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

2. 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

3. 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.

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

8. 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

9. 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)

10. 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

11. pH

12. 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

13. 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

14. 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

15. 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

17. 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

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

19. 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

20. 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

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

22. 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.

23. 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.

24. 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

25. Oxygen Concentration Cell

26. 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

27. 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.

28. 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

29. 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.