Materials Selection for Corrosion Prevention

8/10/2019 Materials Selection for Corrosion Prevention

1/61

CORROSION

CONTROL

MATERIAL SELECTION

ALTERATION OF ENVIRONMENT

PROPER DESIGN

CATHODIC PROTECTION

ANODIC PROTECTION

COATINGS & WRAPPING


2/61

(1) MATERIAL SELECTION

(selection of proper material for a particular

corrosive service)

Metallic [metal and alloy]

Nonmetallic [rubbers (natural and synthetic),

plastics, ceramics, carbon and graphite, andwood]


3/61

Metals and Alloys

No Environment Proper material

1 Nitric acid Stainless steels

2 Caustic Nickel and nickel

alloys

3 Hydrofluoric acid Monel (Ni-Cu)

4 Hot hydrochloric acid Hastelloys (Ni-Cr-Mo)(Chlorimets)

5 Dilute sulfuric acid Lead


4/61

No Environment Proper material

6 Nonstaining atmospheric

exposure

Aluminium

7 Distilled water Tin

8 Hot strong oxidizing

solution

Titanium

9 Ultimate resistance Tantalum

10 Concentrated sulfuric acid Steel


5/61

NORSOK


6/61

NORSOK


7/61

NORSOK


8/61

NORSOK


9/61

NORSOK


10/61

Table 5 - Materials selection for sub-sea production and flowline systems


11/61



12/61



13/61


14/61


15/61


16/61


17/61

E.g : Stainless Steels

Stainless steels are

iron base alloys that

contain a minimum

of approximately11% Cr, the amount

needed to prevent

the formation of rust

in unpollutedatmosphere.wt.% Cr

Dissolutionrat

e,cm/sec


18/61

Alloying elements of stainless steel :

Other than Ni, Cr and C, the following alloying elements mayalso present in stainless steel: Mo, N, Si, Mn, Cu, Ti, Nb, Taand/or W.

Main alloying elements (Cr, Ni and C):

1. Chromium

Minimum concentration of Cr in a

stainless steel is 12-14wt.%

Structure : BCC (ferrite forming element)

* Note that the affinity of Cr to form Cr-carbides is very

high. Chromium carbide formation along grain

boundaries may induce intergranular corrosion.


19/61


20/61

2. Nickel

Structure: FCC (austenite forming element/stabilize

austenitic structure)

Added to produce austenitic or duplex stainless steels.These materials possess excellent ductility, formability

and toughness as well as weld-ability.

Nickel improves mechanical properties of stainless

steels servicing at high temperatures.

Nickel increases aqueous corrosion resistance of

materials.


21/61

Ternary diagram of Fe-Cr-Ni at 6500and 10000C

AISI : American Iron and Steel Institute


22/61

Anodic polarization curves of Cr, Ni and Fe in 1 N

H2SO4solution


23/61

Influence of Cr on corrosion resistance of iron

base alloy


24/61

Influence of Ni on corrosion resistance of iron base alloy


25/61

Influence of Cr on

iron base alloy

containing 8.3-

9.8wt.%Ni


26/61

3. Carbon

Very strong austenite forming element (30x more

effective than Ni). I.e. if austenitic stainless steel

18Cr-8Ni contains 0.007%C, its structure will

convert to ferritic structure. However the

concentration of carbon is usually limited to

0.08%C (normal stainless steels) and 0.03%C (lowcarbon stainless steels to avoid sensitization during

welding).


27/61

Minor alloying elements :

Manganese

Austenitic forming element. When necessary can be used tosubstitute Ni. Concentration of Mn in stainless steel is usually 2-

3%.

Molybdenum

Ferritic forming element. Added to increase pitting corrosionresistance of stainless steel (2-4%).

Molybdenum addition has to be followed by decreasing chromium

concentration (i.e. in 18-8SS has to be decreased down to 16-

18%) and increasing nickel concentration (i.e. has to be increasedup to 10-14%).

Improves mechanical properties of stainless steel at high

temperature. Increase aqueous corrosion resistance of material

exposed in reducing acid.


28/61

Tungsten

Is added to increase the strength and toughness of martensiticstainless steel.

Nitrogen (up to 0.25%)Stabilize austenitic structure. Increases strength and corrosion

resistance. Increases weld ability of duplex SS.

Titanium, Niobium and TantalumTo stabilize stainless steel by reducing susceptibility of the

material to intergranular corrosion. Ti addition > 5x%C.Ta+Nb addition > 10x%C.


29/61

Copper

Is added to increase corrosion resistance of stainless steel

exposed in environment containing sulfuric acid.

Silicon

Reduce susceptibility of SS to pitting and crevice corrosion as

well as SCC.


30/61

Influence of alloying elements on pitting

corrosion resistance of stainless steels


31/61

Influence of alloying elements on crevice

corrosion resistance of stainless steels


32/61

Influence of alloying elements on SCC

resistance of stainless steels


33/61

Five basic types of stainless steels :

Austenitic- Susceptible to SCC. Can be hardened by only bycold working. Good toughness and formability, easily to bewelded and high corrosion resistance. Nonmagnetic except afterexcess cold working due to martensitic formation.

Martensitic- Application: when high mechanical strength and

wear resistance combined with some degree of corrosionresistance are required. Typical application include steamturbine blades, valves body and seats, bolts and screws, springs,knives, surgical instruments, and chemical engineeringequipment.

Ferritic- Higher resistance to SCC than austenitic SS. Tend tobe notch sensitive and are susceptible to embrittlement duringwelding. Not recommended for service above 3000C becausethey will loss their room temperature ductility.


34/61

Duplex (austenitic + ferritic)has enhanced resistance toSCC with corrosion resistance performance similar to AISI316 SS. Has higher tensile strengths than the austenitic type,are slightly less easy to form and have weld ability similar tothe austenitic stainless steel. Can be considered as combining

many of the best features of both the austenitic and ferritictypes. Suffer a loss impact strength if held for extended

periods at high temperatures above 3000C.

Precipitation hardening- Have the highest strength butrequire proper heat-treatment to develop the correctcombination of strength and corrosion resistance. To be usedfor specialized application where high strength together withgood corrosion resistance is required.


35/61


36/61


37/61


38/61


39/61


40/61


41/61


42/61


43/61

Stress Corrosion Cracking of Stainless Steel

Stress corrosion cracking (SCC) is defined as crack nucleationand propagation in stainless steel caused by synergistic actionof tensile stress, either constant or slightly changing with time,together with crack tip chemical reactions or otherenvironment-induced crack tip effect.

SCC failure is a brittle failure at relatively low constant tensilestress of an alloy exposed in a specific corrosive environment.

However the final fracture because of overload of remainingload-bearing section is no longer SCC.


44/61

Three conditions must be present simultaneously

to produce SCC:

- a critical environment

- a susceptible alloy

- some component of tensile stress


45/61

Tensile

stress

Corrosive

environment

Susceptible

material

Stress

corrosion

cracking

Tensile stressis below yield

point

Corrosive

environment is

often specific to

the alloy system

Pure metals are more

resistance to SCC but not

immune and susceptibility

increases with strength


46/61

Typical micro cracks formed during SCC of

sensitized AISI 304 SS

Surface morphology


47/61

Example of crack propagation during transgranular stress

corrosion cracking (TGSCC)brass


48/61

Example of crack

propagation during

intergranular stresscorrosion cracking

(IGSCC) ASTM A245

carbon steel


49/61

Fracture surface oftransgranular SCC on

austenitic stainless steel in

hot chloride solution

Fracture surface of

intergranular SCC oncarbon steel in hot nitric

solution


50/61


51/61

Electrochemical effect

pitting

passive

active

cracking

zones

Usual region for

TGSCC, mostly is

initiated by pitting

corrosion

(transgranular cracking

propagation needshigher energy)

Usual region for IGSCC,SCC usually occurs where

the passive film is

relatively weak

Zone 1

Zone 2


52/61

Note that non-susceptible alloy-environment combinations, will

not crack the alloy even if held in one of the potential zones.

Temperature and solution composition (including pH, dissolved

oxidizers, aggressive ions and inhibitors or passivators) canmodify the anodic polarization behavior to permit SCC.

Susceptibility to SCC cannot be predicted solely from the anodic

polarization curve.


53/61

Models of stress corrosion cracking

Slip step dissolution model

Discontinuous intergranular crack growth

Crack nucleation by rows of corrosion micro-tunnels

Absorption induced cleavage

Surface mobility (atoms migrate out of the crack

tips)

Hydrogen embrittlementHIC


54/61

Control/prevention :

Reduce applied stress level

Remove residual tensile stress (internal stress)

Lowering oxidizing agent and/or critical speciesfrom the environment

Add inhibitor

Use more resistant alloys Cathodic protection


55/61

Alteration of Environment

Typical changes in medium are :

Lowering temperaturebut there are cases whereincreasing T decreases attack. E.g hot, fresh or salt water is

raised to boiling T and result in decreasing O2solubility

with T.

Decreasing velocityexception ; metals & alloys thatpassivate (e.g stainless steel) generally have better

resistance to flowing mediums than stagnant. Avoid very

high velocity because of erosion-corrosion effects.


56/61

Removing oxygen or oxidizerse.g boiler feedwaterwas deaerated by passing it thru a large mass of scrap steel.

Modern practicevacuum treatment, inert gas sparging, or

thru the use of oxygen scavengers. However, not

recommended for active-passive metals or alloys. Thesematerials require oxidizers to form protective oxide films.

Changing concentrationhigher concentration of acidhas higher amount of active species (H ions). However, for

materials that exhibit passivity, effect is normally negligible.


57/61

Environment factors affecting

corrosion design :

Dust particles and man-made pollutionCO, NO,

methane, etc.

Temperaturehigh T & high humidity accelerates

corrosion.

Rainfallexcess washes corrosive materials and

debris but scarce may leave water droplets.

Proximity to sea Air pollutionNaCl, SO2, sulfurous acid, etc.

Humiditycause condensation.


58/61

Design Dos & Donts

Wall thicknessallowance to accommodate for corrosion effect.

Avoid excessive mechanical stresses and stress concentrations in

components exposed to corrosive mediums. Esp when using

materials susceptible to SCC.

Avoid galvanic contact / electrical contact between dissimilar

metals to prevent galvanic corrosion.

Avoid sharp bends in piping systems when high velocities

and/or solid in suspension are involvederosion corrosion.

Avoid crevicese.g weld rather than rivet tanks and other

containers, proper trimming of gasket, etc.


59/61

Avoid sharp cornerspaint tends to be thinner at sharp

corners and often starts to fail. Provide for easy drainage (esp tanks)avoid remaining

liquids collect at bottom. E.g steel is resistant against

concentrated sulfuric acid. But if remaining liquid is exposed

to air, acid tend to absorb moisture, resulting in dilution andrapid attack occurs.

Avoid hot spots during heat transfer operationslocalized

heating and high corrosion rates. Hot spots also tend to

produce stressesSCC failures.

Design to exclude airexcept for active-passive metals and

alloys coz they require O2for protective films.

Most general rule : AVOID HETEROGENEITY!!!


60/61

Protective Coatings / Wrapping

Provide barrier between metal and environment.

Coatings may act as sacrificial anode or release substance that

inhibit corrosive attack on substrate.

Metal coatings :

Noble

silver, copper, nickel, Cr, Sn, Pb on steel.

Should be free of pores/discontinuity coz creates small

anode-large cathode leading to rapid attack at the

damaged areas.

Sacrificial

Zn, Al, Cd on steel. Exposed substrate willbe cathodic & will be protected.

Application hot dipping, flame spraying, cladding,

electroplating, vapor deposition, etc.


61/61

Documents

Materials Selection for Corrosion Prevention