Hot corrosion in GT

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1/83

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ODDRE ltr 19 Oct 1970


2/83

NMAB-2^0

MAY

^70*

:

Hot

Corrosion

in

Gas

Turbines

...

Mechanisms

Alloy & Coating

Development

Environmental

Effects

Evaluation

A

Repor t

o f

|h e

\

NATIONAL

MATERIALS

'ADVISORY OARD

j.

ATIONAL

REStARCH

COUNCIL

N iTIONAL

ACADEMY

OF

SCIENCES-NATIONAL

ACADEMY

OF

ENGINEER1NC


3/83

«CESSIMMor

_

C F S T I

HIT SC3TIM

Q

O O C U F F

E C T I O «

U N A H H O W C E D

J U S T I F I C A r i W I

B Y

DISTRIBUTIV

/AVAIUBIl in T Ü E

H I S T .

AVAIL

4 / l (

NATIONAL ATERIALS

DVISORY OARD

DIVISION

F

NGINEERING

-

NATIONAL ESEARCH

OUNCIL

Chairman

iiam J. Harris,

Jr.

sident of Research

m

nd

T


4/83

HOT

CORROSION

N

GAS

TURBINES

' MECHANISMS

ALLOY COATING

DEVELOPMENT

*' ENVIRONMENTAL

EFFECTS

' EVALUATION

REPORT

OF

THE

COMMITTEE ON

ATMOSPHERIC

DETERIORATION OF SUPERALLOYS

NATIONAL

MATERIALS

ADVISORY

BOARD

Division of Engineering

-

National

Research

Council

This document is

subject

to special

export

controls nd

each ransmittal

o

oreign

governments

or foreign

nationals

may

e

made only with

prior

approval

of

he

Office

of

he

Director

of Defense

Research

and

Engineering

Chief, Materials Division,

OADCT) .

Publication

NMAB-260

National

Academy

of Sciences National

Academy

o f

Engineering

Washington,

. .

May

1970


5/83

This

report

is

one

of a series in studies undertaken

by

he

National

Mater'

s

Advisory

Board

for th e National Academy of Sciences and th e

National

Academy

of

Engineering in

partial

execut ion

of

work under

Contract

No. A-49-083

OSA-

3131

with

th e

Department

o f

Defense .

A s

a

part

of th e National Research Counci l , he National Materials Advisory

Board

performs

tudy,

evaluation,

or

advisory

funct ions

hrough

groups

com-

posed

of

individuals selected from academic, governmental, nd

industrial

sources

for

their

competence

or

interest

in

th e

subject

under

consideration.

Members

of

these groups serve as ndividuals contributing

their personal

know-

ledge

and

judgment s

and

no t

as

representatives of

an y

organization

in

which

they are emp loyed or with

which

they

may be associated.

The

quantitative data publ i shed in this report are

in tended

only o illustrate he

scope

and substance of

information considered

in

th e

study,

nd

should

not

be

used for an y other purpose,

uch

as n specifications

or

in design, nless so

stated.

No

portion

of this

report

may

e

republished

without

prior

approval

of th e National

Materials

Advisory

Board.

Copies of

this

report

are no t

available

from

th e

Clearinghouse for Federal

Scientif ic and Technical

Information CFSTI).

Qualif ied requesters

m ay pply

th rough

Defense

Documen ta t ion

Center.

Those

who

do

no t

use

he

Defense

Documen ta t ion Center may pply o

th e

National

Materials

Advisory

Board.

ii


6/83

NATIONAL

MATERIALS ADVISORY

BOARD

COMMITTEE ON ATMOSPHERIC DETERIORATION OF SUPERALLOYS

Chairman: Dr.

Robert I.

affee, enior

Fellow

and

Associate Manager, Materials

Science

and

Technology, Department

o f

Physics,

Battelle Memorial

Institute,

05

King

Avenue,

Columbus,

Ohio

3201.

Members:

Mr.

Charles C.

Clark, n

Charge, Aerospace Power, The Inter-

national Nickel Company, nc., 7 Wall Street,

ew

York,

ew

York 10005.

Dr. Matthew

J.

Donachie,

Jr.,

Development Metallurgist, MDL-J

Building,

Pratt

& Whitney Aircraft, East

Hartford,

Conn.

06108.

Dr. William R. reeman, Jr., Director, Materials Laboratories,

AVCO

Lycoming

Division,

50

S.

Main

Street,

Stratford,

Conn.

6497.

Dr. William

Hagel,

Head,

Metallurgy Division,

Denver

Research

Institute,

University

o f

Denver, University

Park,

Denver,

Colo.

0210.

Mr.

aul

E.

Hamilton,

ection Chief,

nvestigations Development

Metallurgy, Aircraft

Engine

Operations, Allison Division, General

Motors

Company,

. . ox

894,

ndianapolis,

ndiana

6206.

Dr.

Robert A .

Rapp,

Professor, Metallurgical

Engineering,

College

of Engineering,

h io

State

University, Columbus,

Ohio 43210.

Dr.

Alan

U.

Seybolt,

Metallurgy Ceramics

Laboratory,

General

Electric Company,

& D

Center,

P.O.

Box

8, Schnectady,

ew

York

12301.

Dr.

William

E.

Young,

Manager, Combustion, Corrosion

& MHD

Section,

Heat Transfer Power

Generator

R&D ,

Research

and

Development

Center,

Westinghouse Electric

Corporation,

Churchill

Borough,

Beulah

Road,

ittsburgh,

ennsylvania

15235.

iii


7/83

Liaison

Representatives:

Dr.

Hubert

B.

robsc,

Head, Oxidation

and

Refractory

Compounds

Section, ASA

Lewis

Research

Center,

1000

Brookpark

Road,

Cleveland, h io

44135.

Mr. Milton

Levy, Army

Materials

Mechanics

Research

Center,

Watertown, Massachusetts 02172.

Mr.

Henry

Morrow, . . Army Aviation Materiel Laboratories,

Fort

Eustis, Virginia

23604.

Mr.

George

J.

Danek,

Jr., Naval

Ship

Research and Development

Center,

Annapolis, Maryland

21402.

Mr. rving Machlin, Materials

Engineering, AIR-52031B,

Naval

Air

Systems

Command, Department of

the Navy,

Washington,

D .

.

20360.

Mr. Norman

M .

Geyer, Air Force

Materials

Laboratory, MAMP )

25-53136,

Wright-Patterson Air

Force

Base,

Ohio

45433,

Mr. ohn

C .

Barrett, DDR&E ,

Office

of

Assistant

Director

for

Chemical

Technology,

Room 3D117,

The

Pentagon,

Washington,

D. . 0310.

NMAB

STAFF:

Dr.

oseph R.

ane,

taff

Metallurgist,

National Materials

Advisory

Board,

Division

of

Engineering, National Research

Council, NAS-NAE, 101

Constitution

Avenue, . W.,

Washington,

. .

0418.

iv


8/83

SUBGROUPS

of the

COMMITTEE ON ATMOSPHERIC DETERIORATION OF SUPERALLOYS

Mechanisms

Dr. Alan

U.

eybolt,

Chairman

Dr.

William

Hagel

Dr. Robert

A. Rapp

Alloy Coating Development

Mr. Charles

C . Clark, Chairman

Dr.

Matthew J.

Donachie,

Jr.

Mr. aul E. Hamilton

Environmental Effects

Dr.

William

E.

Young,

Chairman

Dr. William R. reeman

Aided

by Contributions From

Mr.

Norman

Bornstein, United Aircraft

Research Center

Mr.

.

A . DeCrescente,

United Aircraft

Research Center

Mr.

rving Machlin,

Naval

Air Systems

Command

Mr. Warren Kentz,

AVCO

Lycoming

Div.

Miss

E. J.

MacNair,

Ministry

of

Defense,

Bath, England

D .

R.

Carlisle,

Rolls

Royce

Industrial,

Ansty,

England

Henry

L.

Morrow, . S. Army Aviation

Materials

Laboratory

Jeremy

J. Walters,

VCO Lycoming

Irving

Machlin,

Naval Air

Systems

Command

R.

M .

Shir'mer,

hillips

Petroleum

Alan

U.

eybolt,

General

Electric

G. . Danek, Naval Ship Research and

Development

Laboratory

Hubert

B. Probst, NASA

Lewis

Research

Center

W. T.

Reid,

Battelle

Memorial

Institute

W. E. Somgrs,

Pujlic

Service

Electric

and Gas Company

G.

.

Widers

urn,

Philadelphia

Electric

Company

Evaluation

Dr.

William

R.

reeman, Chairman

Mr. aul E. Hamilton

Mr. George J. Danek,

Naval

Ship

Research

and Development

Laboratory


9/83

CONTENTS

Abstract ii

I.

INTRODUCTION

H .

MECHANISMS

A .

ot

Corrosion Attack

B.

igh-Temperature

Oxidation

1

IE. MATERIAL AND

COATING

DEVELOPMENT

4

IV.

ENVIRONMENTAL EFFECTS 0

V.

EVALUATION 9

A .

Recommendations

9

VI.

SUMMARY OF

CONCLUSIONS

AND RECOMMENDATIONS

1

Appendices

I.

Recommendations

from

Coating Systems for

Gas Turbine Engines 4

1 1 .

Round Robin

Hot

Corrosion

Testing

Program 7

vi


10/83

ABSTRACT

Hot

corrosion (sulfidation)

n

gas

turbine

engines

has

become

a

major

problem

because

of

the

increased

use of

al loys

lo w

in

chromium and th e operation

in

environments

containing

alkali

metal

salts,

especially

near sea water.

The

mechanism

of

attack

is

understood

to

some

extent,

ut

more

work

is

needed. It

is

clear

that

sodium

and other

alkali

metal

salts are

i nvo lved .

Sodium

sulfate

is ngested

with

the

combustion

air

or

formed

from

the

sulfur

in

th e

fuel,

and reacts with the metal oxide scale

acting

as n

Na20

sink to

form

a

comp lex

sodium

salt. After scale

breakdown,

sodium sulfate

may

attack

the

underlying

metal, orming

sulfides. The

presence of NaCl in the

gas

nd

a

l iquid

salt film

seem to

be

required

for accelerated attack.

Coatings,

l loy modification, nd

additives

to

the

fuel

all

help

alleviate the

immediate

problem.

Al loy development,

dispersion

hardened and

fiber

strengthened

alloys,

nd

rare

earth additions to

superalloys m ay offer longer-range

solutions.

Target

performance

properties

are

needed

to

focus

the

future

research

and

development

work.

Sulfur

can

enter a

gas turbine

from

the

fuel, nd

chloride

and

sulfate

salts

from

th e

air.

There

is

little

prospect

for

removal

of

these

contaminants

to such

a

degree

as

to

eliminate

th e

problem.

However ,

rom

10

to

75% of

sea salt

can

be

removed

from

the air

intake

to retard

the

attack.

Data on

tolerable

levels

of con-

taminants are

needed.

Engine

testing

is still

th e only

reliable

evaluation

method

to

check

im-

provements

in

the

hot

corrosion

resistance

of

materials.

Test

rigs are common ly

used

fo r

a preliminary

evaluation of

hot

corrosion

resistance. Reproducibility

n

test

results

from

test

rigs

n

different laboratories

s very poor,

nd

further effort

toward

test

rig

standardization,

correlation

of

the data

between

different

tests,

nd

interpolation

of tests results nd

extension

to

different

testing

conditions, all are

needed.

Specific

recommendations for attacking th e hot

corrosion

problem are

detailed in

th e

report.

vil


11/83

I.

INTRODUCTION

The Problem

A s temperatures in

th e

turbine section

of

gas turbine

engines

have

increased,

n

increasing

amoun t

of

environmental

attack

has been

encountered

in

th e

superalloy

parts.

Operational

personnel

generally

have ascribed the problem

to

hot corrosion

or

sulfidation,

whereas

the

real

problem

m ay

have

been

excessive

oxidation

du e

to

overtemperature. For whatever cause,

he

results of this

attack

penalizes engine

performance

by

a

severe

restriction

in

operating

life.

The major

limitation

on

turbine

inlet temperature

is the

strength

of

the

turbine

rotor-blade and

stator-vane

materials.

There

have

been

major

improve-

ments in

the

high-temperature

strengths

of

these

materials,

primarily

through

th e

use of strong casting alloys,

ardened

by

a substantial

vo lume

fraction

of

the

coherent

y' precipitate,

Ni,

Co)

AI,

Ti).

However , he major

means

for

achiev-

in g

higher

gas

temperatures in

the

turbine

is

through the use of air cool ing ,

where-

by a

small

percentage

of

the

inlet

air

is diverted

through

cooling

passages

in

the

ho t

turbine

blading

an d vanes.

Through

this means,

he

temperature

of these

parts

is

owered

about 400-500

o

F

222-288

0

C) below

the gas

temperature

to

the

poin t

where

their strength capability is sufficient.

Up to th e

present time,

orrosion

and

oxidation

resistance have no t

been design considerations in

aircraft

gas

turbine

material selection

other

than

that

the

materials

resist th e

environment,

hich

superalloys

ontaining abou t

20%

chromium

do adequate ly

up to

the

temperature

at

which strength

is

maintained,

abou t

leSO'F (899

0

C ). However , he

development of the

stronger

gas

turbine

alloys

was

facilitated

by

a

reduction

in

chromium

content

to abou t 10%

chromium,

to

permit

an increased

amoun t

of

y' hardening

from

Increased contents

of

aluminum


12/83

and titanium.

The

high

aluminum

content maintained

th e

oxidation

resistance

but

not sulfidation resistance

equivalent

to th e higher chromium content.

Hot

corrosion

is an

accelerated oxidation

in

the

presence

of

NaCl and

Na

SO

usually

due

to operation

in marine

environments.

It

also

includes

attack

by

lead

vanadium

and

other fuel

contaminants.

The reduced oxidation

and

hot

corrosion

resistance

has

been

combated

by

coating th e

hot componen t s

with

aluminide-type

coatings.

The

coating

procedure entails

additional

expense and

a

problem with

reliability

compared with

th e former situation

where

uncoa ted

alloys

with adequate oxidation

and

corrosion

resistance

were

operated

at

lower

gas temperatures.

The Commi t t ee

The Department of

Defense ,

ecognizing th e

seriousness

of

th e

ho t

corrosion

problem, sked

th e

National

Research Counc i l to organize a

committee

to review th e problem. The Committee, which

was

assembled

by th e

National

Materials Adviso ry

Board,

s shown on

page

iii.

Members

of

th e Committee,

act ing

as

individuals

and not

as representatives of their employers,

ontributed

their services.

The

Committee

reflected

varying

viewpoints

ranging from

basic

researchers

to

those of

stationary

and aircraft

gas

turbine manufacturers.

Method

of

Operation

After

initially considering their individual positions

and

viewpoints on

th e

problem,

he

Committee

divided their

consideration into

four

areas:

mechanisms, materials

deve lopment ,

nvironmental

effects,

nd testing and

evaluation,

each

of

which

was assigned to a

subgroup.

The remainder of this

report

comprises th e

findings

of

th e

four

subgroups,

ol lowed

by

overall conclu-

sions

and

recommendations.


13/83

II.

MECHANISMS

Introduction

In this entire report, mphasis has been

placed on hot corrosion or

sulfidation of superailoys,

s opposed

to

their oxidation,

ecause

this

ype of

reaction

is

a

matter

o f

the

most immediate

concern and

study in

current

engine

service.

The

subject of

hot corrosion is discussed first.

It is

generally

agreed that

the basic corrosion

attack occurring

in

hot

corrosion or sulfidation is

oxidation which

occurs in

a

very rapid or

near-

catastrophic

manner

because

of the

disruption

caused

by the existence o f a salt

film of the normally protective scale. In addition,

ulfides

form

inside

the sur-

face grains

and

sometimes

deep along grain

boundaries

o

cause

serious

degrada-

tion

o f structural integrity of the alloy.

As

has

been pointed

out

in many publications see, or

example,

he

recent

ASTM

Symposium

on this

subject),

he

immediate

agent

responsible

for

hot corrosion in aircraft

engines

s

Na SO which forms from chemical reactions

taking place between NaCl

n sea air and sulfur in the luel, s

well

as from

the

appreciable

content

of Na„SO, n sea air. There is also evidence that NaCl-Na^SO,

2

4

4

mixtures

are

more

corrosive

han

Na

SO

alone.

While

the

same

type

of

hot

corrosion

attack

has been

observed

in

stainless steels

subjected

to

SO

atmos-

pheres ttention here s focused

on the

problem

caused

by Na SO

in

aircraft

jet

engines and

marine

gas urbines.

2

DeCrescente and

Bornstein

have shown that Na^SO, must be present

2 4

on

the attacked surface as a condensed

phase,

rdinarily

a liquid,

o

cause hot

corrosion.

a

SO apor

did

not

cause hot corrosion. or

this

reason,

most

investigations

have

found

that

for

pressures

near

atmospheric

his

accelerated


14/83

attack

occurs

over

th e

temperature

range

of

about

HOO-ISOOT.

In

qualitative

agreement

with

th e effect

of pressure

n

th e

dew po in t

of

Na

SO he temperature

range

of attack

is increased

with

higher

total

pressures.

There has been

comparatively

ittle

work

aimed

at

understanding

th e

mechanism

of oxidation as

altered

by

th e

presence

of

contaminants uch

as salts

like

Na

SO

NaCl , or

low-melting

oxides uch

as V O

These

materials can

form

l iquid

slags

and

hence

cause

catastrophic

oxidation

by

virtue

of

removal

of

protective scales. This

field of

high

temperature

corrosion

has

recently been

3

reviewed by Hancock, In

this review,

nearly

all th e investigations

reported

were

concerned with examination of field-returned

samples

or

with

attempts

in

th e

laboratory to

make

high-temperature

corrosion tests

which

essentially

dupli-

cated field conditions. Very

little work

aimed

at elucidating

mechanisms

has

been

reported,

particularly

in

simple systems amenable

to

scientific analysis.

A .

Hot

Corrosion

At tack

Characteristics

The

alloys of main concern

here

are used

as first-

or second-stage

blades and

vanes

in

aircraft

engines,

nd

are

th e

nickel-base

superalloys con-

ta ining

typically

6-22%

Cr,

-26%

Co, -10% of either

Ta, o or W, -6%

A l,

0-5%

Ti, nd

small fractional

percentages of carbon and other

elements. A

sample showing

severe hot corrosion attack exhibits a heavy surface

layer

of

oxides,

nd

just

below

this

layer,

xides

intermingled

with

alloy-depleted

nickel

matrix, nd below

this

level, hromium

sulfide

particles

which

m ay

contain

appreciable percentages

of

Ni,

o, A l,

Ti, nd refractory

metals.

A s

corrosion

proceeds,

he

chromium

sulfides

are converted

to

oxide

with

th e

sulfur

atoms

t hus released diffusing more deeply

into th e alloy to form more sulfides. The

alloy

matrix

surrounding th e

chromium

sulfides

is

depleted

in

chromium

to

an

appreciable

extent and would be

expected

to

oxidize

more

readily

than

th e

original


15/83

alloy.

However ,

his

effect

is considered to

be

of less

importance

than

th e

rapid

oxidation attack proceeding at

th e

oxide/alloy

interface because

th e oxide

scale

fails

to act

as

a diffusion barrier, s would occur in normal

air oxidation. Chro-

mium sulfides

both

in grains and in

grain boundaries cause loss

of structural

integrity,

nd

in cases where

such sulfides

are

formed deeply along grain bound-

aries,

severe

reduction

in

mechanical

properties results. Such

reductions in

mechanical properties

are

particularly

severe

in

blades,

which

are

highly

stressed

as compared to vanes.

Some recent

ho t

corrosion experiments

on

a

variety

of

alloys

appear

Q

to

confirm

th e

original

suggestion of

Simons

et

al

hat

th e

catastrophic

nature

of th e corrosion attack

invo lves

rapid

oxidat ion of

Ni S

Ni

eutectic

liquid.

This

liquid

usua l ly

is short-lived,

nd

may not

be

visible

in

an

alloy cooled

to room

temperature because of

th e displacement reaction

(7)

see

below).

On th e

other

9

hand,

Bornstein

an d

DeCrescente

have

found

that

oxidation

attack

of

several

different

alloys in th e presence

of NaNO

salt

closely

simulates

hat for NaSO

attack. The conclusion reached is

that

sulfur

and

alloy

depletion are

not he

important

aspects

of

hot

corrosion,

ompared

to

th e presence

of

sodium

com-

pounds.

Hot-corrosion resistance

can be

achieved

by

preventing

attack

by

th e

salt

of

th e

initial oxide

layer on

th e

alloy.

This

salt-oxide reaction

will

be

considered

first.

Scale

-

Salt Interaction

Since

Na

SO

is a very

stable

compound

there is

very little

thermal

decomposition

of

Na

SO

according

to

th e

fo l lowing reaction:

Na

2

S0

4

= a

2

0 + S0

2

+i

2

. 1)


16/83

However, f there is available an

Na O

sink which

effectively

retains

an

Na

activity

well below unity which

is assumed by equation 1) , hen Na O

can

continually

decompose. Formation

of

complex oxides

involving

Na

O and

oxides

normally

present

in the oxide

scale,

uch

as Cr O Al

iO

r

WO

an

4

serve

as sinks

for

the

Na„0

by such

reactions as:

2

2Na

2

S0

4

Cr

2

0

3

|0

- »

2N a

2

Cr0

4

2S0

2

2)

Experimental

corroboration of

Na

O-oxide

interaction

has

been observed

by

5

Bergman

and Kaufman

who

found

X-ray

diffraction patterns

for

sodium ungstate,

sodium

tantalate,

nd

sodium

titanate in corrosion products on hot-corroded

superalloys.

imilarly

an

NiO

scale breakdown by

Na SO would

occur

according

to

the

following

reaction:

Na SO + 2NiO

Na NiO

+

SO

3)

2 4

6

NaNiO

has been

reported

by

Bornstein.

Alternatively

NiO

may

break

down

d

according

to

the

following reaction:

Na SO +

NiO (N a O in

NiO)

SO + ̂ O 4)

Z r

A

combination of both

reactions, 3) and 4) , may

be

operative.

That

NiO

can

1

dissolve some

Na

O

has been suggested by

Danek , and Quets

and Dresher.

2

The

actual mechanism of

scale breakdown

is

probably

more complex than

that

suggested, ut it seems

obvious hat, or an understanding of

the

hot

corrosion

mechanism,

he

area

of

salt/oxide

interactions

should

be

carefully

investigated.

Although

they

have

not as yet

developed

a

detailed

theory,

DeCrescente

7

and Bornstein indicate

from

their

work on molten

salts

EMF measurements and

SO

evolution)

hat

they

have evidence hat the presence

of

oxides

such

as

Cr

O

3

on

an

alloy

reduces

the

Na

O

concentration by

formation

of

Na CrO

and

thus

2

4

lessens

hot corrosion attack.


17/83

Alloy-Salt

Interaction

Once

th e

oxide scale

has

been

penetrated, he

next

stage

of

attack

is

reaction

between

condensed

Na^SO,

or

Na„SO

NaCl

and

th e alloy itself. Various

2 4 4

investigators have

suggested

reactions based

on th e early

work

of

Simons who

suggested

a

reaction

equivalent

to

th e

tcilowing:

4M

+

Na

SO

=

a

O

+

3MO

+

MS.

5)

4

However ,

Quets

and

Dresher

have shown by

th e

use

of Pourbaix-EUingham

diagrams

that

pure

Na

O,

which

has

been considered

as

an

important

product

of

th e

hot

corrosion

attack

by Na SO

cannot

exist

at

unit

activity

with

th e

products

Ni

S

iS

or

C rS

as

would

be

required

by

equat ion

5).

For

this reason,

quation

(5) must

be modified for both th e

case

of

M =

Ni

or

M

=

Cr.

If

th e Na„SO. reacts

2 4

with

the metal

to form

a

ternary sodium

oxide

and

binary sulfide,

a modified

reaction

becomes

possible:

xM+NaSO, =Na„MO

+M S

6)

2

4

y (x-y)

In support

of

this

mechanism,

he

stabilities

of

th e

ternary sodium

oxides

extend

to

much higher

values

of

P

than

those for

pure

Na

O.

This mechanism

is

very similar to

th e

one

proposed

for

th e

oxide

scale

breakdown.

However , ince th e reaction

occurs

at

th e metal-scale inter-

face,

ts

oxygen

potential

is

set

by

that

interface.

Dur ing

sulifidization,

he

oxygen potential at th e metal-scale interface should

be approximately th e same

as

for

a

Ni-NiO

interface

because

of

th e chromium

depletion

in

th e

alloy.

For

th e formation

of

a

ternary

sodium

salt,

he

sulfur

potential is

high enough to

allow

th e formation

of nickel

sulfides.

However ,

ue

to th e displacement reaction

Cr,. ..

+NiS

rS

+

Ni,

„ 7)

(in

al loy)

in

al loy)

NiS

shou ld

be reduced

by

th e matrix

chromium

to

form

the

more stable chromium

sulfide.

But

if

th e

chromium

content

in

th e

al loy

s

rather

low,

eaction

7)

does


18/83

not

occur

apidly

or

completely

enough

to

reduce all

th e

NiS.

Therefore, n

severe cases

of hot corrosion,

iS actual ly

Ni on cool ing

to

room

tempera-

O It

ture) m ay be left

in

th e

structure.

Effect

of

Al loy Compos i t ion*

A s

might be expected for a class of alloys with a very

complex and

widely

variable composition, he individual

effects

of al loy components

on

re-

sistance

to

ho t

corrosion

is

no t

unambiguous .

Certain

generalizations

can

be

made, such as

that resistant alloys

usually

contain fairly high chromium

con-

tents,

around 15% Cr

or

higher.

There appears

to

be

agreement that

molybdenum

is

harmful,

bu t th e behavior

of

other

al loying

elements

s

ess

consistent, nd

therefore not clearly

established. Better understanding of compositional

effects

migh t

be

obtained

after

some of

th e

scale-Na

SO

and

alloy-Na

SO interactions

are

further

elucidated.

Research for

Defining

Mechanisms

The most important

areas

for

additional

research are

th e

salt/scale

and salt/alloy interactions

n th e temperature

range

of hot. corrosion, nd oxygen/

al loy

nteractions

at

higher

temperatures. Ano the r

area

which

needs

attention

is th e

stabilities

thermodynamics)

of

ternary

odium

oxides.

Water

vapor,

product

of

combus t i on , as

no t

been

seriously

con-

sidered

thus far

as corrosion

variable,

but

it

seems possible

that

water vapor

in th e

eng ine

might be

significant in view of

th e

ability

of

water vapor to cause

oxidation at

eng ine

operation

temperatures. Further

study

is

ndicated.

7

The

suggestion

of

DeCrescente

and

Bornstein

hat

additions

of

cer-

tain oxides

affect th e Na SO

alloy

reaction

should be investigated

further,

s

this

obviously

would be

significant

with regard

to

th e

type

of

corrosion

mechanism

operating.

*A more

detailed discussion of

th e

effect of

al loy

composition

of ho t corrosion is

given

in

th e

next

section

on

Material

and

Coat ing

Deve lopment .


19/83

Since there is

th e

possibility of using vanadium-containing fuels in

gas

turbines associated

with

ship

propulsion,

o

0 contamination or

possibly contam-

2 5

ination

by other fuel

components

should be studied. Some

work

along

these lines

has

already

been

done,

as

shown

in

reference

3,

but as

in

th e

Na

SO contamina-

t ion case,

uch

more work

on

mechanisms remains o

be

done.

Closely

associated

with

ph enomena

already

mentioned

is

he

question

of sulfur

diffusion

a long

grain

boundaries

an d in th e grains of

nickel-base

super-

alloys. This solid-state

diffusion

process

is

of prime

importance

in

th e

formation

of

th e chromium sulfide inclusions so characteristic

of hot corrosion. One

aspect

that

is

of

considerable

practical

importance is

o

ascertain

those

conditions which

lead to excessively

deep

grain

boundary

penetration

by sulfur.

Like sulfur, oxy-

gen

tends

to

concentrate

in grain

boundaries and

to

form

internal

oxides

much

deeper

along the

al loy

grain

boundaries than

in

th e bulk

grains.

Research on Reducing

Hot

Corrosion

There appear

to

be

four

main

avenues

of

research

aimed

at

reducing

hot corrosion

attack.

Alloy

Composition

By comparatively

minor changes

in

al loy

composition,

t

has

been

found that

considerable improvement in

hot

corrosion

resistance is possible,

usually at a small sacrifice

in high-temperature

strength. However ,

s ncreased

demands are made

upon

th e

mechanical capabilities of

nickel-base

superalloys,

this road to

corrosion

resistance appears

less

feasible.

Coat ings

NiAl- type

coatings are

now

standard

in

military jet

engines,

where

th e

temperatures are

somewha t

higher

than

in commercial engines.

Through

coating,

considerable

improvement in

both oxidation resistance and hot


20/83

10

corrosion resistance is

secured.

However ,

ncreased

demands for

higher

metal

temperatures approaching

1800

o

F

982

0

C)), or

longer times between

engine over-

hauls,

nd for improved coating

reliability

make current coatings

less and less

acceptable.

Research activity

in th e

coating

field

shou ld

be

cont inued; hopeful ly

some

research

would

be

aimed

at

th e fundamental

aspects nvolved,

uch

as

measurement

of

diffusion

rates in moderately complex systems.

Rare Earth Addit ions

Fractional percentages of

rare earth metals, uch as cerium '

lanthanum

, nd

gadolinium

have

been found

to generally

improve hot

corrosion

resistance

in

binary

Ni-Cr

alloys

and in

superalloys.

Much

of this work

has been

carried out in

laboratory

tests, often of

th e

simple

crucible

type, but th e few

available

hot

corrosion

tests

in

hot-corrosion

rigs,

imulating

to

some

degree

actual

engine

tests,

ave shown

marked

improvement.

One

cannot

expect

that

rare

earth

additions

cou ld

add

more

than

about

1000

hours

of

life

to

jet

eng ine

blades and vanes.

The

principal

difficulty

with

th e

use

of rare earth additions

is

that a low melting phase

associated

with

th e

Ni

M/Ni

eutectic

where

M is a

rare

earth) occurs to

reduce

high-temperature

strength

properties.

Recent

laboratory

work,

owever ,

hows

hat

greater

improvements

in

corrosion

resistance

are

offered

by

rare

earths

when added in oxide

form.

If

verified,

his fact

m ay

mply

a

change

in

processing

techniques

hrough

which

rare

earths would

be

present

in

th e

al loy

as

a

well-dispersed

oxide

phase.

Mos t

important,

heoretical

rationali-

zation

and

understanding

of

th e

effect

of

rare

earth

additions

hou ld

be a t temp ced.

Fue l

Addit ives

Thisis

discussed

in

th e section

under

th e

heading

of

Environ-

mental Effects.


21/83

11

B.

High-Temperature

Oxidation

Imp roved

oxidation

resistance for

strong superalloys

is

desirable

in

blade applications. Higher

turbine inlet

temperatures

could be accommodated,

and

th e

requirement for coating

constricted

air-cooling

passages could be

avoided.

The problem

s

complicated

because

ew

strong

alloys,

uch s

B1900,

ave

excellent

oxidation

resistance

but suffer

hot

corrosion

attack.

Therefore, sep-

aration

of

th e

two

mode s of attack

is

not

realistic.

In

th e

past,

l loy

oxidation

i

has been a

research

area with an emphasis on

short-range

improvement of

exist-

in g

alloy

compositions,

with th e best theory and experiments in

alloy

oxidation

coming

from

other countries,

namely Germany,

ngland,

nd

France.

Sugges t ed

al loy

oxidation

research, with

only

short

bridges

to

th e improvement of commer-

cial

superalloys, ncludes:

a)

th e

use

of

radiotracers

and

h igh

resolution

techniques

to

establish

th e

role

of

rare

earth and

other

additions

ike

Si

and

Mn

on binary

and

ternary

alloy

oxidat ion;

b)

a

study

of th e role

of carbon

in dissolved

and combined

form

in

al loy

oxidation;

c) th e role

of th e common ly used

strengthening components,

W

and Mo,

n

alloy

oxidation particularly

for

Co-base

alloys;

d)

further attempts to study

th e

complex morpho logy

and

transport

in

th e metal/scale

interface

when combined

internal

oxidat ion

and

external scale formation are

occurring;

e) further

tudy

of

th e modes

of

failure

of

external Cr O

and

A l O

scales

and th e healing

processes;

f)

specific

studies

relating

th e

evaporation rate

of Cr

O to

th e

vapor pressures of th e several

vapor

species

for

th e

oxide;

g)

further

studies

of

the phases formed and

th e kinetics

of

internal

oxidation

of

alloys;


22/83

12

h)

studies

of

th e

high-temperature

mechanical

properties

of oxides

i.

e.,

creep)

and

th e

relation

of

these

properties to

scale

fracture;

i) preferential grain

boundaiy

attack

in pure

metals

nd

alloys

and

its

prevention;

j)

many

of

th e

more

fundamenta l aspects of

al loy

oxidat ion:

nucleation of oxides

on

clean

al loy

surfaces,

oping effect,

olu-

bilities and diffusivlties of

oxygen

in

nickel and cobalt, olubilities

and

diffusivlties

of

aliovalent

impurities

in

simple

oxides,

particu-

larly

CoO,

NiO, Cr

O and A l O.


23/83

13

REFERENCES

1.

Hot Corrosion Problems Associated with

Gas Turbines, ASTM

Specia l

Publication

No.

421

1967).

2. DeCrescente, . A .

nd

Bornstein,

.

S., Corrosion

,

4,

1968), 27 .

3.

ancock,

P., Corrosion

of Al loys

at High

Temperatures

in Atmospheres

Consisting

of

Fuel Combus t ion Products and

Associated

Impurities, Her

Majesty's

Stationery

Office, ondon, 1968).

4.

Quets,

J. M . nd

Dresher,

W.

H.,

ournal

of

Materials ASTM)

,

4,

(1969), 83.

5.

e r gman , aul

an d

Kaufman, Murray,

hompson

Engineering

Laboratory,

General

Electric

River

Works,

W.

ynn ,

Massachusetts

personal

com-

municat ion) .

6.

Bornstein,

.

S.,

nited

Aircraft Research Laboratories, . Hartford,

Connec t i cu t (personal

communicat ion) .

7.

DeCrescente,

.

A .

nd

Bornstein,

N. S., nited Aircraft Research

Laboratories,

. Hartford, onnect icut To

be

published).

8. imons , . L.,

rowning,

. V., nd Liebhafsky, . A., Corrosion, ,

(1955),

05.

9.

Bornstein,

. . nd DeCrescente, M. A., Trans. Met . oc. AIME, 45,

(1969), 947.

10.

eybol t ,

A . U., Trans. Met . oc. AIME, 42, 1968), 955.

11 . Wasielewski, . . Materials and

Processes

Laboratory,

General

Electric Company,

chenec tady,

N.Y., Report no. 68AEG430.


24/83

14

HI.

MATERIAL

AND

COATING

DEVELOPMENT

Introduction

Although

there are

many

types

of hot

corrosion

which

are

potentially

of interest

to industrial

gas

turbine

producers,

h is

portion of th e report

will

only consider those means

that

might lead to th e development of improved sulfi-

dation and

oxidation-resistant

alloys for turbines

operating

on diesel quality or

better

fuels.

This

section will

discuss

th e

current

status of

corrosion-resistant

cobalt-, nickel-, nd iron-base

al loy

developments,

ttempt

to

predict

what

realistic

improvements can

be expected in th e near

and

distant future,

nd

sug-

gest areas

for potentially fruitful deve lopmen t

activity,

A Committee on Coat ings of th e NMAB has

made recommendations

regarding

coatings

for

superalloys. We are

in agreement

with

these recommen-

dations. The pertinent recommendations

of th e

Coat ings

Committee are

con-

tained

in

Appendix

I.

For

this reason,

coatings

have

not

been

g iven

detailed

consideration

in

this

report

except

as

required

to

place

he

need for

coat ing

deve lopment in

perspective

with

the

need

for al loy

development.

Current

Status

In

th e

early

1960's, t was

apparent that th e

then-existing advanced

alloys did no t

possess

dequate

suifidation resistance

to

permit long-time opera-

t ion in

environments conducive

to

this type of

attack.

Hence ,

he

gas urbine

industry

devoted

considerable

attention

to

coat ing

deve lopment .

Alumin ide coat-

ings

were

found

to

provide

good corrosion

protection.

However , s overhaul

lives

were

extended,

he existing coatings broke down after

several thousand


25/83

15

hours

and

did

not provide adequate protection. This was

especially

true of those

alloys

developed for maximum

strength capability,

hich

required

compositions

of lower

chromium

content.

It soon

became

apparent

that

more inherent

sulfidation

resistance

was

required

of

turbine

blade

and

nozzle

guide

vane alloys.

The

Naval Ships Systems

Command

established

a

program

in

1964

with

th e

fol lowing

objectives:

1)

evelop a turbine

blade

alloy combin ing th e ho t corrosion

resistance

of

Udimet

500

in

1 %

sulfur diesel

fuel

combustion

products

with the

strength

and

ductility of alloy

713C

at

1600

o

F

100

hr rupture

life

at43,000psi).

2)

evelop a cobalt-base turbine nozzle alloy with th e 1900

C

F

strength of WI-52 100 hr

rupture

life at 10,

000

psi)

but

with improved

hot-corrosion

resistance.

These targets were accepted

by

th e metals industry, because, f met,

th e

alloy

would

offer

useful

properties

for all types

of turbines. Soon

several

alloys appeared,

uch

as MAR-M-421 Mart in Metals), d ime t 710

Specia l

Metals), N-738 The Ihtemational Nickel Company) ,

nd

more

recently,

AK-

M-432 (Martin Metals). * These

met

or came close to meeting th e

above

targets

for blades.

They

are currently being

evaluated by

he

gas

turbine

industry and

one, MAR-M-421,

s being

used in

production.

Several

cobalt-base

alloys

de-

veloped

in th e Navy-General Electric program meet

the

targets for a cobalt-base

nozzle material.

Extensive preliminary

studies

conducted by

several

investigators

on

simple

all

» y systems

suggested

that

chromium,

nd,

o

a lesser

extent,

itanium

and

cobalt

were

th e only

elements

that

contributed

in

a

beneficial

way to

sulfidation

Compos i t ions of all

alloys

mentioned

in

this

report

are

contained

in

Table

1.


26/83

16

W

ffl

<

S

tr -

io

IM

Iß

in

o i

^ 3

Ü

OI

I

I

I

in

o

o o

o

Tf

00

00 C O

t>

in

CD

t

o

o

o

o

o

o

o

o

J

b

—

^

o

1

r—i

1

00

e d

H

in

t>

00

h

o

oi

00

T— t

—

00

1

3

1

C O

00

eg

o

C «

in

C O

T ̂

05

O

d

N

Oi

CO

Oi

C O

w w

. .

.

|̂ „•

-•

in

N

Oi

5

00

CO

I

Oi

CO

I

M

—

O

CD

CO

00

o

d

in

0i

CD

O

OJ

Oi Oi

o

in

o

o

u

in

CO

tN

t^

O J

t>

s

^

2

* —

in

T;

o

eg

o

O -1 H H -1

u

N

o

C

a

o

.2

e

o

ü

to

o

o

o

o'

(M

ai

s

a

o


27/83

17

resistance of nickel-base alloys. Some elements,

uch as

tantalum,

were

bene-

ficial

solely

with regard

to

oxidation resistance.

Some ,

such as

molybdenum

and

a lum inum,

were

considered

detrimental

to

sulfidation

resistance. Other alloying

elements

were

generally considered

neutral

or

ineffective.

These

effects

are

no t

necessarily

true

for

more

complex

alloys. Within

the

last

two

years it

has

been

found

that

a

proper

balance

of

elements,

particularly

the

refractory

metals,

can

substantially improve

sulfidation

resistance

even at relatively

low

chromium

levels. For

example, preliminary

tests ndicate

that

a new

alloy, N-792

(12.7

Cr),

possesses

nearly

the

corrosion resistance

of

Udimet

500

(17.5

C r)

bu t

with

rough ly 50

C

F

ncreased temperature capability based

on

rupture

strength)

over alloy 713 C. It

is

still under

evaluation

and has yet

to be

proven

for

turbine

applications.

The rare

earths, as

will be

ment ioned

later, m ay

also

be useful.

While th e alloys currently

be ing

developed

offer

improved

high-

temperature

capability,

hey do

not

exhibit

a

corresponding improvement

in

intermediate

temperature

1000 - 1500

o

F)

strength.

Similarly, lloys developed

for

improved

sulfidation resistance

of ten

lack

good

high-temperature

oxidation

resistance,

nd,

ence ,

ay

have

to

be

coated.

Clearly, herefore,

he

al loy

developer

must give

more

consideration

to

achieving

an

optimum balance of

properties.

Smal l evolutionary improvements can be expected

to

occur

in

nickel-

and

cobalt-base

alloys,

but

th e

lack of clearly defined

requirements

and markets

for future needs

tends

o

limit

th e support

for development work by

th e al loy

industry.

An

industry-government

effort

is

needed

to def ine requirements in

terms

of strength,

corrosion

resistance, nd economics. Present generalities

such

as,

We need

an

alloy that

can

be

used

uncoated

in

place

of

th e

coated

alloy

XXX

which

we are

now

using, s n insufficient

base

upon

which

to

establish

an

objective

al loy deve lopment program.

This

will

be

particularly important in th e

deve lopment

of

fiber

composites

or

dispersion-hardened alloys.

These

materials


28/83

18

will

not

have th e same balance of

properties

with

which designers

are

familiar.

What

will

constitute

a satisfactory

olut ion

to

th e

hot-corrosion

problem

shou ld

be stated

in terms

commensurate

with

prudent

economics. This can

be

done only

when th e different

t ypes

of

gas turbines are

considered

individually , ecause an

alloy

adequate

for

one ype of service m ay be completely

inadequate

for another.

Some specific requirements of aircraft

turbine

builders are contained in

References

3,

4,

nd

5.

Alternate Approaches

A s

indicated, prior

efforts

to

improve

th e

hot-corrosion xesistance

of

turbine blade an d nozzle

guide

vane alloys

have

been largely

in th e area of

alloy-

in g element modifications and/or coating application.

Alternate

approaches o

th e

deve lopment

of turbine

blade

an d

guide vane materials with better corrosion

resistance/temperature capability

migh t

be possible if

new strengthening

mechanisms

were

evolved, which

would

permit

th e

use of

base systems

and/or

al loying elements

with

greater inherent

hot-corrosion resistance.

A

number

of

approaches have

been considered for use

with

nickel-

cobalt-,

or iron-base

turbine

blades an d

guide vanes. For the

purpose

of

this

report, n improvement was defined as

a

system

having :

c

Sulfidation

an d

oxidation

resistance

Udimet

500

0

Temperature-strength

capability

for

blades

>

N-100

0

Temperature-strength

capability

for

vane s

s WI-52

In

order

to

guide

establishment

of

future deve lopmen t programs

o

that

efforts will

be

concentrated

in

th e most fruitful

areas,

n assessment

was

made of

th e capability and probability of each

approach. Capability

was

considered

as an

indication

of th e theoretical

possibilities that

an

approach

offers for achieving

Improvement,

whereas Probability is

realistic

but

empirical assessment

of

th e

chances

for realizing that

capability.

The


29/83

19

considerations

were

made

in

keeping

with

current

technology and

a

realistic

extrapolation

of

that

capability

into

th e

near

future, .

e.,

975.

Results

of

this

estimation are summarized

in

Tables

an d

3

nd

are discussed

in

th e fo l lowing

paragraphs.

Dispersion Harden ing

Dispersion

hardening

is

effective

in strengthening

alloys

at

high

tem-

perature > 1600

o

F)

but

contributes little to low temperature


30/83

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Documents

Hot corrosion in GT