to determine the loss of weight. The rate of corrosion (R) is calculated using the formula:-

R=Kw/(DAT)


where "k" is a constant, "W" is the weight loss of the metal in time "T", "A" is the surface area of the metal exposed, and "D" is the density of the metal (in g/cm³).

The rate of corrosion is usually expressed in terms of mg/dm²/day or inches per year(IPY)

Economic impact

The collapsed Silver Bridge, as seen from the Ohio side

The US Federal Highway Administration released a study, entitled Corrosion Costs and Preventive Strategies in the United States, in 2002 on the direct costs associated with metallic corrosion in nearly every U.S. industry sector. The study showed that for 1998 the total annual estimated direct cost of corrosion in the U.S. was approximately $276 billion (approximately 3.2% of the US gross domestic product).[12]

Rust is one of the most common causes of bridge accidents. As rust has a much higher volume than the originating mass of iron, its build-up can also cause failure by forcing apart adjacent parts. It was the cause of the collapse of the Mianus river bridge in 1983, when the bearings rusted internally and pushed one corner of the road slab off its support. Three drivers on the roadway at the time died as the slab fell into the river below. The following NTSB investigation showed that a drain in the road had been blocked for road re-surfacing, and had not been unblocked so that runoff water penetrated the support hangers. It was also difficult for maintenance engineers to see the bearings from the inspection walkway. Rust was also an important factor in the Silver Bridge disaster of 1967 in West Virginia, when a steel suspension bridge collapsed in less than a minute, killing 46 drivers and passengers on the bridge at the time.

Similarly, corrosion of concrete-covered steel and iron can cause the concrete to spall, creating severe structural problems. It is one of the most common failure modes of reinforced concrete bridges. Measuring instruments based on the half-cell potential are able to detect the potential corrosion spots before total failure of the concrete structure is reached.

References

  1. Worldoil webcast:MONITORING AND MITIGATING OILFIELD CORROSION.
  2. "Industrial Corrosion Monitoring." Department of Industry Committee on Corrosion. HMSO (1978)
  3. "Corrosion Monitoring in the Oil, Petrochemical and Process Industries." (Ed. J. Wanklyn). Oyez Scientific and Technical Services Ltd. London (1982)
  4. "Proceedings of the Second International Conference on Corrosion Monitoring and Inspection in the Oil, Petrochemical and Process Industries." London 1984. Oyez Scietific and Technical Services Ltd. London (1984)
  5. R H Hausler, H P Gaddart "Advances in CO2 Corrosion": Volume I NACE 1985; Volume II NACE 1986
  6. Galvanic Corrosion
  7. Methods of Protecting Against Corrosion Piping Technology & Products, (retrieved January 2012)
  8. Intergranular Corrosion
  9. Crevice Corrosion
  10. Management of Accelerated Low Water Corrosion in Steel Maritime Structures, JE Breakell, M Siegwart, K Foster, D Marshall, M Hodgson, R Cottis, S Lyon. ISBN 0-86017-634-7
  11. R. Zuo, D. Örnek, B.C. Syrett, R.M. Green, C.-H. Hsu, F.B. Mansfeld and T.K. Wood, Inhibiting mild steel corrosion from sulfate-reducing bacteria using antimicrobial-producing biofilms in Three-Mile-Island process water Appl. Microbiol. Biotechnol. (2004) 64:275–283 http://dx.doi.org/10.1007/s00253-003-1403-7
  12. FHWA Report Number: FHWA-RD-01-156.

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