Corrosion Basics Chapter 2 - PETEX Book Common Corroding Agents PTRT 1309 Corrosion Basics Chapter 2 - PETEX Book Common Corroding Agents Prepared by Dr. Capone

Objectives Name several of the corroding agents that commonly cause problems in oil and gas production Give several reasons why brine is such a troublesome corrodant Define splash zone Describe a few ways in which corroding agents contribute to the formation of oxygen-concentration cells Explain how the presence of carbon dioxide and hydrogen sulfide can create compounds that are harmful to production structures Explain the difference between aerobic and anaerobic bacteria and how bacteria affect corrosion Define synergistic effect

Common Corroding Agents THE CORROSION OF MOST CONCERN IN OIL AND GAS PRODUCTION IS THAT WHICH OCCURS INSIDE: - Piping - Fittings - Separators - Treaters - Tanks - Downhole structures THE OUTSIDE SURFACE OF ABOVE-GROUND STRUCTURES MAY ALSO BE SUBJECT TO SEVERE CORROSION, BUT ARE MORE READILY AVAILABLE FOR INSPECTION AND MAINTENANCE.

Corrosion Reactions At the anode we create free electrons: At the cathode we consume free electrons:

Corrosion Reactions Corrosion rates depend upon: Conductivity pH If oxygen is present we consume the hydrogen: Corrosion rates depend upon: Conductivity pH Dissolved gases H2S

Conductivity More conductive electrolyte means easier current flow Results in faster corrosion rates as long as nothing else interferes Amount of metal that dissolves is proportional to the amount of current (ions) 1 A of current represents 20 lbs of iron per year Distilled water vs salt water (next weeks lab)

pH pH is negative log of hydrogen ion concentration pH of 6, 5 and 4 are 10, 100 and 1,000 times MORE acidic than pH of 7, respectively Corrosion rates generally increase as pH decreases BUT

pH

Troublesome corrodants Most concern is that which occurs inside piping, fittings, separators, treaters, tanks and downhole structures. Outside surfaces of above ground structures are more readily accessible Common corrodants Brine Carbon dioxide Organic acids Hydrogen sulfide Oxygen Soil moisture bacteria

Brine FOUND ALONG WITH CRUDE OIL AND GAS IN PRODUCING FORMATIONS OFTEN REFERRED TO AS WATER OR SALT WATER THEY ARE WATER SOLUTIONS OF SODIUM CHLORIDE WITH ADDITIONAL IONS Positively charged cations such as calcium, magnesium and hydrogen Negatively charged anions such as sulfate, bicarbonate, sulfide ( from hydrogen sulfide) and hydroxyl GENERALLY 10,000 PARTS PER MILLION OR THREE TIMES THE SALINITY OF SEAWATER BECOMES AGGRESSIVELY CORRODING COMBINED WITH OXYGEN. STRUCTURES LIKE DRILLING / PRODUCTION PLATFORMS, UNDER-WATER STORAGE TANKS, BOATS, BARGES AND PIPELINES CARRYING OIL AND GAS ASHORE HAVE BECOME A CORROSION THREAT BECAUSE THEY ARE IN OR AROUND SEA WATER MOST SERIOUS CORROSION OCCURS ON EQUIPMENT THAT IS REGULARLY WETTED, BUT NOT COMPLETELY SURMERGED

Brine Water or saltwater (Note: the term usually is reserved for salt concentrations greater than typical seawater) Positively charged ions (cations) Sodium Calcium Magnesium Hydrogen Negatively charged ions (anions) Chloride Sulfate Bicarbonate Sulfide Hydroxyl Typical brine concentrations about 10,000 ppm salinity (about 3x seawater) Excellent electrolyte due to low resistivity

Seawater Plenty of oxygen also present makes seawater an aggressive corrodant Corrosion most problematic on surfaces that are intermittently wetted These so-called splash zones require difficult control measures that are expensive Exterior surfaces can use cathodic protection Interior surfaces in contact with saturated brine usually see little oxygen

CO2 and Other Organic Acids CO2 can form weak acids from oil or gas condensate like the reaction below: CO2 + H2O → H2CO3 AND H2CO3 + Fe → FeCO3 + H2 Carbonic acid is only slightly ionized and is therefore a weak acid Corrosive ability has been found to be proportional to its partial pressure (Table 3 in text book) Corrosion cell activity is enhanced by high temperature and pressure in deep formations Tremendous losses can occur at critical locations where corrosion takes place

Carbon Dioxide and other acid-formers Note: 1000psi flowing pressure at 5% CO2 yields a partial pressure of 50 psi

Carbon dioxide (cont) In deep wells producing gas condensate a small amount of corrosion can result in tremendous losses (weight of tubing string exaggerates the stress at corroded sections) Near the base of the wellhead temperature decrease can cause water to condense with dissolved CO2 Turbulent flow sweeps away protective hydrogen films causing catastrophic failure

Dissolved Gases – CO2 Partial pressure above 30 psi typically triggers corrosion control measures Below 3 psi typically not a problem and corrosion likely caused by other factors See photos in text

Dissolved Gases – CO2 Typical CO2 partial pressures in produced water varies with depth

Hydrogen Sulfide Reacts with iron in the presence of water( either brine or condensed water) Results in a black porous substance which is cathodic to iron making way for galvanic cells that cause corrosion to continue Iron Sulfide is pyrophorric Oxygen is usually absent at sites of such corrosion, so there is no tendency for corrosion products to oxidize into protective films.

Hydrogen Sulfide H2S reacts readily with iron in the presence of water Iron Sulfide (FeS) is a black porous substance that is cathodic to iron Oxygen is usually absent at sites of such corrosion, so there is no tendency for corrosion products to oxidize into protective films. FeS is pyrophoric and can ignite in the presence of oxygen.

Oxygen Accounts for a large proportion of all metal corrosion Omnipresent in the atmosphere and has a strong tendency to form metal oxides Oxygen is by far the worst corroding agent Causes severe corrosion at very low concentrations Dissolved oxygen (DO) accelerates the corrosivity of H2S and Co2 gases Oxygen concentration cells may result from oxygen deficient areas becoming anodic to oxygen-saturated cathodic areas. An example of this would be pipe in varying soil conditions or areas beneath pipe that contains sand, scale, tubercles, or trash One advantage of oxygen is its oxidation products sometimes form protective coverings that resist further corrosion blue – green patinas on copper roofs Dense oxide films on steel Often- invisible oxide films on aluminum

Oxygen Concentration Cell

Soil Moisture Other than well casings, only a small amount of production equipment is in contact with soil moisture. The principal surface structures exposed are flow lines and tanks Production tubing is sometimes exposed downhole. The elongated nature of these structures brings them into contact with different types of soil with different characteristics, allowing for corrosion cell formation. Disadvantages are tank-bottom failures, and casing & tubing failures which are expensive and extremely disruptive. Corrosion failures in flow lines can be readily detected and located

Bacteria Two classes depend on how they use oxygen Aerobic live in oxygen environment and can produce an oxygen concentration cell or enmesh sands and cause plugging Anaerobic live without oxygen by consuming sulfate and converting it to sulfur which then combines with hydrogen to form hydrogen sulfide Desulfovibrio desulfuricans (SRB) produce microbiologically induced corrosion (MIC) using a process still not fully defined Low pH caused by H2S formation FeS production that is cathodic to iron.

Bacteria Bacteria is divided into two classes based on how they utilize oxygen Aerobic bacteria Flourish in the presence of oxygen produces slime or scum that accumulates on metal surfaces and creates oxygen-concentration cells. sticky slime can trap sand particles (especially in areas of low flow velocity) create deposits that are not easily removed by increased flow rates or other cleansing procedures this type of bacteria is found in producing wells, water( brine) disposal systems, water flood products, pipelines and casings it has also been found in unlikely places like gasoline storage tanks and airplane fuel tanks. Anaerobic bacteria this is bacteria that lives in the absence of free oxygen this form of bacteria obtain the oxygen they need for their survival by reducing the sulfate ion to sulfur, which combines with hydrogen ions in water to form hydrogen sulfide anaerobic bacteria thrive an cause hydrogen sulfide corrosion in closed water systems where oxygen is excluded and a good supply of sulfates are present in the water they live under deposits of sand, rust scale, and other trash that excludes oxygen making these conditions favorable for the creation of oxygen-concentration cells

Combinations of corroding agents The combination of corroding agents is encountered and causes an increase in corrosion rates Corrosion rates may also increase due to synergistic effect synergistic effect is an incremental effect of two or more agents working together. Sometimes corrosion rates increase geometrically when one agent prevents the limiting action of another agent over a period of time when the protective qualities of corrosion products are altered.