fundamentals of corrosion

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Fundamentals of Corrosion

Jinhee Lee, PE01

GS Prepared by Jinhee Lee, PE E-mail : [email protected]

(Consulting) (Material Selection for Corrosion Service)

INDEX1. Introduction 2. What is Corrosion 3. Forms of Corrosion 4. Refinery Corrosion overview 5. Corrosion Prevention 6. References 7. QA

(Welding & Heat Treatment Technology)

(Corrosion & Fouling Control)

(Failure Analysis and Trouble Shooting)

(DT & NDT Technology)

(21), 2000 6, 689 (), 2003 1, 288 2 Prepared by Jinhee Lee, PE E-mail : [email protected] 3

Prepared by Jinhee Lee, PE

E-mail : [email protected]

1. IntroductionEven though this brief presentation material does not contain sufficient information and reference data, I hope it can be a good educational material for the non metallic material engineer who willing to study and design economic and reliable plants. Corrosion can be minimized and preventable with fundamental knowledge of the corrosion engineering. Prepared by Jinhee Lee, PEMaterial & Corrosion Specialist4 Prepared by Jinhee Lee, PE E-mail : [email protected] 5

What is Corrosion? Corrosion is the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties - ASTM, Compilation ofASTM Standard Definitions, Fourth Edition, 1979, pp. 152, 759.

Corrosion is the destruction of a material by reaction with its environment. Corrodere (Latin) To eat awayPrepared by Jinhee Lee, PE E-mail : [email protected]

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Corrosion is important step in the overall metal cyclemetal extraction copper metal products such as piping and wiring product utilization recycling Discontinued use, or corrosion/failure End of useful product lifePrepared by Jinhee Lee, PE E-mail : [email protected] 7

How Important is Corrosion?Corrosion leads to loss of productivity, product contamination, part over design, and in some cases loss of life. It is conservatively estimated that $30 billion could be saved through proper use of corrosion minimization technology each year in the U. S. (M. G. Fontana, Corrosion Engineering, 3rd ed., McGraw-Hill, NY, p. 1-5, 1986)

6 copper-bearing minerals found in the earths crust

Under atmospheric conditions, corrosion leads to the decomposition of materials into their natural state. The natural decomposition products of metals are minerals.6



Direct Corrosion Cost in USA

Indirect Corrosion Cost

Corrosion Cost = Corrosion Loss + Corrosion Prevention Effort + Research + Etc.8 Prepared by Jinhee Lee, PE E-mail : [email protected]

Indirect costs of corrosion are conservatively estimated to be equal to the direct costs, for a combined cost of approximately $551 billion annually, or 6.3 percent of the GDP.9 Prepared by Jinhee Lee, PE E-mail : [email protected]

Cost of Corrosion in Industry Categories ($137.9 Billion)

Corrosion Failure

This data set indicates that only 8% of corrosion failures is unforeseeable. In other words 92% of the corrosion failures could be preventable10 Prepared by Jinhee Lee, PE E-mail : [email protected] 11 Prepared by Jinhee Lee, PE E-mail : [email protected]

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How does metal corrosion occur?Mo Environment High energy

Anodic and Cathodic ReactionAnodic reactions involve oxidation of an element to a higher oxidation state or valence Iron can be oxidized from elemental metallic iron to ferrous ions by means of electron transfer away from iron: Fe Fe2+ + 2eCathodic reactions involve reduction of an element to a lower oxidation state or valence Hydrogen can be reduced from a + 1 oxidation state to a 0 oxidation state as follows: 2H+ + 2e- H213 Prepared by Jinhee Lee, PE E-mail : [email protected]

In order for corrosion to occur, metal atoms must leave the lattice. Metal atoms must generally have high energy or lose or gain electrons to leave the solid.Prepared by Jinhee Lee, PE E-mail : [email protected]

Metal (Mo)Electron transfer Environment Mn+ or MxOy12

Electron Transfer is Essential to Nearly all Metal CorrosionWith the notable exceptions of high energy and liquid metal systems, metal corrosion requires electron transfer. Overall reactions do not show intermediate electron transfer steps that are necessary and often rate controlling. Electrical circuits must be completed (both anodic and cathodic reactions must occur).14 Prepared by Jinhee Lee, PE E-mail : [email protected] 15

Electrical Circuit Corrosion Diagram+cathodic reaction resistance Counter Reaction V Corrosion Reaction solution and other resistances electrons

-

anodic reaction resistance

An electrochemical potential gradient is necessary for electron flow and corrosion to occur in most systems.Prepared by Jinhee Lee, PE E-mail : [email protected]

Typical Corrosion Scenario2H+ H2O 0.5O2 M 2eM2+

Each environment has elements that have specific tendencies to acquire electrons.Gases and such as oxygen and chlorine, and ions such as ferric and hypochlorite, that have strong tendencies to acquire electrons are known as oxidants. Oxidants generally provide the driving force and electron acquisition that are necessary for corrosion.

MetalNotice the electron transfer from one reaction to another.16 Prepared by Jinhee Lee, PE E-mail : [email protected] 17



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Forms of CorrosionGeneral/Uniform Corrosion Atmospheric, Galvanic, Stray-current, General biological, Molten salt, Liquid metals, High-temperature (Oxidation,Sulfidation, Carburization, etc.)

Forms of Corrosion

Localized Corrosion Metallurgically influenced corrosion Mechanically assisted degradation Environmentally induced cracking1819

Filiform, Crevice, Pitting, Localized microbiological Intergranular, Dealloying Erosion, Fretting, Cavitation & Water drop impingement, Fatigue Stress cracking, Hydrogen damage, Liquid metal embrittlement, Solid metal embrittlementPrepared by Jinhee Lee, PE E-mail : [email protected]

Uniform Corrosionoriginal metal level

Uniform Corrosion

metalMicroscopic view of the effect of uniform corrosion on a metal surface.20

Macroscopic top view of a uniformly corroded surface.



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Carbon Steel Lamellar Corrosion or ExfoliationCarbon steel beam and the bolts, exposed in a wastewater plant. Located above the water level, but exposed to some levels of H2S. If you observe carefully, you will see that some steel has lifted in its metallic state.

Pack RustPack rust is a form a localized corrosion typical of steel components that develop a crevice into an open atmospheric environment. This expression is often used in relation to bridge inspection to describe built-up members of steel bridges which are showing signs of rust packing between steel plates.

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Eiffel Tower MaintenanceSince it was built (for the International Exhibition of Paris in 1889), the tower has been painted once every seven years. The application of an anticorrosion treatment lasts a good year, so that the tower can stay open and continue to greet visitors. This legendary structure comprises 220,000 m2 of surfaces that have to be maintained and repainted (7,300 tons of structural metal, and 18,000 metal parts held together by 2,500,000 rivets). Some of these surfaces are very difficult to reach. The budget for the job totals 20 million francs.

The Normandy bridge

Many other parts of the Normandy Bridge have been protected by hot galvanization, sometimes supplemented with a coat of paint. The following requirements were specified: For parts to be galvanized but not painted: 80 micron coating For parts to be both galvanized and painted: same specification as above, plus a 200 micron coating of powder paint25 Prepared by Jinhee Lee, PE E-mail : [email protected]

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Galvanic Corrosion ()

Each metal element has a unique tendency to lose electrons within a given environment.Metals such as gold and platinum that do not lose electrons easily are often referred to as noble metals

more reactive metal

less reactive metalMacroscopic view of galvanic corrosion on a pipe flange. P. S. Pao, R. P. Wei, Metals Handbook, Failure Analysis, Vol. 11, 9th ed., ASM, Metals Park, OH, Fig. 13, p. 187, 1986.Prepared by Jinhee Lee, PE E-mail : [email protected] 27

Macroscopic view of galvanic corrosion which preferentially corrodes the more reactive of two connected metals.

Metals such as magnesium and aluminum that lose electrons easily are often referred to as active metalsPrepared by Jinhee Lee, PE E-mail : [email protected]

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Noble or Active in CorrosionWhat is likely to happen to gold, copper, iron, zinc, titanium and magnesium in a corrosive environment such as 10 % hydrochloric acid at room temperature? Use the following qualitative information?Gold Copper Iron Zinc Titanium Magnesium28

Metal Corrosion in Hydrochloric Acid (10 wt %)Metal Gold Copper noble/ active very noble somewhat noble somewhat active moderately active very active very active test result little or no reaction little or no reaction slow reaction fast reaction little or no reaction fast reaction explanation low reactivity restricts corrosion low reactivity restricts corrosion slow reaction due to metal reactivity reacts rapidly due to reactivity and H+ passivation layer restricts corrosion reacts rapidly due to high reactivity and H+E-mail : [email protected]

Very Noble Somewhat Noble Somewhat Active Moderately Active Very Active Very Active29

Iron (Steel) Zinc Titanium MagnesiumE-mail : [email protected]



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Dissimilar Metal CouplesOK

Area Effect

OK

Graphite Titanium Alloy C-276 Stainless Steel Copper Cast Iron Steel Aluminum Zinc

Can Be Bad

* Area Effect : .E-mail : [email protected] 31 Prepared by Jinhee Lee, PE E-mail : [email protected]

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Galvanic Corrosion Steel / Copper Low-Velocity Seawater

Galvanic Corrosion Steel / Copper Low-Velocity Seawater

The area ratios of cathode to anode determine the rate of corrosion of the anode. This photo is an example where unfavorably high current density at the anode causes very rapid corrosion of steel rivets in a copper plate, which was exposed to seawater. The area ratio of the anode (steel) - the rapidly corroding part - to the cathode (copper) is very important in design of process equipment to control galvanic corrosion. A couple with a large cathode & small anode is bad design and should be avoided.32 Prepared by Jinhee Lee, PE E-mail : [email protected]

In the opposite galvanic condition, a small cathode to anode area ratio, the copper rivets are protected. There is only a little corrosion on the steel plates which are the anodes. This is good design, i.e. the fasteners and welds should be cathodes relative to the surrounding metal.33 Prepared by Jinhee Lee, PE E-mail : [email protected]

Good Galvanic Design

Bad Galvanic Application

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The Statue of Liberty CaseThe galvanic reaction between iron and copper was originally mitigated by insulating copper from the iron framework using an asbestos cloth soaked in shellac. However, the integrity and sealing property of this improvised insulator broke down over the many years of exposure to high levels of humidity normal in a marine environment.36 Prepared by Jinhee Lee, PE E-mail : [email protected]

The Statue of Liberty RestorationThe Statue of Liberty renovation was indeed one of the greatest undertakings of the twentieth century, requiring $230 million in private funding. The Statue of Liberty many corrosion problems were associated with: Thousands of holes pitting the copper surface caused by a century of salt-air exposure Distortion of the iron framework produced by continuous stress and fatigue Previous repair attempts that created different problems and more deteriorationThe Statue of Liberty, established in 188437 Prepared by Jinhee Lee, PE E-mail : [email protected]

Corrosion Sank the Titanic?The Titanic was held together by 3 million rivets made with a different type of iron than the hull plates, he notes. And once the hull was finished, the ship sat in seawater for a year while the inside was furnished. The dissimilar metals of the hull and rivets, bathed in electrically conductive seawater, might have created a circuit that slowly flecked away and weakened the rivets. The Titanic collision with the iceberg could have popped the weakened rivets, which would explain a clinking sound reported by survivors.

Corrosion Sank the Titanic?Baboian points out that the following information supports his theory that corrosion contributed to the sinking of the Titanic: She was launched to be fitted out at dockside almost a year before her maiden voyage, During the year in seawater, there is a strong possibility that stray currents from DC equipment caused accelerated corrosion, The rivet iron was different from the hull plate iron by design. The rivet iron needed to be maleable and therefore consisted of a higher level of slag inclusions, During the year at dockside, corrosivity of the rivet iron could therefore have been higher than the hull plate iron. Galvanic corrosion of the rivet iron is likely during that period of time, Photos and video taken by Robert Ballard during his 1985 and 1986 expeditions to the Titanic show preferential corrosion of the rivets in some areas, Inspection of the "big piece" which was retrieved from the ocean bottom also shows this same preferential corrosion in some areas. All of this evidence points towards weakening of the hull/rivet structure, enhancing the rivet popping mechanism which caused the Titanic to sink.39 Prepared by Jinhee Lee, PE E-mail : [email protected]

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Galvanic Corrosion

Galvanic Corrosion Material: Carbon steel, coated with coal tar epoxy. System: Sewage treatment plant. Part: Skimmer in settling tank. Phenomenon: Galvanic corrosion. Appearance: Local flaking-off of the coating, with pitting attack of the steel. Time to Failure: year after fitting stainless-steel cap.

Material: Hot-dip galvanized steel. System: Hot water system, 160C. Part: Heating tube from make-up water vessel. Phenomenon: Galvanic corrosion (reversal of potential). Appearance: Pitting attack. Time to Failure: 1 year.40 Prepared by Jinhee Lee, PE E-mail : [email protected] 41



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Prevention of Galvanic Corrosion Select combinations of metals close together in galvanic series (useful as first approximation only). Provide for complete dielectric insulation of dissimilar metals. Avoid unfavorable area ratio effects (i.e., small anodic and large cathodic areas should be avoided) Avoid threaded joints between metals far apart in galvanic series; use welded, brazed, or fused joints instead. Apply coatings, but with caution (coatings contain pinholes!). Do not fully rely on painting. Corrosion inhibitors may be helpful, since they reduce the aggressiveness of the environment. Prevent access of air and/or water to the joint of the two metals.42 Prepared by Jinhee Lee, PE E-mail : [email protected] 43

Pitting ()

Illustration of Pitting Corrosion

Macroscopic view of pits inside a pipe. Forms of Corrosion Recognition and Prevention, Ed. Dale McIntyre, Vol 2., p. 32.Prepared by Jinhee Lee, PE E-mail : [email protected]

Formation of Corrosion Pit

Pitting corrosion forms on passive alloys like stainless steel when the thin oxide film is broken and does not immediately repassivate. Pits can become wide and shallow or may rapidly perforate the metal.

Pitting occurs in stainless steels in neutral or acid solutions containing halides, primarily chlorides (CI ), for example seawater. The attack most often takes place at points where the passive layer might be weakened, e.g. by slag inclusions or surface defects. Once the attack has started, the material may be completely penetrated within a short time.44 Prepared by Jinhee Lee, PE E-mail : [email protected] 45

An example of pitting due to chlorides of an iron-nickelchromium alloy pickling hook

Undercutting of the surface of a stainless steel by pitting



Pitting on Aluminum Alloy

Pitting Corrosion Material: Titanium. System: Effluent tank. Part: Heating coil. Phenomenon: Cavitation corrosion. Appearance: Pitting corrosion in the form of numerous small cavities, the pipe being perforated at various places. Time to Failure: 8 months.

Oxygen Pitted Boiler Feed Water Pipe, 304SS

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Pitting Corrosion Material: Aluminium. System: Air conditioning plant. Part: Part of an aluminium air duct. Phenomenon: Salt corrosion. Appearance: Pitting attack. Time to Failure: Unknown.

Effect of Temperature and PRE on Pitting

Critical temperatures evaluated in 6% ferric chloride per ASTM G48 and in 1M NaCl.

Critical pitting and crevice corrosion temperatures for austenitic stainless steel related to PRE numbers.48 Prepared by Jinhee Lee, PE E-mail : [email protected] 49 Prepared by Jinhee Lee, PE E-mail : [email protected]

PRE-VALUESThe chromium content of stainless steel grades is important and alloying with molybdenum and nitrogen has proved very beneficial for the pitting resistance. From experimental data, relations between elemental composition and pitting resistance have been developed.

Crevice Corrosion ()

PRE = %Cr + 3.3% Mo + 16%NUNS S30400 S30403 S32100 S316001 S316032 S316031 S316353 S316534 S3170350

PRE 19 19 18 26 24 26 24 29 29

Sandvik 2RK65 Sanicro 28 254 SMO 2RE10 2RE69 Sanicro 41 SAF 2304 SAF 2205 SAF 2507Prepared by Jinhee Lee, PE

PRE 35 39 > 41 25 34 31 25 35 > 41E-mail : [email protected] 51 Prepared by Jinhee Lee, PE E-mail : [email protected]

Schematic diagram of crevice corrosion.

Filiform crevice corrosion of aluminum under paint, Metals Handbook, Vol 13, Corrosion, 9th edition, p. 1034, 1987. Used by Permission, ASM-International.

Crevice corrosion is most common in chloride-bearing solutions, such as seawater. In seawater coolers, therefore, the water should flow inside the tubes (as design crevices are more easily avoided there) rather than on the shell side. The risk is greatest in stagnant solutions. At flow rates over 1.5 m/s the risk decreases since there will be no deposit formation and build-up of an aggressive environment. If required, the water should be decalcified. Otherwise, the tubes should be cleaned regularly. Stainless steel should normally not be painted, because a crevice will result if the paint is damaged. Crevice corrosion often occurs at lower temperatures and at lower chloride concentrations than for pitting corrosion. Up to a certain limit, the risk for attack increases the more narrow the crevice is.52 Prepared by Jinhee Lee, PE E-mail : [email protected] 53

Preference site for Crevice Corrosion



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Mechanism of Crevice Corrosion

Crevice Corrosion of Type 304 Fastener in Marine Tide After 6 Months

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Crevice Corrosion on a Flange Gasket Contact Surface

Crevice Corrosion Material: Stainless steel (AISI 316). System: Hot tapwater system. Part: Pipe sections. Phenomenon: Weld defect. Appearance: Pitting attack in the welds. Time to Failure: 5 years

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Corrosion of steel as a function of water pH

Corrosion of Steel vs. pHIn the range of pH 4 to pH 10, the corrosion rate of iron is relatively independent of the pH of the environment. In this pH range, the corrosion rate is governed largely by the rate at which oxygen reacts with absorbed atomic hydrogen, thereby depolarizing the surface and allowing the reduction reaction to continue. For pH values below 4.0, ferrous oxide (FeO) is soluble. Thus, the oxide dissolves as it is formed rather than depositing on the metal surface to form a film. It is also observed that hydrogen is produced in acid solutions below a pH of 4, indicating that the corrosion rate no longer depends entirely on depolarization by oxygen, but on a combination of the two factors (hydrogen evolution and depolarization). For pH values above about pH 10, the corrosion rate is observed to fall as pH is increased. This is believed to be due to an increase in the rate of the reaction of oxygen with Fe(OH)2 (hydrated FeO) in the oxide layer to form the more protective Fe2O3 (note that this effect is not observed in deaerated water at high temperatures).

Condition : exposure of iron to aerated water at room temperature58 Prepared by Jinhee Lee, PE E-mail : [email protected] 59 Prepared by Jinhee Lee, PE E-mail : [email protected]

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Effect of Temperature and Chloride on Initiation of Crevice Corrosion

Critical Crevice Temperature for 6% Mo Alloys in FeCl3 ASTM G 48

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Evaluation of CCT, CPTEvaluation of crevice corrosion resistance may be done according to the ASTM G48B test. As in the pitting test (practice A) samples are immersed in a 6% FeCl3 solution. The critical temperature for crevice corrosion, CCT, may be determined in the same way as for CPT. Where the temperature is increased by 2.5C.

Susceptibility of various alloys to pitting and crevice corrosion in an acid brine media

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Critical crevice & pitting corrosion temperature

Nickel Alloys PRE Vs Crevice Corrosion in Seawater

Pitting Resistance Index, PRE PRE = %Cr + 3.3(%Mo + 0.5%W) + 30%N64 Prepared by Jinhee Lee, PE E-mail : [email protected] 65 Prepared by Jinhee Lee, PE E-mail : [email protected]

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Corrosion of 90Cu-10Ni in Seawater

Pillowing of a lap jointAn advanced form of crevice corrosion is called pillowing. Notice how the rivet heads appear to be lower than the surrounding skin surface.

Corrosion processes and the subsequent build-up of voluminous corrosion products inside the lap joints lead to so-called "pillowing", whereby the faying surfaces are separated.The structural failure on April 28, 1988 of a 19 year old Boeing 737, operated by Aloha airlines,66 Prepared by Jinhee Lee, PE E-mail : [email protected] 67 Prepared by Jinhee Lee, PE E-mail : [email protected]

Aloha Airline Disaster

Filiform Corrosion

The crawling under paint corrosion called 'filiform'

Aloha Airlines, 4/28/1988. The aircraft lost 1/3 of its roof due to a stress fracture while cruising at 24,000 feet.68 Prepared by Jinhee Lee, PE E-mail : [email protected]

A special form of crevice corrosion in which the aggressive chemistry build-up occurs under a protective film that has been breached. This type of corrosion occurs under painted or plated surfaces when moisture permeates the coating. Lacquers and "quick-dry" paints are most susceptible to the problem.69 Prepared by Jinhee Lee, PE E-mail : [email protected]

ASTM G48-Standard Guide for Crevice Corrosion Testing

Crevices in S30400 stainless steel... after 30 days in 0.5 FeCl3 + 0.05 M NaClThe number of sites showing attack in a given time can be related to the resistance of a material to initiation of localized corrosion, and the average or maximum depth of attack can be related to the rate of propagation. The large number of sites in duplicate or triplicate specimens is amenable to probabilistic evaluation71

washers make a number of contact sites

In this test, washers make a number of contact sites on either side of the specimens. The number of sites showing attack in a given time can be related to the resistance of a material to initiation of localized corrosion, and the average or maximum depth of attack can be related to the rate of propagation.70 Prepared by Jinhee Lee, PE E-mail : [email protected]

ASTM G 78 testE-mail : [email protected]


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Dealloying, Selective Leaching

Selective Leaching Selective leaching, also called Dealloying, is the removal of one element from a solid alloy by corrosion processes. The most common example is dezincification: the selective removal of zinc in brass alloys. Aluminum alloying constituents are removed in dealuminification. Another example is "graphitic corrosion" of (gray) cast iron, in which the metallic constituents are selectively leached or converted to corrosion products leaving the graphite intact.73 Prepared by Jinhee Lee, PE E-mail : [email protected]

Microscopic view of typical dealloying. (The metal left as small islands is rich in the more noble metal.)72

Microscopic dealloying in a brass screw thread. Metals Handbook, Vol 13, Corrosion, 9th edition, p. 222, 1987.Prepared by Jinhee Lee, PE E-mail : [email protected]

Combinations of alloys and environments subject to dealloying and elements preferentially removedAlloyBrasses Grey iron Aluminium bronzes Silicon bronzes Tin bronzes Copper-nickels Copper-gold single crystals Monels Gold alloys with copper or silver High-nickel alloys Medium- and highcarbon steels Iron-chromium alloys Nickel-molybdenum alloys74

Selective Leaching

EnvironmentMany waters, especially under stagnant conditions Soils, many waters HCl, acids containing Chloride High-temperature steam and acidic species Hot brine or steam High heat flux and low water velocity (in refinery condenser tubes) Ferric chloride Hydrofluoric and other acids Sulfide solutions, human saliva Molten salts Oxidizing atmospheres, hydrogen at high temperatures High-temperature oxidizing atmospheres Oxygen at high temperature

Element removedZn (dezincification) Fe (graphitic corrosion) Al (dealuminification) Si (desiliconification) Sn (destannification) Ni (denickelification) Cu Cu in some acids, and Ni in others Cu, Ag, Cr, Fe, Mo and T C (decarburization) Cr, which forms a protective film Mo

A classical example of selective leaching or de-alloying of yellow brass where the zinc from the nut was dissolved and left only the spongy layer of copper.75 Prepared by Jinhee Lee, PE E-mail : [email protected]



DezincificationDezincification selectively removes zinc from the alloy, leaving behind a porous, copper-rich structure that has little mechanical strengthLayer type High brass (Zn rich alloy) / acid solution

The service conditions generally present where dezincification occurs include Water with high levels of oxygen and carbon dioxide (uniform attack). Stagnant or slow moving waters (uniform attack). Slightly acidic water, low in salt content and at room temperature (uniform attack). Soft, low pH and low mineral water combined with oxygen, which forms zinc oxide (uniform attack). Waters with high chloride ion content (uniform attack). Neutral or alkaline waters, high in salt content and at or above room temperature (plug-type attack).77 Prepared by Jinhee Lee, PE E-mail : [email protected]

Plug Type Low brass (Zn lean alloy) / weak acid, neutral, alkali solutionCopper Red

Brass76 Prepared by Jinhee Lee, PE E-mail : [email protected]

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Dezincification Material: Aluminium-zinc alloy (5 to 6% zinc). System: Glasshouse irrigation system. Part: Water distributor pipes. Phenomenon: Selective leaching (dezincification). Appearance: Severe uniform attack with pitting to the point of leakage. Time to Failure: 6 months.78 Prepared by Jinhee Lee, PE E-mail : [email protected]

Why Dezincification Occurs Copper-zinc alloys containing more than 15% zinc are susceptible to dezincification. During dezincification, the more active zinc is selectively removed from the brass, leaving behind a weak deposit of the porous, more noble copper-rich metal. Conditions favoring dezincification are contact with slightly acid or alkaline water..79 Prepared by Jinhee Lee, PE E-mail : [email protected]

The service conditions generally present where dezincification occurs include Water with high levels of oxygen and carbon dioxide (uniform attack). Stagnant or slow moving waters (uniform attack). Slightly acidic water, low in salt content and at room temperature (uniform attack). Soft, low pH and low mineral water combined with oxygen, which forms zinc oxide (uniform attack). Waters with high chloride ion content (uniform attack). Neutral or alkaline waters, high in salt content and at or above room temperature (plug-type attack). Relatively high tube-wall temperatures Permeable deposits or coatings over the tube surface80 Prepared by Jinhee Lee, PE E-mail : [email protected] 81

Prevention of Dezincification Make environment less aggressive (e.g., reduce O2 content) Cathodically protect Use a better alloy (common cure - above not usually feasible). Red Brass (> 41

-43140 Prepared by Jinhee Lee, PE E-mail : [email protected] 141

Material: Carbon steel (35.8). System: Vertical water-tube boiler (7.1 MPa). Part: Water tube. Phenomenon: Creep rupture caused by overheating. Appearance: Fractured water tube. Time to Failure: Two years.Prepared by Jinhee Lee, PE E-mail : [email protected]

Comparison of creep and stress rupture testsCreep Test Measures strain versus time at constant temperature and load or stress. Relatively low loads and creep rates. Long duration, 2,000 to 10,000 hours. Not always to fracture. Strain measured accurately using sensitive equipment (inductance gauges) to determine creep rate. Strains typically less than 0.5%142

Temper Embrittlement Mainly Cr-Mo low alloy steel susceptible Long time service within 370 ~ 590 Impurities precipitation in grain boundary become brittle (No other mechanical properties change excepttoughness value)

Stress Rupture Test Measures stress versus time to rupture at constant temperature. Higher loads and creep rates. Shorter duration, less than 1,000 hours typically. Always to fracture. Simper less sensitive strain measuring equipment (dial gauges). Time and strain to fracture measured. Strain typically up to 50%143

Sensitivity to T.E1/2 Mo steel : not sensitive 1 Cr - 1/2Mo : not sensitive 1 1/4Cr - 1/2Mo : a little sensitive 2 1/4Cr - 1 Mo and 3 Cr - 1 Mo : very sensitive 5 Cr - 1/2 Mo : a little sensitive 9 Cr - 1 Mo : not sensitivePrepared by Jinhee Lee, PE E-mail : [email protected]



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Temper EmbrittlementTemper embrittlement refers to the decrease in notch toughness of Cr-Mo low alloy steels when heated in, or cooled slowly through, a temperature range of 400C to 600C. Watanabe J factor for base and weld metalJ=(Mn + Si)(P + Sn) x104 (in wt %) < 150 (for 1Cr), 100 (for 2Cr)

Transition Temperature Charpy Impact TestPlotting impact test value as test temperature Show S shape for carbon and alloy steel

Bruscato X factor for weld metalX=(10P + 5Sb + 4Sn + As)/100 (in ppmw) < 20

DBTT(ductile brittle transition temperature)Temperature showing 50% brittle and 50% ductile fracture

Sugiyama equation for weld metalPE = C + Mn + Mo + Cr/3 + Si/4 + 3.5 (10P + 5Sb + 4Sn + As) < 2.8 ~ 3.0 for coarse grain weld metal

15ft-lbs TT (15ft-lbs transition temperature)Generally 15ft-lbs considered as enough value to resist brittle fracture and this is a temperature showing 15ft-lbs of impact value145 Prepared by Jinhee Lee, PE E-mail : [email protected]

Material will not suffer a temper embrittlement in serviceAF + 2.5(SC - AF) < 38C where AF=As formed Charpy 54J temperature SC=Step cooled Charpy 54J temperature144 Prepared by Jinhee Lee, PE E-mail : [email protected]

Temper Embrittlement Criteria of brittleness 15ft-lbs (20joules) of impact energy FATT (Fracture appearance transition Temperature)

Charpy V-notch energy absorption curve for a typical carbon steel

Degree of embrittlement TT54 (54joules transition temperature) TT54 + XTT54 24 (X : correction factor : 2.5~3.5, depends on specification. TT54 = (TT54[step cooled]-TT54) FATT (Fracture appearance transition Temperature)146 Prepared by Jinhee Lee, PE E-mail : [email protected] 147

Lower shelf 15ft-lbs TT

Upper shelf : 100% ductile fracture Lower shelf : 100% brittle fracturePrepared by Jinhee Lee, PE E-mail : [email protected]

Effect of Temper Embrittlement on Impact Characteristics

Temper Embrittlement Susceptibility of Various Cr-Mo Steels

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MPT for High Pressure Equipment MPT (Minimum Pressuring Temperature) Temperature at which equipment can withstand 40% of MAWP for Div.1 equipment, 25% of MAWP for Div.2 equipment (according to ASME)

Define MPT MPT generally suggested by the equipment manufacturer from experimental data but in general as follows For 2.25Cr - 1Mo steel : MPT FATT For 1.25Cr-.5Mo, 1Cr-.5Mo, .5Mo FATT+20

Material is very susceptible to brittle fracture below MPT MPT is related to S/up and S/D at cold temp. Consider MPT for hydrostatic test150 151





Biocorrosion (Bacteria-Assisted Corrosion)tubercule formation

Microbiological CorrosionMIC will actively or passively attack metal. Active MIC Byproducts of Active MIC are acids and/or ammonia. Active MIC is localized and normally the point in which pinhole leaks are formed. Passive MIC Passive MIC is normally produced by the biological waste products forming a gel like mass. This mass can extend over large areas, which will accumulate additional deposits. These deposits will cause flow obstruction within fire sprinkler systems. Passive MIC will attack large areas and accounts for the greatest metal loss.153 Prepared by Jinhee Lee, PE E-mail : [email protected]

metalSchematic diagram of a typical biologically assisted corrosion feature (tubercule). Macroscopic view of a piece of tube with tubercules that are caused by bacterial activity. W. P. Iverson, ASTM STP 741 p. 40, 1981.

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Bacteria related to MIC Acid Producing Fungi Aerobic Slime Formers Iron/Manganese Oxidizing Bacteria Methane Producers Organic Acid Producing Bacteria Sulfate reducing bacteria (SRB) Sulfur/Sulfide Oxidizing Bacteria154 Prepared by Jinhee Lee, PE E-mail : [email protected] 155

Microbiologically Influenced Corrosion (MIC)There are three major types of bacteria which can cause corrosionSpecies Oxygen Desulfovibrio No Thiobacillus Gallionella Yes Yes Metals Fe, Al, Cu Fe, Cu Fe Corrosive Sulphide Sulphuric Acid Fe++ to Fe+++ Mn++ to Mn+++



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Bacteria Effects - MIC

Bacteria Effect - MIC

Before CleaningThis is an example of a failure of a type 304 stainless steel pipe due to MIC caused by Gallionella bacteria in plant waste water service. Note the tuberculation, that is, the mound of corrosion products from the bacteria, and the crevice, located on the weld, shown here by an arrow. Leaks at welds were observed after only 6 months.156 Prepared by Jinhee Lee, PE E-mail : [email protected] 157

After Cleaning No Attack

Severe corrosion occurred underneath the bacteria colony. The 6 mm (1/4 inch) thick walled piping is perforated.Prepared by Jinhee Lee, PE E-mail : [email protected]

Prevention of MIC in Water Keep The System Clean Keep Water Flow > 2 m/s (6fps) Use Bactericide : Chlorine Chlorine Dioxide Hyperchloride Ozone Non-oxidizing158 Prepared by Jinhee Lee, PE E-mail : [email protected] 159

Prevention of MIC in Water Remove Heat Tints After Welding Drain and Dry Equipment After Hydrotesting Use Continuous Cleaning Use High Pressure Hydrolancing Use Stainless Steel Scrapers (Hard to Remove of Heavy Deposit) Use Alloy Resistant to MICPrepared by Jinhee Lee, PE E-mail : [email protected]

High Temperature Corrosion Types of high-temperature corrosion Oxidation, Sulfidation Carburization, Decarburization, Nitriding Hydrogen attack, Halide attack

Corrosive High Temperature Environment Oxidation implies oxides Sulfidation implies sulfides, Sulfidation/oxidation implies sulfides plus oxides, and Carburization implies carbides.

More Peculiar Corrosion Phenomena 160

Metal Dusting Hot Corrosion Green Rot Black Plague161





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Corrosive high temperature environments Corrosive gases Air, oxygen : oxidation Steam : oxidize the iron Carbon, carbon oxides and methane : Carburization Sulfur containing Gases : Sulfidation Hydrogen : C(Fe) + 2H2 CH4 Nitrogen : form nitrides below 540oC Combustion gases Chlorine and hydrogen chloride162 Prepared by Jinhee Lee, PE E-mail : [email protected]

Corrosive high temperature environments Ash: Vanadium pentoxide and sodium sulfate present in fuel ash can attack stainless steels due to the formation of a molten oxide phase that fluxes the protective oxide scale. Molten salts: Molten salts generally act as fluxes, removing possible protective scales of corrosion products. Combined with the high temperature and the high ionic conductivity of molten salts this will result in high corrosion rates. Molten metals: Contact between molten metals and condensed metals frequently results in severe corrosion attack, which in some cases can be due to temperature differences or concentration gradients in the system used.163 Prepared by Jinhee Lee, PE E-mail : [email protected]

High Temperature Oxidationhigh temperature corrosive environment

Six types of oxidation phenomena At low temperature, diffusion of oxygen and metal species through a compact oxide film At moderate and high temperatures, a combination of oxide film formation and oxide volatility At moderate and high temperatures, the formation of volatile metal and oxide species at the metal-oxide interface and transport through the oxide lattice and mechanically formed cracks in the oxide layer At moderate and high temperatures, the direct formation of volatile oxide gases At high temperature, the gaseous diffusion of oxygen through a barrier layer of volatilized oxides At high temperature, spalling of metal and oxide particles.165 Prepared by Jinhee Lee, PE E-mail : [email protected]

fully oxidized metal layer partially oxidized metal pure metal

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Scaling Temperatures in AirAlloy 0.1% Carbon steel 5% Cr - 0.5% Mo 7% Cr - 0.3% Mo 9% Cr - 1% Mo 12% Cr martensitic 17% Cr ferritic 21% Cr ferritic 25% Cr ferritic 18-8 CrNi stabilized 18-8 18-8 CrNi + Mo 23-13 CrNi 25-20 CrNi 70-28 NiMo 59%Ni-16%Cr-16%Mo166

Volume Ratio (P-B Ratio) For an oxide to be protective the oxide-metal volume ratio must be somewhat larger than unity. Volume ration < 1, the oxide does not totally cover the metal surface, permitting free access of the corrosive atmosphere to the metal on certain locations. Volume ratio is > 2, compressive stresses will develop in the oxide layer, which could result in spalling of the oxide and in exposure of the metal to the corrosive atmosphere.

Matl Code 1010 430, 410 430 442 446 302, 304 321, 347 316 309 310 Alloy B Alloy C

Temperature C F 480 900 620 1150 650 1200 675 1250 705 1300 845 1550 955 1750 1095 2000 900 1650 900 1650 900 1650 1095 2000 1150 2100 760 1400 1150 2100E-mail : [email protected] 167

This rule is called the Pilling-Bedworth theory




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Pilling and Bedworth RatioProtective oxides Be 1.59 Cu 1.68 Al 1.28 Cr 1.99 Mn 1.79 Fe 1.77 Co 1.99 Ni 1.52 Pd 1.60 Pb 1.40 Ce 1.16168

Weathering Steel

Non protective oxides K 0.45 Ag 1.59 Cd 1.21 Ti 1.95 Mo 3.40 Hf 2.61 Sb 2.35 W 3.40 Ta 2.33 U 3.05 V 3.18169

Gugenheim Museum, Nevada

P-B Ratio : 1 1 Prepared by Jinhee Lee, PE E-mail : [email protected] Prepared by Jinhee Lee, PE E-mail : [email protected]

Weathering SteelLow Carbon : 0.21% with Cu, Mo, Cr, P, Al Moisture & Air is essential to have protective hard scale Do not use costal environment

Oxidation Resistance vs. Cr Content

170



171



Damaged Surface of High Temperature Corrosion

Carburization Carburization is the increase of the carbon content of (the surface of) a steel due to interactions with the environment at elevated temperatures. Results in the formation of a very hard top layer that is more brittle than the material at the core. Carburization can have an influence on the corrosion behavior as well, as carbon can form carbides (like Cr23C6, Cr3C 2 or Cr7C3 ), depleting the metal matrix locally of chromium and making it more sensitive to corrosion.

172



173



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Carburization: Prevention Minor alloying elements can exert an influence on the susceptibility to carburization of various alloys. Good : Si(1.5 to 2.0%), Nb, W, Ti, Ni and the rare earths Bad : Pb, Mo, Co, Zr and B

Carburization of Modified HK-40 Furnace Tubing

Because carburization is based on carbon transport across the metal/gas interface, preoxidation and the subsequent formation of an oxide film will increase the resistance against decarburization. Austenitic steels carburize more readily than ferritic steels because of the high solubility of carbon in austenite174 Prepared by Jinhee Lee, PE E-mail : [email protected] 175 Prepared by Jinhee Lee, PE E-mail : [email protected]

Decarburization Decarburization is the decrease of the carbon content of (the surface of) a steel due to interactions with the environment at elevated temperatures. The decreasing carbon content causes a degradation of mechanical properties, as the hardness as well as the strength decrease. However, the elongation of the metal when subjected to a tensile stress increases. Any compound that can influence the carbon level of the steel will influence the mechanical properties of the steel.176 Prepared by Jinhee Lee, PE E-mail : [email protected] 177

Decarburization by Hydrogen Surface decarburizationSurface decarburization is the formation of hydrocarbons at the metal surface which causes a migration of carbon atoms to the surface. Due to surface decarburization the hardness, room temperature strength and creep strength of a steel decrease, whereas the ductility increases. Surface decarburization is accelerated by moisture.

Internal decarburizationHydrogen permeated into the steel can react with carbon, resulting in the formation of methane which will accumulate in voids in the metal matrix. The gas pressure in these voids can generate an internal stress high enough to fissure, crack or blister the steel (hydrogen attack). Due to internal decarburization the tensile strength and the ductility will drop dramatically.Prepared by Jinhee Lee, PE E-mail : [email protected]

Hydrogen partial pressure, MPa1500 1400 3.45 6.90 10.34 13.79 17.24 20.7 34.5 48.3 62.1 75.8 800

NitridingTemperature, oC

Surface decarburization Internal decarburization (hydrogen attack)6Cr-0.5Mo 1.25Cr-0.5Mo 3Cr-0.5Mo 1Cr-0.5Mo 2.25Cr-1Mo 2Cr-0.5Mo 1.25Cr-0.5Mo 1Cr-0.5Mo Carbon Steel 200 0 500 1000 1500 2000 2500 3000 7000 11000 400 500 700

Temperature, oF

1300 1200 1100 1000 900 800 700 600 500 400 300

Beneficial Effect of Nitriding Obtain high surface hardness Increase wear resistance Improve fatigue life Improve corrosion resistance (except for stainless steels) Obtain a surface that is resistant to the softening effect of heat at temperatures up to the nitriding temperature179 Prepared by Jinhee Lee, PE E-mail : [email protected]

600

300

Hydrogen partial pressure, psi

Nelson Chart : Material Selection for Hydrogen Service178 Prepared by Jinhee Lee, PE E-mail : [email protected]

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Nitriding Active nitrogen produced from ammonia can form nitrides below 540C, resulting in a decrease of the corrosion resistance in aqueous solutions.

Metal Dusting Metal dusting is a form of rapid localized degradation that occurs in environments containing carbon and hydrogen compounds, but almost no oxygen. Due to carburization of the metal iron carbides can form which ultimately, if enough carbon is available, can decompose into iron and graphite. These products can act as catalysts for the decomposition of carbon monoxide into carbon and oxygen, resulting in localized accelerated attack and the production of voluminous amounts of carbon, iron, iron carbides and other carbides.181 Prepared by Jinhee Lee, PE E-mail : [email protected]

Although copper and nickel show good resistance against nitriding, the relative poor oxidation resistance of these metals at high temperatures and the susceptibility to sulfur attack does not make them a good choice. Normally a high nickel chromium alloy is used as a construction material; e.g., 18-8 CrNi, 25-20 CrNi or Inconel.180 Prepared by Jinhee Lee, PE E-mail : [email protected]

Metal Dusting - Prevention All metals that can form stable carbides are, in principle, susceptible to metal dusting. Metal dusting can be prevented by building and maintaining a stable, continuous oxide layer, which can be realized by pre-oxidation and periodic steam injection. It has been suggested that resistant materials should contain enough chromium and silicon to obey the following rule:%Cr + 2 x %Si > 24%182 Prepared by Jinhee Lee, PE E-mail : [email protected] 183

Stress Corrosion Crackingstress + mildly corrosive environment

metalMicroscopic cross-section of a typical stress corrosion crack in metal. Microscopic cross-section of a stress corrosion crack in metal. ASM Handbook, Vol. 13, Corrosion, p. 354.



Combinations of some alloys and environments that have been shown to promote stress corrosion crackingMaterial Al alloys Mg alloys Cu alloys C steels Austenitic steels High strength steels Ni alloys Ti alloys184

Environments Chlorides, moist air Chloride-chromate mixtures, moist air Nitric acid, fluorides. Sodium hydroxide Ammonia, moist air, moist sulfur dioxide Nitrates, hydroxides, carbonates Anhydrous ammonia Chlorides, sulfur acid Moist air, water, chlorides, sulfates, sulfides Hydroxides Halides, methanolPrepared by Jinhee Lee, PE E-mail : [email protected] 185

Factors Favoring SSC-Stainless Steels Temperature < 55 Chloride Oxygen Evaporative Condition Pit Stress Riser Residual or Applied StressPrepared by Jinhee Lee, PE E-mail : [email protected]

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6. REFERENCES

Prevention of SCC Select suitably resistant materials of construction. Keep stress levels as low as possible (stress-relief where possible). Shot peening may eliminate (residual) surface tensile stresses. Be aware of all of the constituents in the service environment. SCC problems are often caused by trace impurities ! Avoid designs that allow stagnant regions where impurities can concentrate or deposit. Use lower operating temperatures if possible. Apply cathodic protection (for chloride-induced SCC only !). Use inhibitors.186 Prepared by Jinhee Lee, PE E-mail : [email protected]

References List1. ASM Handbook Vol 13. Corrosion 2. ASM Handbook Vol 20. Materials Selection and Design 3. Metal Corrosion Damage and Protection Technology - Masaminchi Kowaka 4. Corrosion Control in the Chemical Process Industries - C.P. Dillon 5. Stress Corrosion Cracking : Material Performance and Evaluation - Russell H. Jones 6. Advances in Corrosion Control and Materials in Oil and Gas Production - P.S. Jackman and L.M. Smith 7. Corrosion Atlas - E.D. During 8. Corrosion Engineering Handbook - Philip A. Schweitzer, P.E. 9. Corrosion Engineering - M.G. Fontana 10. Corrosion of Austenitic Stainless Steels : Mechanism, Mitigation, and Monitoring - H. S. Khatak and B. Raj 11. Technical Report Published by NiDi (Nickel Distributes Institute) 12. Technical Report Published by NACE (National Association of Corrosion Engineer) 13. H2S Corrosion in Oil and Gas Production : A Compilation of Classic Papers - R.N. Tuttle and R.D. Kane 14. Materials Selection for Petroleum Refineries and Gathering Facilities - R.A. White 15. Corrosion Prevention Manual - Chevron 16. Metallic Material Engineering - UOP 17. Microbiologically Influenced Corrosion Handbook - S.W. Borenstein 18. Pipeline Risk Management Manual, Third Edition - W. Kent Muhlbauer 19. Materials Selection for Hydrocarbon and Chemical Plants - David A. Hansen and Robert B. Puyear 20. International Code (NACE, API, etc.)370



Q&A

Jinhee, Lee. PE Jinhee, Lee. PE Jinhee, Material & Corrosion Specialist Material & Corrosion Specialist [email protected] [email protected] [email protected] Prepared by Jinhee Lee, PE E-mail : [email protected]

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fundamentals of corrosion

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