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

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

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

Structure types

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

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

Grain boundaries

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

Grain boundaries

Ferritic and Pearlite Grain Structure

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

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

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

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)

Martensite

Ferrite-Pearlite

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

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

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”

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

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

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

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

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

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

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

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

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)

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)

Typical Terms used for Steel

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

Martensitic Stainless Steels Cr is the principle alloying element 12% common but can be as high as 18% Cr C ranges from 0.08% - 1.10% 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

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

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

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

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.

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

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

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 

When in doubt ASK

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

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

Copper-nickel solid solution

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

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

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