1. Corrosion Basics Chapter 4 – NACE Book Materials Selection - Metals
PTRT 1309
Corrosion Basics
Chapter 4 – NACE Book
Materials Selection - Metals
Prepared by Dr. Capone

2. Basic Metallurgy
Oil and Gas Facilities primarily constructed of metals
Terminology
Microstructures
Compositions
Physical properties and characteristics
Unified Numbering System (UNS) and common names will be used

3. Crystalline Structure
Nearly all metals exhibit crystalline structure
Atoms arrange into an orderly three-dimensional structure
Most common structures found:
Body-centered cubic
Face-centered cubic
Hexagonal close packed
Unit cell is the smallest building block from which the metal is constructed

4. Structure types

5. Crystallization When a metal cools from the liquid state
Atoms arranged into crystals which grow steadily until they meet another crystal growing simultaneously
As the blocks of crystals meet that join with random alignments
These random arrangements of crystal blocks are referred to as grains

6. Grain boundaries
The interface between the grains are referred to as grain boundaries
High stress at these boundaries
Higher corrosion rates
Can be viewed under microscope after acid etching

7. Grain boundaries

8. Grain boundaries Typical size 0.001 – 0.1” in diameter
Sample prepared by grinding then polishing followed by etching
Grain sizes are quite varied and the average grain size can be determined by averaging measured cross-sections

9. Grain boundaries

10. Ferritic and Pearlite Grain Structure

11. Alloys
Pure metals actually contain a variety of imperfections and impurities
These are an inherent cause of corrosion (increased stress regions)
High-purity metals generally have low mechanical strength and are rarely used in engineering applications
Alloying is the blending of two or more different metals to form a mixture with superior mechanical properties
Corrosion properties of these alloys is also a factor

12. Steel Primarily an alloy of iron and carbon
Pure iron is relatively weak but ductile (able to be deformed without breaking)
Alloying with 0.2 – 1% Carbon greatly increases the strength
Carbon chemically reacts with some of the iron and the resulting alloy is actually a mixture of pure Fe grains embedded with iron carbide (Fe3C) regions
The type of steel and its properties is determined by the size and shape of the Fe3C regions

13. Steel
The heat treatment (e.g. heating and cooling cycles) and mechanical working (e.g. forging, drawing, rolling, etc) determine the size and shape of these regions of Fe3C
While these structures are primarily introduced to give particular mechanical properties they also factor into the corrosion properties of the steel

14. Types of Microstructures of Steel
Four common structures:
Austenite – Face centered cubic phase
Ferrite – Body Centered cubic phase (magnetic)
Martensite – “Supersaturated” solid solution carbon phase characterized by needle-like structures
Pearlite – A mixture of ferrite and cementite (Fe3C)

15. Martensite

16. Ferrite-Pearlite

17. Heat Treating
Carbon Steels are typically found in one of four heat-treatment conditions
Annealled
Normalized
Spherodized
Quenched then tempered

19. Annealing
Heating the steel to that above “critical temperature” ~ 1600 F (871 C)
Followed by slow cooling in the furnace
Cooling can take 8 – 10 hours
Resulting microstructure is a relatively coarse Pearlite
Fe3C forms platelets that are rather thick and widely spaced

20. Normalizing
Steel is normalized by heating above the critical temperature
Steel is allowed to cool in still, ambient air
This typically takes 10 to 15 minutes
Fe3C platelets are now much thinner with a finer spacing
Often called “fine pearlite”

21. Spheroidizing
Involves holding the steel at a high temperature (but below critical temperature) about 1200 F (649 C)
24 hours or more is required
This allows the Fe3C phase to coalesce into it’s most stable globular form
Not typically encountered in Oil and Gas Applications

22. Quenching and Tempering
Often used for Oil and Gas industry products
Two step process:
Quenching
Heat to a temperature above the critical temperature
Rapidly cool by immersion in water or oil (quenching)
Tempering
Hold at temperature below critical for about 1 hour
Resulting structure has very fine Fe3C precipitates

23. Carbide Distribution
Heat treatment has a striking effect on the distribution of carbide (Fe3C)
Relatively small but NOT negligible effect on corrosion
Platelets seem to be somewhat non-reactive and hence improve corrosion resistance
Globular distributions do not produce the same effect

24. Localized Effects Ringworm corrosion
Severe attack of tubular goods along a narrow band of demarcation between two heat treatments (e.g. upset tubing not fully heat treated after upset)
Ends are heat treated prior to upset but not the bulk of the tubing

25. Localized Effects Weld Line Corrosion
Heat affected zone along a weld bead has different microstructure
Yields globular microstructure more susceptible to corrosion
Specific location of the corrosion depends on the specific of the alloy and the weld being performed

26. Alloys Not usually used to describe carbon steels Two kinds of alloys
Homogeneous
Heterogeneous
Solid solutions
Single phase – e.g stainless steel
18% chromium, 8% nickel, fraction carbon with the balance iron
Atoms are dissolved completely like milk (and sugar) into coffee
Uniform composition

27. Alloys
Heterogeneous
Not solid solutions
Separate phases exist
Composition is NOT uniform
e.g. carbon steels
Separate phases build a galvanic cell right inside the alloy
Ferrite and iron carbide are close together in the galvanic series = low corrosion

28. Phases in Steel Phase Symbol Austenite γ Ferrite α Pearlite P
Martensite α'
Cementite θ

30. Alloys Ferrous Alloys Non-ferrous Alloys Major component is iron
Other constituents present as well but majority is iron
Non-ferrous Alloys
May still contain iron but NOT the major component
Aluminum alloys
Brass (copper-zinc alloys)
Superaustenitic stainless (nickel or cobalt)

31. Ferrous Alloys Carbon Steels Alloy steels
May contain up to 1.65% Mn and 0.6% Silicon
Alloy steels
When content exceeds:
1.65% Mn
0.6% Si
0.6% Cu
When other elements are added such as: Al B Cr
Many others
Low and medium alloy steels generally are >95% iron
High alloy steels are generally < 95% iron (e.g. stainless)

32. Typical Terms used for Steel

33. Stainless Steels NOT one set of alloys
Group of alloy systems with widely different microstructure and mechanical properties
One common feature is corrosion resistance due to the presence of Cr
Stainless usually has > 10.5% Cr
All stainless steels are NOT the same but all get corrosion resistance from passivation layer of CrO on the surface
Five distinct classes
Martensitic
Ferritic
Austenitic
Precipitation hardened
duplex

34. Martensitic Stainless Steels
Cr is the principle alloying element
12% common but can be as high as 18% Cr
C ranges from 0.08% %
Other elements (e.g. Ni, Nb, Mo, Se, Si and S) are added in small amounts for special purposes in some grades
Can be hardened using the same heat treatments as the carbon alloy steels
Strongly magnetic under all conditions of heat treatment
Primary microstructure is Martensite

35. Martensitic Stainless Steels
Comprise part of the 400 series
Most common are:
Type 410 (UNS S41000)
Type 420 (UNS S42000)
Both contain about 13% Cr
Type 410 fulfills requirements for standard material for tubes
Type 420 for forgings
Widest range of use of any of the CRA’s (Corrosion resistant alloys)
API L-80 type 13 Cr is the tubing of choice for <300F sweet wells with low saltwater production

36. Ferritic Stainless Steels
Similar to Martensitic but with higher Cr content
Range from 13% to 27% Cr
Generally low carbon
Not hardenable by heat treatment
Used for good corrosion resistance and high temperature properties
Only limited use in oil and gas operations
Also part of the 400 series
Type 405 (UNS S40500)
Type 430 (UNS S43000)
Type 436 (UNS S43600)
Strongly magnetic with ferrite microstructure

37. Austenitic Stainless Steels
Two principle alloying elements – Cr and Ni
Austenite – Face centered cubic phase of iron
Minimum of 18% Cr and 8% Ni
Up to as high as 25% Cr and 20% Ni
Highest corrosion resistance of any of the stainless steels
Not hardenable by heat treatment (although they can be hardened somewhat by cold work)
The make up the 300 series
Type 304 (UNS S30400)
Type 316 (UNS S31600) – High Cr and Ni, also some Mo
Type 303 (UNS S30300) – Free machining
Type 347 (UNS S34700) – Stabilized for welding and corrosion resistance
Not generally used for high strength applications

38. Precipitation- Hardened (PH) Stainless Steels
Two principle alloying elements – Cr and Ni
Combine high strength of martensitic with good corrosion resistance of austenitic.
Most are proprietary alloys with special heat treatment cycles
Common name often seen in field equipment is “17-4 PH Stainless Steel” (UNS S17400)
Approximately 17% Cr and 4% Ni
Most are formed and machined before heat treatment
Heat treatment then hardens the finish part.
Corrosion-resistant and wear-resistant parts.

39. Duplex Stainless Steels
Recent additions to the CRAs for use in the oil field
Mixture of Austenite and Ferrite
Combine corrosion resistance of Austenitic Stainless Steels with the strength of Ferritic Stainless Steels
More expensive than 13% Cr or other CRAs
Most have compositions in the range of 20-29% Cr and 3-7% Ni
Gain high strength through cold working and can achieve yield strengths exceeding 110 kpsi
Duplex has been successfully used in sour wells with as much as 1.5 psi partial pressure.
Annealed duplex is more resistant to H2S than cold worked versions

41. Cast Iron Most of the carbon is NOT in solution with the iron
Free carbon present in the microstructure
Very low ductility as compared to steel
Strength is generally lower than the higher strength carbon and alloy steels
Big advantage is relatively low cost
Different types with differing compositions, heat treatment, microstructure and properties
Grey cast iron
White cast iron
Malleable cast iron
Nodular cast iron
Less ductile cast iron not normally seen in oil field service

42. Cast Iron
Microstructure of white cast iron containing massive cementite (white) and pearlite 100x
Ductile iron as-cast. Nodules of graphite, pearlite (dark islands) and ferrite (light background)
Flake graphite Grey cast iron 
Nodular cast iron 

43. When in doubt ASK

44. Non Ferrous Alloys Nickel-based alloys
Ni content > 50%
Bulk is usually Cr and Fe
Most are commonly referred to by trade names (e.g. Hastelloy, Monel, Inconel and Inconel-X)
Typically only used in very severe conditions
Nickel-copper alloys (Cupronickel)
Good resistance to corrosive waters
Commonly used in pump parts

45. Non Ferrous Alloys Copper-based alloys Aluminum and Aluminum Alloys
Most common are brasses
Brasses are Copper-zinc alloys
Improved mechanical performance over copper
Sensitive to corrosion (dezincification and SCC)
Copper-nickel alloys (10-30% Ni)
90/10 and 70/30 Copper nickel are both common
Resist SCC better than brasses
Aluminum and Aluminum Alloys
High strength to weight ratio
Tenacious passive oxide layer (pH between 5 and 7)
Highly corrosive at both ends of pH scale
Used in temporary water lines and heat transfer equipment

46. Copper-nickel solid solution

47. α phase – FCC structure
β phase – BCC structure (disordered)
β’ phase – BCC structure (ordered)
γ phase – complex structure (brittle)

48. Specs and Standards Specs are generally purchasing documents
Detail of particulars
Materials
Dimensions
quality, etc.
Standards are generally guidance on HOW to do something
Level of Requirements
Level of Excellence
Level of attainment

49. Oilfield Standards Common standards quoted in specifications
API – various types of oilfield equipment and components
ASTM – standards for metals and material properties including testing methods
ASME – cover materials, manufacturing, and operating limitations most specifically relating to pressure piping and vessels
NACE – prevention and testing for corrosion