Corrosion101slidesJan2005[1].pdf

CORROSION CORROSION 101101

Why we are here• To discuss the problems of corrosion and learn about

corrosion control• To meet others and share thoughts, knowledge and

concerns• To see how to avoid failures, excessive costs and

other problems through corrosion control

Outline• Introduction–economics• Applications to various industries• Corrosion –what is it and what causes it

• Examples of corrosion problems

Outline• Methods of Corrosion Control

– Materials Selection– Coatings and Linings– Cathodic Protection– Inhibitors– Engineering Design– Monitoring and Maintenance

• NACE Resources

Introduction

• Problems in Recognizing Corrosion• Costs of Corrosion• Economics of Corrosion Control• Safety• Environmental Factors • Value of Corrosion Control

Problems In Recognizing Corrosion

• Not mobilized by public opinion• Not recognized as an engineering discipline• Not recognized as controllable

Cost of CorrosionTotal Direct Cost of Corrosion in the

U.S.B$276 / year = 3.1% of GDP

Source: Corrosion Cost and Preventative Strategies in the United States, September 2001, Report FHWA-RD-01-156

Summary of Costs per SectorCost of Corrosion per Analyzed Economic Sector, ($x billion)

$8.3$7.0

$0.3$7.0

$-$-

$5.0$36.0

$6.9$-

$23.4$2.7

$2.2$0.5$0.9$1.4

$0.1$3.7

$1.7$6.0

$1.1$2.1

$-$1.5

$20.0$0.1

1.00

Highway BridgesGas and Liquid Transm. PipelinesWaterways and PortsHazardous Materials StorageAirportsRailroadsGas DistributionDrinking Water and Sewer SystemsElectrical UtilitiesTelecommunicationMotor VehiclesShipsAircraftRailroad CarsHazardous Materials TransportOil and Gas Expl.and ProductionMiningPetroleum RefiningChem., Petrochem., Pharm.Pulp and PaperAgriculturalFood ProcessingElectronicsHome AppliancesDefenseNuclear Waste Storage

How much money is lost to corrosion annually in the USA?

It costs U.S. households, businesses, and government agencies more than $300 billion each year.

That’s $1,100 for every man, woman, and child in this country.

(These are direct costs but there are also indirect costs of corrosion.)

Direct Corrosion CostsExcessive Maintenance / Repair / ReplacementLost Production / DowntimeProduct ContaminationLoss of ProductLoss of Efficiency (Oversizing & Excess)Energy CostsAccidentsIncreased Capital Costs – OverdesignEnvironmental Cleanup – Fines

Indirect Corrosion CostsSafetyStructural CollapseLeaksFire / Toxic ReleasesProduct ContaminationFoods / Pharmaceuticals WaterConsumer ConfidenceLoss of RedundancyAppearance Increased Regulation

Costs are about 5% of U.S. GNP

• Costs add up to about $2,000 per person per year– $500 Billion in the USA– $58 Billion in Canada

• About 35% of this could be saved through better corrosion control

• % of GNP is about the same in other developed countries

Corrosion Affects You!

• Your Environment

• Your Company

• Your Job

• Your Safety

• Your Pocketbook

Economics of Corrosion Control

• Avoid excessive maintenance costs• Avoid unexpected shutdowns• Avoid financial losses and lawsuits

Economic Analyses• Long range costs of project vs. initial cost• Maintenance costs (operations) vs. capital

investment (new construction)• Return on investment is usually ten times the cost

Safety

• Safety is everybody’s business• Fire and explosions• Bridge collapses• Falls from corroded ladders or gratings• Machine parts• Automobile and aircraft parts

– arouses public opinion

Safety (cont)

• Loss of electrical grounding due to corrosion of connections

• Overheating of electrical contacts due to corrosion (fire hazard)

• Collapse of reinforced parking deck• Falling of building decorations and facia• Loss of fire protection from corroded piping

Environmental Factors• Leakage• Pollution of water and earth• Atmospheric corrosion and deterioration from acid

rain• Preservation of resources

Value of Corrosion Control• Preservation of materials and products• Gain desired product life• Avoid safety and environmental problems• Avoid financial losses• Avoid inconvenience to customers• Avoid shut downs and loss of production• Economic advantages of control over losses

Application to Various Industries


• Pipelines• Underground Storage

Tanks• Industrial Plants• Power Generation• Transportation

• Marine • Waterworks• Petrochemical Plants• Pulp and Paper• Real Estate• Private Homes


• Pipelines - transmission and distribution– Leak prevention– OPS rules (USA)– NEB rules (Canada)


• Underground storage tanks– Pollution control– EPA and NEB rules


• Industrial plants and manufacturing– Underground structures– Mechanical equipment– Process equipment– Fire protection, heating and cooling systems– Product liability – Customer satisfaction


• Real Estate– Plumbing systems– Mechanical equipment– Air conditioning


• Transportation– Safety and performance of vehicles, aircraft, etc.– Pipelines, trains, etc.


• Power generation– Boilers– Condensers and heat exchangers– Underground structures– Nuclear reactors


• Marine– Docks and wharfs

• saltwater• freshwater

– Ships– Offshore platforms and structures

• Environment• Safety


• Water works– Transmission and distribution piping– Water storage tanks– Water and waste water treatment plants– Sewers


• Chemical and petrochemical– Refineries– Marketing terminals– Aboveground Storage Tanks– Underground piping and Tanks– Product handling equipment– Safety of carriers - trucks, rail, ships


• Pulp and paper– Reactors, clarifiers, etc.– Paper making machinery


• Oil and gas• Production wells• Crude oil equipment• Offshore platforms• Treaters/Process Vessels• Storage Tanks


• Private homes– Water heating tanks– Metal siding– Aluminum screens, doors, etc.– Steel garage doors– Appliances– Burglar and fire alarms

What is Corrosion?

Corrosion• Definition:

The destruction of a substance, usually a metal, or its properties because of a reaction with its surroundings environment).– The natural tendency of a refined metal to return to its

original state as an ore

Elements of a Corrosion Cell

CathodeAnode

ElectrolyteMetal Path

An Electrochemical Cell Must Have:

• Anode - where oxidation occurs• Cathode - where reduction occurs• Electrolyte - where ion migration occurs• Metallic path - where electrons migrate from the

anode to the cathode

Are all of these Corrosion?

Single Corrosion Cell

e-e- e-

e-e-e-e-

e-

e- e-

Fe++ Fe++

Fe++

Fe++

OH-

OH-

OH-

Fe(OH)2

Fe(OH)2

Fe(OH)2H+

H+

H+

H+

H+

H+

H+HH

H

CATHODICSITE

ANODIC SITE

Fe++Fe++e-

Microscopic View

The Corrosion Cell• Metallic corrosion is an “electrochemical reaction”

which involves :– a transfer of electrons– oxidation - loss of electrons (corrosion)– reduction - gain of electrons (protection)– migration of ions

ee

Cu Fe

ee

ee

Metallic Path

Cathode Anode

Electrolyte

Fe++

H+

H+

H+OH—

H+OH—

Actual Case Showing Ion and Electron Flow

e eStructure (Metallic Path)

EFe Fe++

2H+ + 2e H2H2O H+(OH)—+2e

H+ H+Fe++

Electrolyte

Practical CaseShowing Conventional Current Flow

Corrosion Current Electrolyte

(soil)

Cathode

E

Anode

Structure (metallic path)

Electrochemical ReactionsAnode: (Oxidation)

Fe Fe++ + 2e—

Cathode : (reduction) 2H+ + 2e— H2

3O2 + 6H2O + 12e– 12OH—

O2 + 4H+ + 4e— 2H2O

ElectrolyteH2O H+ + OH—

Fe ++

Fe ++

Fe++

Fe ++

Fe ++

Fe ++Fe ++

Fe++

Fe++

e-

e - e -

e-

e-e-

e-

e-e-

e-

e-

e-e-e-

e-

e -e- e-

ANODE

Oxidation Reaction

ELECTROLYTE

Anodic Process

Reduction Reaction

H+

e-

e -

e-

e-

e -e-

e - e-

e-

e-

e-e-

e-

e-e-

e-

e-

CATHODE H

H+

H+

H+

H+

H+

HH

H+H+

H+H+

H+H+

H+

HH

H

ELECTROLYTE

Cathodic Process

Why Does Corrosion Occur?

Corrosion is driven by the voltage difference between irregularities on the surface of the structure.

Forms of Corrosion

Forms of Corrosion• General Corrosion• Localized Corrosion• Galvanic Corrosion• Environmental Corrosion Cracking• Flow-assisted Corrosion• Intergranular Corrosion• Dealloying• Fretting• High-temperature Corrosion

General CorrosionCorrosion that proceeds more of less uniformlyover the exposed surface without appreciablelocalization of attack.

General Attack Corrosion

GeneralCorrosion

General Corrosion• Dissimilar surfaces• Dissimilar soils/electrolytes/environments• Dissimilar oxygen concentration• Dissimilar metal ion concentration

General Corrosion due to Dissimilar Surfaces

• Mill Scale• New–Old Pipe• Stressed Surfaces• Welded Sections (Heat Affected Zone)

Corrosion Caused by Dissimilarity of Surface Conditions

Scratches Caused by Pipe Wrench(Anode)

Threads Bright Metal (Anode)

Pipe(Cathode)

New—Old Pipe Cell

Old Pipe (cathode)

New Pipe (anode)

Old Pipe (cathode)

Different Stress

The portion of the metal under higher stress, (eg. cold worked) will be anodic to the portion of the metal at a lower stress level.

– High Stress Anode– Low Stress Cathode

Stress Corrosion

Pipe

Lower Stressed Area (cathode)

Higher Stressed Area (anode)

Lower Stressed Area (cathode)

Coupling

General Corrosion due to Dissimilar Electrolytes

In most cases, the metal immersed in the moreconductive solution will be the anode, and the metal immersed in the less conductive solution will be thecathode.

– More Conductive Anode– Less Conductive Cathode

General Corrosion due to Oxygen Concentration

• The metal immersed in the oxygen lean electrolyte will be the anode, and the metal immersed in the oxygen rich electrolyte will be the cathode.

–Oxygen lean Anode–Oxygen rich Cathode

Dissimilar Metal Ion Concentration

• Cathode–in saturated solution• Anode –in unsaturated solution

Corrosion Caused by Differential Aeration of Soil

Poor or No Aeration (anode)

Aerated Soil

Structure

Localized Corrosion• Pitting

A deep, narrow corrosive attack in a localized region which often causes rapid penetration of the substrate thickness.

• Crevice CorrosionA form of localized attack in which the site of the attack is an area where free access to the surrounding environment is restricted..

• Filiform CorrosionA special form of oxygen cell corrosion occurring beneath organic or metallic coatings on materials. The attack results in a fine network of random “threads” of corrosion product developed beneath the coating material.

PittingSeveral pits that became interconnected and still partially covered by corrosion product.

Individual Pit

Crevice Corrosion Mechanisms

Corrosion Deep Inside Crevice

Oxygen Concentration Cell CorrosionAnodic area inside crevice

Crevice Corrosion

Filiform Corrosion

Galvanic Corrosion

Corrosion due to the potential differencebetween metals.

Galvanic Corrosion

Galvanic SeriesPotassiumMagnesiumBerylliumAluminumZincChromiumIronNickelTinCopperSilverPlatinumGold

MostEnergy

Required

LeastEnergy

Required

Graphite-Zinc Battery

Protective casing

Electrolyte paste(ammonium

chlorideand zinc chloride)

Zinc

Separator

Negative terminal

Pitch SealAir Space

Carbon and manganesedioxide mixture

Carbon rod

Water Heater

Copper Water Service

Wat

erG

as

Steel Gas Service (Anode)

Gas Meter with no or shorted insulation

Environmental Corrosion Cracking

• Stress Corrosion CrackingA brittle failure mode in an otherwise ductile metal, resulting from the combined action of tensile stress and a specific corrosive environment.

• Hydrogen-Induced Cracking (HIC) Results from the combined action of a tensile stress and hydrogen in metal.

• Sulfide Stress Cracking (SSC)A type of hydrogen-induced cracking in which sulfide is the primary poison for

hydrogen evolution. Liquid Metal Embrittlement (LME)A decrease in the strength or ductility of a metal or alloy as a result of content with a liquid metal.

• Corrosion FatigueResults from the combined action of a cyclic tensile stress and and a corrosive environment.

Stress Corrosion Cracking

Stress Corrosion Cracking

SCC of Underground P/L

Corrosion Fatigue

A smoothed area that appears as if it were

peened is the result of the opening and closing of the

crack on each cycle

A crystalline appearing area, where the crack progresses too

fast for the peening effect.

The sharp tip (on each piece) resulting when the rod’s cross section is too

small to carry the load and the metal fails as a pure tensile stress..

Flow-Assisted Corrosion• Erosion-corrosion

Occurs when the velocity of the fluid is sufficient to remove protective films from the metal surface.

• Impingement CorrosionLocalized erosion-corrosion caused by turbulence or impinging flow.

• Cavitation CorrosionMechanical damage process caused by collapsing bubbles in a flowing liquid.

Erosion Corrosion

Cylinder Lining Cavitation

Impingement

Intergranular Corrosion

Corrosion due to the preferential attack at, or adjacent to, the grain boundaries of a metal.

Intergranular Corrosion

Dealloying

The corrosion process, in which one constituentof an alloy is removed preferentially, leaving analtered residual structure.

Dealloying

Fretting CorrosionFretting corrosion is defined as metal deteriorationcaused by repetitive slip at the interface between twosurfaces in contact.

Fretting

Fretting

High-Temperature Corrosion• A form of material degradation that occurs at

elevated temperatures. • Direct chemical reactions, rather than the reactions of

an electrochemical cell, are responsible for the deterioration of the metals.

High-Temperature Corrosion

Stray Current Corrosion• The flow of current, along a path other than the

intended path.

• Discharge of current to the environment results in metal loss (corrosion).

Stray Current Corrosion• Concern for rapid corrosion because of large currents

– Dynamic• Rail Transit• Mine Railroads• Direct current arc welding• Telluric currents

– Dynamic - changes with time• eg. DC Powered Rail Transit

– When train travels near a foreign structure, the foreign structure can pick up direct current.

– This direct current has to discharge back to the power source.– At point of discharge, large current - rapid corrosion!

Fluctuating Current Source

+Overhead Positive Feeder

Load Current Required to Operate Train

Moving Current Pickup Area

Current Discharge Area Due to Corrosion

Current Flowing Around High Resistance or Insulating Joint

DCSubstation

_

Tracks Negative Return

Stray Current Corrosion• Concern for rapid corrosion because of large currents

– Steady state• Cathodic interference• Hvdc transmission

– Steady state• eg. Foreign structure pick-up of current from cathodic protection

system.• At point of discharge, concentrated large current - RAPID

CORROSION!

Cathodic Protection Stray Current(Interference)

Interference (corrosive) current flowing through the

earth to return toprotected line

Protective Current picked up by Foreign

Line

Foreign or Affected Line

Protected Line

Cathodic Protection Installation Protective Current

Examples of Corrosion Problems

Copper Piping• Natural protection from oxide film• Deposit corrosion

– If contaminants deposit onto the copper surface, differential aeration cells are set up, and pit the surface

• Low concentrations of CaCo3 can actually assist in resisting corrosion because of film.

Copper Pipe• Natural protection from oxide film• Deposit corrosion from oxygen• Cold water attack

– Well water– Low velocity– Low hardness– pH less than 8.5

Copper Pipe• Cold Water Attack

– L.S.I. slightly (+) to strongly (-)– High oxygen and carbon dioxide content– High sulfates, chlorides and dissolved solids

• Flux corrosion• Erosion corrosion• Lack of reaming tube ends• Turbulence at partially open valves• Velocity• Electrolysis

Galvanized and Black Steel• Galvanic action with connected copper pipe• Copper ion deposition from copper pipe in system

causes pitting• Oxygen/deposit corrosion - tuburculation• Corrosion accelerates at higher temperatures,

generally >160°F

Boilers and Heat Exchanger Applications

• Dissolved oxygen• Corrosion fatigue• Caustic embrittlement• Caustic gouging/crater

corrosion• Acid corrosion

• Chelate attack• Hydrogen cracking• Scaling• Lay up procedures• Firebox attack from

humidity

Boilers/Heat Exchangers• Dissolved gases cause problems, especially oxygen

(use of oxygen scavengers such as hydrazine)• Corrosion accelerated by corrosion fatigue at

elevated temperatures and pressures.

Boilers/Heat ExchangersCaustic embrittlement is the intercrystalline cracking ofsteel, caused by a specific concentration of stress, andfree NaOH in the boiler water.

Boilers/Heat Exchangers• Caustic gouging occurs under porous deposits.

Extensive, general and severe• Acidic water (low pH) extremely corrosive• Chealant attack concentrates on areas of stress in

the boiler, and occurs when excess concentration of sodium salt is maintained over a period of time

Boilers/Heat Exchangers• Hydrogen cracking

– lower end of pH operation– high pressure (>1800 psi)– hydrogen absorbs into steel– reacts with carbon– molecule large, steel ruptures

• Scaling creates problems in deposit corrosion, plugs, flow, etc.

Effect of pH on Corrosion• Acids• Alkalis

Effect of Low pH on Corrosion(example: Iron attack by hydrochloric acid)

In the electrolyteHCl = H+ + Cl–

Anodic area on iron:

Fe = Fe++ + 2eCathodic area:2H+ + 2e = H2

Effect of High pH on Corrosion• Alkalis: Strong bases that produce OH- ions in water

– Particular problems for metals are KOH, NaOH, and NH4 OH– Nickel and nickel alloys best– “Caustic embrittlement”

• Oxidizing Salts– Increase conductivity of electrolyte– Increase corrosion rate

Methods of Corrosion Control


Materials SelectionCoatings and LiningsCathodic ProtectionWater TreatmentDesign

Metallurgy and Metal Behavior

• Different metals corrode differently due to their metallurgy

• Some effects– Steel (pitting)– Gray cast iron (graphitization)– Ductile iron (usually pitting, some graphitization)

• Stainless steel– Passivity due to oxide film– Susceptible to corrosion in various environments– Need to select the proper stainless for the

environment

Metallurgy and Metal Behavior

• Alloys– Corrosion as a unit– Selective attack

• Weathering Steel– Depends on alternate wetting and drying to develop oxide

film– Susceptible to corrosion in chloride or polluted atmosphere – Underground - behaves as carbon steel

Stainless Steels• Martensitic

– 12% – 17% Cr– minor other elements– moderate corrosion resistance in mild environments

• Ferritic– higher Cr composition than Martensitic (12% – 30%)– better corrosion resistance to high temperatures

Stainless Steels (cont)• Austenitic

– Cr: 17% – 25%– Ni: 9% – 10%– highly resistant to wet oxidizing environments

• Duplex– mixture of ferritic and austenitic– increased strength and corrosion resistance

Non Metallics• Concrete

– Freeze thaw cycle– Rebar corrosion– Attack by de-icing salts and acids– Hydrogen sulfide attack in sewers

• Plastics– UV attack– Thermal stresses and cracking– Stress cracks from some thermal insulation’s – Solvent attack

Corrosion of Concrete

Corrosion of Concrete

Non Metallics (cont)• FRP

– Creep– Delamination

• Other Concerns– Mechanical damage– Gnawing by animals

Fiberglass Material

Corrosion of FRP Plastic

Pipe

Summary of Materials Selection

• No one perfect material• Factors affecting choice:

– Corrosion resistance (metallurgical)

– Mechanical properties (metallurgical)

– Availability– Ease of

installation/fabrication

– Environmental– Operational issues– Cost: capital vs. long

term– Sometimes a finite life

with replacement is the most economical approach

Factors Affecting Corrosion Resistance of a Material

Environmental Environmental FactorsFactors

Corrosion Corrosion ResistanceResistance

Protective Protective Treatments

Metallurgical Factors Metallurgical Factors Resistance TreatmentsResistance

MechanicalMechanicalPropertiesProperties

Safety FactorsSafety Factors

ApplicationApplication



Functions and qualities of coatings

• Separate material from the environment• Provide good adhesion to the metal• Serve as primer for topcoats• Provide sacrificial protection to the metal• Resist abrasion, impact and soil stress• Hold an inhibitor at the metal surface• Resist water absorption• Prevent contamination• Protect against high temperature oxidation • Be safe to use• Be environmentally acceptable• Provide good electrical insulation for underground structures

Coating Considerations• Cost• Life expectancy• Recoating

– Cost– Effect on structure performance

• System approach is needed

Atmospheric Coatings• Sacrifical - galvanizing• Dielectric - barriers

– Oils– Phenolics– Acrylics– Alkyds– Vinyls– Epoxies– Urethanes

Hot dip Galvanizing

Underground Coatings• Holidays• Typical Materials

– Coal tar enamel– Asphalt enamel– Extruded polyethylene– Liquid epoxy– Fusion bonded epoxy– Mastics– Polyethylene and other cold applied tapes– Hot applied tapes - usually coal tar based

Process Coatings(Example: Water Storage Tank)

• Sometimes used– Recoating expense– Tank downtime

• Environmentally acceptable re. water standards• Usually combined with cathodic protection

– maintenance of proper cp levels– danger of disbonding from overprotection

Coating Application Considerations

• Surface preparation• Atmospheric conditions• Application techniques• Inspection

– Wet and dry film thickness– Electrical (holiday) inspection

Surface Preparation–Blasting

Coating Inspection

Holiday Detection



Cathodic Protection• Electrochemical theory• Polarize the cathodic sites on the structure to the

same potential as the anodic sites.

Microscopic View of a Corrosion CellMicroscopic View of a Corrosion Cell

AnodeCathode

Microscopic Corrosion Cell on the Surface of a Pipeline

Cathodic Protection on a Structure

(Macroscopic view)

Cathodic Protection on a Structure

(Macroscopic view)

AnodeCathode

Metallic Connection

Electrolyte

Cathodic Protection Anode

Cathodic ProtectionCurrent Applied

Conventional Current Theory

• Apply current to the surface of the metal• Overcome the corrosion current on the metal• The structure becomes the cathode of a new

corrosion cell• The new anode is allowed to corrode to protect the

structure• Coatings reduce the protective current

requirement• Protects the holidays

Types of Cathodic Protection

• Galvanic• Impressed Current

Galvanic Cathodic Protection• Galvanic (sacrificial anodes)

– Magnesium and zinc most common– Aluminum alloys used offshore– Typical uses

• Well coated structure• Electrically isolated structures• Facilities with low current requirements• Little flexibility, so current requirements need to be fairly

constant

Galvanic Anode Cathodic Protection SystemGalvanic Anode Cathodic Protection System

ANODEANODE

STRUCTURESTRUCTURE

CURRENTCURRENT

CURRENTCURRENT

Cast, Packaged and Extruded Magnesium Anodes

Tank with anode attached at bottom

Impressed Current Cathodic Protection

• Impressed Current– Rectifiers– Anodes

• High silicon iron• Graphite• Mixed metal oxide

Impressed Current SystemImpressed Current System

ANODEANODE

STRUCTURESTRUCTURE

CURRENTCURRENT

CU

RR

ENT

CU

RR

ENT

PowerSource

+-

CU

RR

ENT

CU

RR

ENT

Impressed Current Cathodic Protection

4Anodes

4 Power Source

4Wiring

Rectifier Unit Schematic

V

+-

A

-

+

AC Power Input

Step-DownTransformer

AdjustingTaps on

SecondaryWinding Rectifying Stacks

AC Breaker Switch

Current Shunt Output Ammeter

Output Voltmeter

To Structure To Anodes

Grounding

Housing

Rectifier Unit

Impressed Current Cathodic Protection(cont)

• Typical Uses– Large current requirements– Non-isolated structures– Flexible current output

• Concern with cathodic interference

Typical Application of Impressed Current Cathodic Protection

• Underground pipelines• Underground storage tanks• Bottoms of on grade storage tanks• Interiors of water storage tanks

Typical Application of Impressed Current Cathodic Protection (cont)

• Condenser and heat exchanger water boxes• Reinforced concrete• Doesn’t work in the atmosphere (Automobiles)


Materials SelectionCoating and LiningCathodic Protection

• Water TreatmentDesign

Water TreatmentChange of Water Characteristics

• Oxygen removal• Minor species removal• pH control• Scale Control• Addition of Corrosion Inhibitors

Corrosion InhibitorsA substance when added to an environment, decreasesthe rate of attack by the environment.

Corrosion Inhibitors• Inhibitors for protective films by:

– Adsorption– Formation of precipitates– Passive layer on metal surface

Corrosion Inhibitors• Five different classifications of Inhibitors

– Anodic– Cathodic– Ohmic– Precipitation– Vapor Phase

Inhibitors form films by:• adsorption• formation of precipitates• form passive layer on metal surface• Five different classifications of inhibitors:

Inhibitors (cont)• (i) Anodic

– increase anodic polarization (current density exceeds that required for passivation)

– can increase localized corrosion if entire surface not covered

– eg. Zinc ion compounds (ZnCl)• (ii) Cathodic

– increase cathodic polarization– slows the cathodic reaction– some cause precipitates– eg. Phosphate, nitrates

Inhibitors (cont)• (iii) Ohmic

– form a film on the metal surface– increases circuit resistance– eg, amines – not used in fresh/pure water– used in oil/gas/salt water

• (iv) Precipitation– create large precipitates that interfere with both

anodic and cathodic reactions– eg. Silicates, phosphates

Inhibitors (cont)

• (v) Vapor phase– compounds carried in the vapor phase of a system– upon contact with the metal, condense and

passivate– eg. ammonia

Cooling Water Applications• Langlier and Stability (Ryzner) Indices• Scaling or non-scaling properties of the water• Corrosive effect of oxygen

Water Handling Systems• Langlier Stability Index - Defines the conditions

(temperature and pH) under which CaCo3 is precipitated from water or dissolved in water.

• CaCo3 a major problem in water (hardness)• The solubility of most scales decreases with

increasing temperature.• Oxygen is extremely corrosive in closed elevated

pressure and temperature water systems.

Water Treatment & Inhibitors

• Once through Systems• Closed Loop Systems• Boiler Systems• Cooling Tower Systems• Heating and Chilled Water

– Biocides–slime control– Inhibitors –corrosion control– Good maintenance of closed loop systems to exclude air

Water Treatment–Once-through Systems

• Usually not economical to treat• pH adjustment• Chlorine residual

Water Treatment–Closed Loop Systems

• Nitrites and/or Molybdates• Good maintenance to exclude air

– Excessive leakage nullifies treatment

Water Treatment–Boiler Systems

• Boiler Feed Water• Boiler Water• Condensate

Water Treatment–Cooling Waters Systems

• Corrosion control–pH• Scale control–dispersants• Microbiological control–chlorine• Side stream filter

High-temperature CorrosionA form of material degradation that occurs at elevated temperatures. Direct chemical reactions, rather than the reactions of an electrochemical cell, are responsible for the deterioration of the metals.

High Temperature Corrosion

• Not “if”, but how fast a structure corrodes• Foundaries, flue gasses, etc.• Both general corrosion and pitting corrosion• Corrosion is not electrochemical in nature• Materials react to form oxides

High Temperature Corrosion

• Oxidation occurs from water vapor, hydrogen, hydrogen sulfide, ammonia, sulfur dioxide, chlorine, sulfur, carbon dioxide and oxygen

• Development of scales• Corrosion products

– solids– liquids– gases


Materials SelectionCoating and LiningCathodic ProtectionWater Treatment

• Design

Structure or Product Design• Corrosion control requires lead time like other parts of the

project• Corrosion control must be planned• Preconstruction corrosion surveys• Lowest initial cost design may be the most expensive in the long

run• Coordination among design, construction and other disciplines• Proper specifications addressing corrosion control

Design Considerations• Ease of painting• Ease of maintenance• Avoidance of traps where water will collect• Avoidance of crevices• Treatment of faying surfaces

Design Considerations (cont)

• Proper welding techniques - problems with stainless, eg., heat-affected zone (HAZ)

• Selection of proper materials for the job• Use of dissimilar metals• Mechanical system design• Underground structures

Design Considerations

CREVICETRAP FORLIQUIDS

CREVICE WELDS

BOLTBOLTORORRIVETRIVET

POOR BETTER BEST

Crevices

AVOID AVOID BETTER PREFERRED

Joining Details

Design ConsiderationsDESIRABLE SPECIAL ATTENTION

REQUIRED No radius

Enough distance for coating

Weld accessible

Radius

Weld not accessible for grinding

Not enough distance for coating

Design –DrainageWater/Debris

Tank Outlet for Drainage

Structural Member Orientation for Drainage

Avoid Preferred

Avoid Preferred

Design ConsiderationsJoining Dissimilar Metals

Pit (anode) Protective coating

Alloy (cathode)

Steel

Welding ConsiderationsSKIP WELDS

Skip weld

Welding ConsiderationsSKIP WELDS

Special attention requiredDesirable

Continuous weld Skip weld

Welding ConsiderationsRough

Flush

Smooth contour (as-welded or ground) for

coating

Undercut Rollover

Porosity

Smooth

Welding ConsiderationsFLANGED OUTLET

Sharp corner

Inside of vessel

Special attention required

Threads

Weld

Inside of vessel

2” Min.

Desirable Round corners

Threads in a collar

Threads on nipples

Angle stiffener

Desirable Special attention required

Inside of vessel Inside of

vessel

RIVETSpecial attention requiredDesirable

Grind smooth, round corners

Continuous fillet weld

Gap

Weld

Inside of vessel

Gap

Monitoring and Maintenance of Corrosion Protection

• Continuous operation is necessary for success• Regular inspections are needed• Coating

– Regular inspections– Programmed recoating as required

Monitoring and Maintenance of Corrosion Protection

• Cathodic Protection– Galvanic anodes - annual surveys– Impressed current

• Rectifier inspections• Annual surveys

– Regulatory requirements– NACE criteria for protection

• Inspections of water boxes, water storage tanks, etc.

Monitoring and Maintenance of Corrosion Prevention

• Water Treatment & Inhibition– Proper Levels

• Other Inspections• Visual• Ultrasonic• Coupons• Corrosion monitoring probes• Electrical resistance

• Record Keeping

Additional NACE Training• NACE International

– Courses for those involved in corrosion control

• Basic Corrosion• Cathodic Protection courses• Designing for Corrosion Control• Protective Coatings and Linings 1 and 2• Corrosion Control in the Refining Industry• NACE Coating Inspector Program• Marine Coating Inspection• Successful Coating and Lining of Concrete

NACE Certification• General Corrosion certifications• Cathodic Protection certifications• Coating Inspector certification

For additional information, visit the NACE website at www.nace.org or contact the NACE CertificationDepartment.

Other NACE Resources• COR•AB• Publications

– Materials Performance– Corrosion– Standards

• Technical Committee Reports• Books

Other NACE Activities

• Technical Practices committees• NACE annual conference• Area and Section meetings and conferences• Colleagues world wide

Other Resources• Short courses around the country• Assistance from manufacturers and consultants• Experience of other owners and operators

Summary• Corrosion control is everyone’s business• Management

– Costs– Concern over failures and replacements– Product Life– Safety

Summary (cont)• Purchasing

– Least expensive to begin with may be most costly in the end– Follow the company’s corrosion control specifications

• Sales– Prevent failures, service interruptions– Have a good product to sell

Summary (cont)• Customer Service

– Good relations with customer - customer satisfaction– Explain what company is doing about corrosion control– Provide good service to customers

• Specification Writers– Include corrosion control in specifications– Write through specifications

Summary (cont)• Design Engineers

– Materials selection– Life of the product– Environmental protection and safety– Long range economics of the design– Design corrosion control into new products

RememberRemember

Corrosion Control Corrosion Control Doesn’t Cost Doesn’t Cost ——

It Pays!It Pays!

Documents

Corrosion101slidesJan2005[1].pdf