Desalination, 21 (1977) 147-164Q Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

BEHAVIOUR OF SOME DESALINATION PLANT MATERIALS UNDERSEVERE CONDITIONS: COPPER ALLOYS, STAINLESS STEELS,TITANIUM, AND PLASTICS* . **

K. R. FROHNERInstilatiu-r Werksto$technotogie der GKSS, Geesthacht (Germany BRD)(Received August 13, 1976)

SUMMARY

Materials for distillation plant, viz_ copper alloys, stainless steels, titanium,fiber reinforced resins, rubber lined steel, and plastic coatings have been com-paratively tested in the desalination materials test loop MEWAK I under condi-tions more severe than encountered in the final heater of common multistage flashevaporators: Maximum brine temperature 125'C, brine concentration factor 3,brine velocity in test condenser tubes 4 m/s, heat transfer through test condensertube walls up to 1 S watts/cm' . Oxygen has been kept below 10 ppb, pH in the 6.5to 7.0 range. The first test lasted 10,000 hours, but during the first 3500 hours theconditions had been reduced to a maximum temperature of 110°C and to the useof normal sea water . During the last 6500 hours of the test, barium sulfate seedingwas applied. Galvanic corrosion of metal components was eliminated by insulatingseat materials. The test results are given in 4 categories and are illustrated bymacrographs and micrographs .

1 . INTRODUCTION : DESCRIPTION OF Itst FACILITY

As the production of potable water by thermal desalination of sea water indistillation plants is relatively highly charged by the costs of plant constructionmaterials, especially condenser tubes, development towards cheaper and/or longerstanding materials plays an important role . To assist the materials industry in

• Paper based on a joint program under management of GKSS. Contributors : F. Bulda, K. R.FrShner, F. 3. Schmitt (GKSS); R. Meyer (Hatter Apparatcwcrkc KG); IL Kopf, IL Stiffer(Wieland-Werke AG); J. Burggraf (Metallgesellschaft AG) ; A. Ismer (TEW-Contimet); G.Lennartz (Thyssen-Edelstahlwerke AG) ; D. Grimme (Vercin Dcutschcr Eisenhiattenleute-VDEh); G. Herbsleb (Mannesmann-Forschungsinstitut GmbH) ; D. Hirschfeld (Fried. KruppGmbH); H. E. Bock, D. Kraatz (Stahiwerke Sudwestfalen AG) ; L. Auslitz (Tegon GmbH,vormals Metallisator); G. Lassak (Deutsche Fibercast GmbH) .** Presented at the Fifth International Symposium on Fresh Water from the Sea, Aighero,May. 16-20, 1976.

148

brine

vacuum

brine--to-air

feed ~ `

k t / heat exchanger

blow down

Fig. 1 . Simplified flow diagram of test loop system MEWAK I .

development and test ng, the test loop system MEWAK I was installed at theGKSS research centre at Geesthacht in 1972 (1). This test loop system (simplifiedflow diagram Fig. 1) consists mainly of 6 experimental heat exchangers holding 8test condenser tubes 16 0 x 1000 mm each, a brine circulation pump, a brine-to-air heat exchanger, and measuring and control instrumentation_ Most parts ofthe system were regarded as test objects : piping, valves, vessels, and pumps . Thetest and corrosion conditions of the first continuous 10,000 hours run of MEWAKI are compiled in Table I .

TABLE I

CORROSION TEST CONDITIONS OF MEWAK I DURING THE F'IRSt

FROHNFR

Test duration About 3,500 hours About 6,600 hoursFeb. 15-Aug. 8, '73 Aug. 8, '73-June 13, '74

Brine concentration About 3,5% About 10°/a (1,000 hoursreduced to 7%)

Maximum brine temp. 110°C 125°CAdditions to the brine None 1% BaSO4, suspendedCa++ concentration About 400 ppm Initial 130 ppm,

later 400 ppmBrine velocity 4 m/s(condenser tubes)Heat transfer (through About 15 W/cm2, temporarily less than 15 W/cIn'condenser tube wall)Oxygen cone. (brine) Generally less than 10 ppbPH of circul, brine 6_5 to 7_0Refurbishing (brine) 3 1/hr (5001 test loop capacity)Inspection breaks After 1500; 3050; 4350; 5800; 7920 hours

DESALINATION PLANT MATERIALS UNDER SEVERE CONDITIONS

1 49

Obviously, these test conditions correspond to those encountered in thefinal heater and the first flash chambers of multistage flash evaporators but aremore severe with respect to brine concentration, brine velocity, and heat transferin order to accelerate corrosion and wear processes . After running for 3,500 hoursunder reduced conditions because of the sulfate scaling problem, this problemseemed to be solved by application of BaSO4-seeding (2) . Positive results oflaboratory seeding experiments enabled us to raise the test conditions to thedesired level as indicated in the right hand part of Table I .

Sea water brine was produced from original sea water (North Sea coast)by vacuum evaporation, preceded by ozonisation, filtration, acidification (H 2SO4)and degassing by washing tower and vacuum deaeration . The circulated brinewas refurbished at a rate of about 3 1/h . With one exception the concentration ofthe Cu" ion was kept below 50 ppm, mostly below 20 ppm. Ni" and totaliron (not regularly determined) remained in the ppb region .

Heat transfer through condenser test tube walls was forced by condensationof saturated steam of at least 130 °C. Four test heat exchangers included electricalheaters to produce steam directly in the lower part of the vessels . Two test heatexchangers were supplied from a steam raising unit . In the course of the 10,000hours run, one "electric" heat exchanger was changed to the process steamoperating mode .

In the heat exchangers the test tubes are detachably sealed by O-rings . Theconstruction includes two sealing points against brine and steam on each end of thetube (1). The actual heated length of the test tubes is thus reduced to about 0.8 m,and comparison between the heat transfer section and the unheated section of atube can be done. During inspection breaks, the test tubes were cleaned, weighed,and reinserted . In some cases the loss or increase of weight was not directly cor-related to corrosion phenomena because of badly removable deposits in the tubes .Galvanic corrosion was generally avoided by insulating gaskets between the testtubes and all components of the test loop system .

The participants of the joint corrosion programme, viz . firms of the Germanmaterials industry, collaborate with GKSS under contracts, covering materials,samples, test conditions, analysis, interpretation and publication of test results .The programme is continued and several test objects reported here continuetesting during the second run of MEWAK I which is now under way . The resultsof the first corrosion test run will be given in 4 groups under this .

Similar corrosion tests are reported by G . Boisde, H. Coriou and coworkers(3, 4). Though their maximum test temperature reached 150°C, their tests werecarried out with sea water of normal salt concentration and without heat transferthrough test condenser tubes .

150

2. COPPER ALLOYS

2.1 General remarksCopper base alloys containing 20 % zinc (± 2 °o Al) or 10 % or 30 % nickel

(+ 1 % Fe) are standard materials for sea water operated heat exchangers, con-densers, and desalination plants. If the well-known operation limits given inTable II are maintained the general service life is satisfactory .

The present study included testing of some modified alloys in reference to thestandard alloys and aimed at more corrosion resistant alloys and more economictube manufacturing. The various copper alloys compiled in Table III have beentested in the form of condenser tubes produced by drawing or by longitudinalseam welding . The enhanced test conditions compiled in Table I included in-creased turbulence at the inflow of the tubes, low oxygen supply from the corrodingfluid, scheduled operation periods interrupted by drying-out periods (inspections)and incrustations that developed more or less unintentionally and under whichseparate corrosion conditions might have developed . The copper alloys testedranged from CuZn20A1 to modifications of this alloy by addition of Ni, Fe, Pand more Al, to the CuNi 10Fe alloy and to a modified CuNi30Fe alloy. Thepost-test examination took place in the laboratories of the two tube suppliers(also partly at GKSS) according to the different aspects aimed at .

2.2 Corrosion resistance and oxide layer formationThe CuZn20A1 alloy developed only thin oxide layers containing traces of

Mg, Si, and Cl, and slightly enriched Al, and suffered from intergranular corrosionbelow this oxide layer (Fig . 2). The depth and severity of this intergranular attackdiffers locally but does not exceed 20 pm in depth, measured from the oxide layer .More interesting is the CuNilOFe alloy . The test tubes showed oxide layers ofabout 16 pm containing traces of Si, Cl, Mg, and Al as was shown by energydispersive electron analysis .

TABLE II

GENERALLY ACCEPTED OPERATION LIMITS OF STANDARD ALLOYS

K. R. FROHNER

Corrosiontendency/sensitivity

Stress corrosion cracking ;ammonia, nitrates, nitrites

Pitting in new condition,with low oxygen supplyand water impurities

Alloy Maximumoperationtemperature °C

Theoreticalmaximumflow velocity, m/s

Cn7n70A1 120 2.5

CuNilOFeCuNi30Fe

150 4

TABLE III

151

DATA OF CORROSION TESTS WITH COPPER ALLOY CONDENSER TUBES

16 m

m ou

ter

diam

eter

, 10

00 m

m in

leng

th.

bd=

brea

kdow

n, b

el =

bel

ated

ins

erti

on i

nto

test

loo

p.

Notes:

•)al

loy

pate

nted DBP 2353 238,

••)J

apan

ese

stan

dard

JIS

H 3

632.

+) 1

st f

igur

e: total loss of weight of 1

.0 m

tub

e le

ngth

; 2nd figure related to the heat transfer zone

(0.8 m) only

. ++) figure

not

cor

rect

, tu

be c

onside

rabl

y sc

aled

.

CuZn20AI (

SoMs

76)

Hecorros 76 ® F 40

0.01z.20

50.1 5_0

.07 2.

08 0

.024

0.9

5

7250

bel

2.7 -

Oxid

e la

yer

of 8

µm

thic

knes

s,DIN 1785/DIN 17 660

900 bd -

-

unde

r it

par

tly

inte

rgra

nula

rAS

TMB

111

6165

bd3.

58-

corrosion

1.0

9020

2.58

1.78++) Erosion at inflow, one perfora-

6000 4.29 2.73 tion directly behind encrusta-

tion

dot

, ox

ide

laye

r pl

ain

and

sleek

CuZn

20AI

DIN

1785/

DIN 17 660

ASTM B 1

11Wi

elan

d-S20

CuZn

20AI

refe

renc

e tu

be

-

se 21.

8

-

<0.0

5 1.95 0.

026

1.25

3570

5.8

-

Perf

orat

ion

near

inf

low

DIN

1785

referencetube

6725

6.3

-

Perf

orat

ion

atin

flow

CuZn20AI + 0

.3%

Fe + 0

.1%

P an

neal

ed,

-

.. 20

-

0 .3

2 .0

0.02

1.0

9320

bd

3.1

6 2

.45

Erosion at inflow and in tube

pick

led'

)

(0.I

P)

9320

bd

3.16

2.40

cent

er b

ehin

d in

crus

tati

on d

ot,

oxidela

yerpl

ainan

dsl

eek

CuZn

20A1

+ 1% Ni**) annealed, pickled

1.0

18

-

-

2.0

0.07

1.0

10,000

2.79

2.04

Eros

ion

at i

nflo

w, o

xide

lay

erpl

ainan

dsl

eek

C 688 00 +

0.0

7 As

induction welded,

-

22.6

-

-

3.4

0.07

1.0

10,0

00

3.05

1.92

Eros

ion

at i

nflo

w, a

t ro

ugh

(+ 0

.4%

Co)

drawn, annealed,

weld

and

beh

ind

incr

usta

tion

pick

led

dotat

infl

ow,weld

perf

ect

CUNI

FER

10

10

-

0.7

1.5

-

1.25

8150

bd

-

-

Oxide layer 15 pm thick,

®F

30

10,000

(2x)

part

lyne

tted

cracks

welded (WIG),

ras 10

0.7

1.5

-

-

0.8

10,0

00

3.52

2.7

8

Little erosion of oxide layer at

gage

d

infl ow,

weld

perf

ect

CuNi30Fc2Mn2 CUNIFER 302 8

31.0

-

1.9

2.0

-

0.9

8850

bel

-

-

Oxid

e la

yer

30 t

o 60

,ur

n th

ick

6950

bel

CuNi

lOFe

DIN 1785/DIN 17 664

ASTM

B 111

anne

aled

, no

t pi

ckle

d

annealed, pickled

weld

ed (

WIG)

, ga

ged,

no s

tres

s-re

liev

ing

5 0

.1

^w20

;90.1 X0

.07 2.

0

0.028

1.0

9000

3.0

2.38

0.5

2780

bd

3.75 -

Intergranular corrosion and

residual stress caused break-

down

of

weld

Alloy description, con

diti

on a

nd r

emar

ksno

mina

lanalysis

(%),

rema

inde

r =

copp

erWall

Test

Loss

of

welg

ht+)

Cor

rosi

on/e

rosi

on r

esul

tsNi

ZnMn

FeAl

Asthick- duration

g/dm

2(1

0,00

0 h)

ness

(mm)

(h)

152

Fig. 2. CuZn2OAi condenser tube. Left : Inner tube surface showing oxide layer developedduring the corrosion test . 200 x . Right : Micrograph section showing intergranular attack belowoxide layer. 1400 x (electron picture) .

Enhanced contrast reproduction of the reflection electron microscopere (Fig. 3, left) shows that the oxide layer is cracked, the cracks forming a net

structure. According to Kievits and IJsseling (5), not-hydrate bound watermolecules are present in the passive oxide layers of CuNiFe alloys, if these layershave developed under the influence of sea water. In the present study the testperiods have been interrupted by drying-out periods. The drying caused contractionand cracking of the oxide layers and resulted partly in the formation of "scabs"(Fig. 3, center) that might flake off . There is enhanced probability of pittingcorrosion (Fig. 3, right), depending on the service conditions after the interrupt

K. R. FROHNER

Fig. S. Inner surface of CuNilOFe condenser tube after 10,000 hours of corrosion testinterrupted by inspections (drying-out) . Left: Oxide layer showing netted cracks . 200 x . Center :Formation of "scabs". 100 x . Right: Pitting corrosion under flaked-off scab . 100 x .

DESALINATION PLANT MATERIALS UNDER SEVERE CONDITIONS

153

period . Impure waters of low oxygen content such as harbour waters might preventthe healing of the oxide layer. With respect to the surrounding perfect oxide layera point of less noble potential is thus formed, causing local anodic dissolution ofthe metal in the form of pitting.

The brine circulated in the MEWAK I system was clean with respect toimpurities found in natural sea water and harbour water, but its oxygen con-centration was generally low. During the start-up periods following an inspectionbreak, viz. when "fresh" brine had been filled into the system, oxygen concentra-tion was presumably enhanced and served for healing the oxide layer in most cases.

The CuNi3OFe2Mn2 alloy developed thicker oxide layers ranging from 30 to60 pm. Alternating service and drying-out conditions caused some more flakingof oxide layer scabs as described in the foregoing section, but altogether thehealing of passive layers seemed to occur satisfactorily.

The copper nickel alloys showed no intergranular corrosion under oxidelayers or incrustations and are consequently superior to the CuZn20Al alloy withrespect to these corrosion phenomena. Emphasis must be laid on clean waterconditions during start-up of a system and on continuous operation respectivelyavoidance of drying-out periods.

2.3 Incrustation tendency, erosion, and loss of freight in different zones of the tubesAs pointed out in the introductory chapter it was the purpose to solve the

sulfate scale problem by barium sulfate seeding, because a complete removal of thescale forming calcium and sulfate by chemical precipitation or ion exchange wasconsidered-by all concerned-to be an unnatural modification of the sea waterbrine corrosion conditions. During the initial period of barite seeding application(several months) the free calcium concentration was found to be continuouslyreduced from 22 to about 2.5 moles per cubic meter respectively 100 ppm, andreally no scale appeared on heated surfaces . After some months the operators werealarmed by increased free calcium concentrations near 10 moles per cubic meter-1 400 ppm, and repetitive application of barite seed did not reduce this concentra-tion . Fortunately no scaling was found during the next inspection break, but asthings went on localized scale appeared on some test condenser tubes. Partly thiscould be related to a failure in the electric control system, viz. no cutoff of heatercurrent in case of stagnant brine flow (failure of the circulating pump), partly thescale formation might be related to insufficient effect of the seeding procedure. Thereason is not yet clear but one has to take into consideration : saturation effectsof the seed material, inactivity of the seed used, formation of a "loaded" andcalcium supplying seed depot in dead corners of the closed looF system, or in-stability of "loaded" seed in the presence of energetically more favorable scaledepots. As a matter of fact, scale did not develop on all heated condenser tubesurfaces. In some cases scale disappeared during the operation period following aninspection break, and in some cases part of thick scale flaked off, causing local

154

turbulence followed by localized erosion and perforation of the test ;ube.The maximum erosion stress was concentrated at the inflow parts of the

condenser tubes. In a few cases CuZn20AI failed because of severe erosion in thispart (Fig. 4) ; the CuNi IOFe tubes showing better erosion resistance (Fig . 5, above) .The outflow parts remained free of erosion in all cases (Fig . 4 and 5). Smallererosion marks occurred close to unsmoothed seam welds and partly nearby deepdrawing grooves. Smoothed and drawn welds remained without erosion marks .

As to the influence of the suspended barite particles on the erosion effectsobserved, it should be mentioned that the erosion marks found at the inflow partsof the tubes are quite similar to those found in experiments with rapid flowingpure well water (6) . In the experiments quoted, severe erosion scars have beenproduced by vortex action at discontinuities of the tube systems . Therefore thedirect influence of the suspended barite particles on erosion stress is consideredto be negligible.

The loss-of-weight figures of Table III (first column) indicate only thetendency of combined corrosion-erosion behaviour of the test tubes because theseeffects could not be separated. There is also much dispersion of the loss-of-weightresults (second column) calculated for the center part of the tubes, i .e. the heatedsection of about 0.8 m in length, by weighing the center part of the tube andcomparing it with the theoretical mass of this part : The tendency is evident that theadditions to the CuZn20A1 alloy improved its corrosion resistance by formation of

K. R. FROHNER

Fig . 4. Above: CuZn2OA1 condenser tube after 9020 hours of corrosion/erosion test .E = inflow, A = outflow. Below: Inflow of CuZn2OAl reference tube, 6725 hours .

DESALINATION PLANT MATERIALS UNDER SEVERE CONDITIONS

Fig. 5 . Above; CuNi10Fe condenser tube, 10,000 hours. E = inflow, A = outflow .Below : outflow of CuZnLOAI reference tube, 6725 hours .

more a

ec i

e corrosion conditions,the base alto

a protective layer. On thecontrary the CuNi lOFe alloy formed an oxide layer with relatively limiteherency to the base material, as can be derived from its loss-of-weight figure .

Summarizing the copper alloy corrosion test results in a common conclusion,the evidence is clear that all of them are able to form protective oxide layers towithstand the severe corrosion/erosion/heat transfer test conditions for a satis-factorily long period, but Zn-alloyed copper tends to intergranular attack whereasNi-alloyed copper is free of unpleasant corrosion phenomena also under extremeconditions.

3. STAINLESS STEELS

The tested materials and the results are compiled in Table IV . Included aretype of test object, test duration, and corrosion phenomena . The abbreviations areexplained in the table.

It is necessary to explain the short test duration of some of the condensertubes. About 2,000 hours were lost since the tubes and the test heat exchangerswere not available when the test run started. Furthermore, in some cases brineleakage at the sealing gaskets was followed by salt deposits and unknown highchloride concentrations outside the tube and in the O-ring seal crevices . This

155

TABL

E IV

COMPILED RESULTS OF MEWAK I

CORROSION TESTSOFVARIOUSTYPES OF STAINLESS STEEL

bd-b

reakdo

wn,

CT-c

ondenser tube, ctd

.-test is continued, nc-nominal composition,'-detailed comment in the text

.

156

rAM

Type

German

Iden

tifi

cati

onof steel

"Werk-

stoff"-

Number

Analyses (%) of actual test objects or "tic"

Type

of

othe

r

test object

constituents

Test

duration

(hours)

Corrosion

phenomena

and remarks

CN

CrNi

Mo

Ferritic-

1.44

17X

2 Cr

NiMo

Si 1

9 5

0.04

4-

18.31

4.65

2.64

Si = 1,76%

2CT, seamless

1256 each

Pitting

-aus

teni

tic

1.44

62X

2 Cr

NiMo

N 22

50 .

022

0 .12

22.7

5.4

2.96

dega

ssin

g ve

ssel

4910

No c

orro

sion

-1.

4573

X 10

CrN

iMoT

i 18

12

0.06

0-

17.2

914

.00

2.82

Ti = 0.3

5%

CT4320

Pitting

CT75

6Pitting

1.4438

X 2

CrNi

Mo 1

8 16

0.02

5-

19.4

016

.87

3.16

-

2CT, seamless

1180 each

Beginningofpitting*

1.44

39X 3 CrNiMoN 17 13 5

0.04

30.

1418

.06

13.4

64.

12-

CT, seamless

3812

No c

orro

sion

Aust

enit

icCT, seamless

CT, seamless

1180

1180

Beginningofpitting*

No c

orro

sion

1.4465

X 2 CrNiMoN 25 25

0 .03

40.

1724

.48

22.3

82 .

05-

2CT

5730

eac

hIn

terg

ranu

lar

atta

ck

1.4511

X 10

CrN

iMoT

i 18

10

<0.0

817

.512

.02 .

3Ti z 0

.4% nc

flow meter

5812

and

pitt

ing

outs

ide

tube*

Seve

re p

itti

ng1.

4580

X 10

CrN

iMoN

b 18

10

<0.0

817

.512

.02 .

3Nb Z 0

.64% nc

flow meter

5812

Seve

re p

itti

ng1.

4448

G-X 6 CrNiMo 17 13

<0.0

617

14.0

4 .5

-nc

ther

mome

ter

well

3050

Seve

re p

itti

ng1.

4449

X 5

CrNi

Mo 1

7 13

1Cr-Mo 20-5

0.003

0.00

119

.93

0.02

4.85

-CT

, we

lded

2000

Crevice corrosion

3CT,

sea

mles

s7100

outs

ide*

Crevice corrosion

"Superferrites"

Cr-Mo 28-2

0.00

20 .

001

28.0

00.

022.

01-

CT,

seam

less

5500

bd

outs

ide*

Crevice corrosion

(W.-No

.1.4133)

outs

ide*

2CT,

sea

mles

sCl

eari

ng o

f te

st2C

T, s

eaml

ess

3600 each

1400 each

obje

ct p

osit

ion,

Cr-Mo-Ni 20-54-Ti

0.00

20.

014

20.0

04.

385 .

16Ti = 0

.22%

CT, welded

8500 ctd

.

inse

rtio

n of

a m

ore

inte

rest

ing

obje

ctNo

cor

rosi

onCT

, we

lded

5500 ctd

.No

cor

rosi

onCr

-Mo-

Ni 2

8.2-4-Cu

0.00

30.

002

27.3

64.

072.

09Cu = 0

.78%

CT,

weld

ed5500 ctd

.No

cor

rosi

onCr

-Mo-

Ni 2

8-2-

4-Ti

150

0.002

0.01

128

.13

4.11

2.27

Ti = 0

.25%

CT,

weld

ed55

00No

cor

rosi

onCT, welded

5500

ctd

.No

cor

rosi

onCT, welded

8500

ctd

.No

cor

rosi

onCr

-Mo-

Ni 2

8-2-

2-Nb

150

0.004

0.01

228

.15

4.00

2.08

Nb = 0

.27%

CT,

weld

ed55

50 c

td.

No c

orro

sion

Cr-M

o-Ni

28.

2-4-

Nb 4

000.

016

0.026

28.0

03.

511.

95Nb = 0

.37%

CT, welded

8500 ctd

.No

cor

rosi

on

DESALINATION PLANT MATERIALS UNDER SEVERE CONDITIONS 157

as followed by pitting corrosion and in two cases by trans-gratin ar stress corrosion cracking. Obviously these phenomena have little valuein the critical examination of the materials behaviour because the corrosionconditions were not well defined . During inspection breaks, the O-ring seals werechanged to packing box O-ring seals . The corrosion process mentioned above didnot occur further.

All stainless steel tubes showed coverings or deposits inside. The depositionstarted in the heat transfer center part of the tubes and reached the inflow andoutlet parts of the tubes in the course of the test run. The deposits grew to amaximum thickness of 4 mm and consisted mainly of about 75% calcium sulfateand 25 % barium sulfate .

The steel surface inside the tubes, when covered with deposits, suffered frompitting corrosion in several cases . W: No. 1 .4573 showed a number of pits withoutperforation . Tube inside of W.-No. 1 .4438 showed many small and shallow pitsmaximal 50 pm in depth . Presumably the pitting process stopped soon after itbegan, showing a re-passivative process. From the tubes of W.-No. 1 .4439 onlyone sample showed a few shallow and small pits indicated by rust spots in thegreyish-white deposit inside the tube. Tubes made of W.-No. 1.4465 showedintergranular attack . In many cases the damaged grain boundaries act as nucleifor pit formation. This behaviour, not common for W .-No. 1 .4465, was examinedin more detail. A structural anomaly was found, obviously arisen during the tubemanufacturing process. A section of 200 pro thickness adjacent to the inner surfaceof the tube showed precipitations of chromium nitride along the grain boundariesand in the grains, too . As the nitrogen concentration in this zone was increased to0.48 wt: %, also lamellar nitrogen-pearlite was formed in the grains . Theseprecipitations caused chromium-depletion in the surrounding matrix, and thecorrosion resistance of W.-No. 1 .4465 was no longer maintained . Consequently,the results of the corrosion tests of W.-No. 1 .4465 in MEWAK I are caused by astructural anomaly and are not representative for this stainless steel quality .

The ferritic-austenitic steel quality W .-No. 1 .4417 (50% ferritic) underwentpitti g corrosion, starting from the inner tube wall, but no perforation was found(Fig. 6) .

The ferritic-auste 'c steel quality W.-No. 1.4462 was tested as constructionmaterial of a thermal degassing vessel . During operation it was filled with seawater brine (concentration factor up to 3.0 i.e. 10.5% salt concentration, reducedcalcium concentration), heated to 102'C by steam, and maintained at that temper-ature ± I °C. No kind of corrosion sign was detected after an operation periodof 4910 hours.

The chromium-molybdenum types of "Superferrit" steel have been testedas condenser tubes in MEWAK I. During the tests crevice corrosion was observedwhich under normal conditions is not typical for these types of steel . Brine pro-truding from the seals formed salt deposits under which high and uncontrolled

Fig. 6. a) Fernttcsample of pitting corrosion oc ti

1K. R. FROHNER

e concentrations have developed. Thus crewtubes, at least after 7,100 hours, although the inn

urfaces o

tubesbeing continuously in contact with the brine flow remained free of corros n, to agreat extent .

A few rust spots had for me

I

with inner surface here

rison with the foregoin resul

nickel-e "Superferrit" (Fig. 7 showe

insideoutside on locations of salt deposit. T

ationmentioned in Table IV was prolonged in the second test run of MEWAK I o over10,000 hours by Oct. 1975.

Test loop components made of common austenitic steels W .-No. 1 .4571,1 .4580, and 1.4448/49 had to be replaced by more corrosion resistant materialsafter relatively short operation periods .

Summarizing the results obtained with the stainless steels investigated itsho id be

1 .4417, 1 .4573, and 1 .4438 sufferred

NDER SEVERE CONDI NS

1 59

Fig. 7. Prepared metallographic sample showing microstructure o

from pitting, the p tting sevent i ciprocally correlated to increasingmolybdenum content with 1 .4439 practically free of corrosion attack under thetest conditions in question. The same result as for W.-No. 1 .4439 should beexpected for 1 .4465 with a high content of both chromium and nickel if a normalstructure is achieved at the inner tube surface . "Superferrit" containing Ni wasfree of corrosion, and Ni-free "Superferrit" showed the same under the intendedcorrosion conditions. Good endurance and corrosion r should also beexpected by steel W.-No. 1 .4462 under the conditions of desaltn

I

4. TITANIUM, TITANIUM ALLOYS

Co rcially pure titanium an ckel alloye nium (1 2%) weretested mainly in the form o nally seam welded con e bes ofdifferent wall thickness down t The tubes and other test objects arecompiled in Table V. Included are characteristics of the metals and alloys and thetest durations . Typical microstructures of cp titanium grade 1, 2, and 3 are shownin Fig. 8.

Fig. 8. Typical microstructure of cp titanium . From left to right: Grade I (W.-No. 3.7025'grade 2 (3.7035), grade 3 (3.7055). Magnification 200x .

brine leakage and' salt deposits sometimes formed a stagnant co osivc environ-ment. The deflector plate (Fig . 9) that turns the hot brine stream downward whenentering the buffer tank and is exposed to erosion stress showed no attack . It wasaltogether not necessary to include a column "corrosion phenomena" into Table V.

In spite of the well-known sensitivity o

alloys to PTFE gaskets

14,The corrosion attack was removed by ma hining, and the object put in

rationagain . (ii) One of four flat titanium rings used with induction type flow meters toprovide a sure earth potential of the circulating brine suffered from crevicecorrosion and probably galvanic corrosion and had to be replaced after a relativelyshort operation period.

According to N. G. Feige (7, 8) pitting corrosion should be expected for cpconditions of temperature and

NaCl cone ntratio ealized in MEWAK L Obviously the Feige diagram is con-firmed in this respect by the above results and gives a posi

stFurther observations marked by asterisks in Table V: (*), The impeller of

st pump -showe s me small blowholes when inspected for the first

cor " in Table V). There waselo ' fabout 4 m/s

K. R. FKtHNER

DESALINATION PLANT MATERIALS UNDER SEVERE CONDITIONS

TABLE V

COMPILATION OF TITANIUM AND TITANIUM ALLOY CORROSION TEST OBJECTS

-comments I

390-540 N/mm2 CT, welded

CT, weldedCT, welded

390-540 N/mm2 Tube with flanges570 x 2.5 x 1120 mm,seam weldedDeflector plate2 flanges

Unknown

Titan Contimet 35 0 3.7035(analogous ASTMgrade 2)

Titanium casting

TIKRUTAN ItT 12TINil ; TiNi2

TIKRUTAN RT 12 0CuNIIOFe; CuNi3OFe

TIKRUTAN RT 12 0TiNi 1TiNi2

Unknown --

4 electrodes forgrounding/earthing

Unknown -•- -

CT, weldedCT, weldedCT, welded4CT, welded

Pump casePump impeller

test pairs

Crevice corrosiontest samples

703370338080 each

9600 ctd.

10,000 ctd .10,000 ctd .

161

ctd.-test is

10,000 ctd . cor.

10,000 ctd .

cor. (i electrode)

43505480815010,000 ctd .

10,000 ctd .10,000 ctd .*

5900"'

time.

blowholes did n

ring the test run although the pumpoperated sometimes with cavitation noise .

(**) Combination of titanium s

er-nickel alloys to form contact corrosion pairs (Fig. 10) showe

aniccorrosion of the copper alloys, titanium being the "nobler" metal. In a separate

Type of alloy, "Werk-stoff'Number

ep titanium grade Icp titanium grade 2 3.7035cp titanium yade 3 3.7055

TIKRUTAN RT 15 0 3.7035

TiNi ITIM 2

TIKRUTAN RT 15 0 3.7035

Ultimate Description of Test durationtensile strength test object (hours)

162

Fig. 10. Titanium specimens after exposure to MEWAK I conditions. (The arrangementswere hanging downwards during the test.) Left : Contact corrosion pairs . Right: Crevice corrosionarrangement .

test, specimens of cp ti

TiNi l, and N

o crevice corrosion(Fig. 10) .

The titanium test results, in general, proved the good usability of the metalbut also-in some cases-its sensitivity to PTFE gaskets.

5. PLASTICS

In this group different non- allic materials are co ' ed .

5.1 Lined steel components5.1 .1 . Piping and valves NW 50, NW 65, and NW 80 win (NW = no

width) of the MEWAK I system are mainly constructed of mild steel lined withBornumharz 6101 0 . This material, based on phenol formaldehyde resin andfilled with anorganic filler (under which graphite), has been applied to the steelsurface (at the contributors works) in uncured condition as a sheet 3 mm in thick-ness, and hardened in pressure autoclaves by polycondensation . The coefficientof thermal dilation of the material is 7 to 10 times 10-6, its adhesion strengtsteel about 100 kg/cm = and its heat distortion temperature according to Martens(temperature of deflection under load, DIN 53 458) is 160°C .

The behaviour of the material was checked at all inspection breaks and wasinspected thoroughly after completion of the 10,000 hours run.

Visual examination: without findings .Check of adhesion : normal adhesion to steel surface .Dye penetration test of pores and cracks : without findings.

The objects were put into service again and continue the test.

K. R. FRt3HNER

DESALINATION PLANT MATERIALS UNDER SEVERE CONDITIONS

163

5.1 .2. The MEWAK I buffer tank is lined with Vulcoferran 2200 c® . Thismaterial, based on butyl rubber and filled with anorganic and organic fillers, is alsoapplied to the steel surface in uncured condition, preferably 3 mm in thickness,and is vulcanized in pressure autoclaves at elevated temperatures in the presenceof sulfur. The resulting lining is elastic, resistant against many acids, caustic andsalt solutions ; hardness 60 Shore A, adhesion strength to steel surfaces 25 kg/cm',normal temperature limit 120°C .

Inspection of buffer tank after the 10,000 hours run gave the following results .High voltage spark test of pores and cracks : without findings .Visual examination_ Many blisters of 10 to 15 mm diameter, partly linked orcombined, probably caused by the well-known steam diffusion . Those parts of thelining which are strained by mechanical pressure such as it-plate gasket seal jointsare deformed by flowage . (Hard rubber gaskets should have been preferably used) .

Result: The buffer tank lining operated to satisfaction for 10,000 hours ; itwas put into operation for some more thousand hours . But it was decided to ordera longer standing material and replace the vessel in the course of the continuationof the tests .

5.1 .3. Materials which have been tested as linings without success : 3 types ofcommercial rubber based materials. One fiber reinforced type of polyester resinwas without success tested as a construction material .

5.2 Reinforced resins (epoxy resins)Part of the MEWAK I piping system was constructed of reinforced resin

units, viz- tube sections, bends, reductions, bushings, flanges, mostly of 65 mminner diameter . Most of these parts were bonded to each other, in a few cases theywere screwed together using fixed flanges . Flange material was pressed reinforcedresin. The stress conditions corresponded to those compiled in Table I, they reachthe upper limits of operation conditions given by the manufacturer .

Results: The reinforced resin pipe system served satisfactorily during the10,000 hours test run under question . Occasionally a bond had to be renewedduring the inspection breaks. At first sight the necessity of repairs was referred toour own lacking experience in bonding techniques . But when the system was putinto operation again after completion of the first 10,000 hours run, those defectsand repairs began to accumulate . Supported by the manufacturer it was decidedto change to the system "fixed collar-loose flange" . All pressed parts were replacedby centrifugally casted parts from Deutsche Fibercast GmbH . The number ofdefects and repairs decreased .

K. it. FRORNER

5.3 CoatingsSeveral types

combinatl s and phenol fo

dresin have been applied to carbon steel components of MEWAK I as protectionagainst corrosion .

The vacuum degassing vessel of 0.15 m3 volume operating at about 50°Cin the 0.1 bar pressure range with slow but continuous flow of sea water brine(concentration factor 3) was initially coated by "Metallogen DT-hell" about200 ym in thickness. The vessel was not heat insulated and consequently thecoating was subjected to a temperature difference of about 20°C . The bakedcoating, sensitive to steam diffusion, behaved insatisfactory as could have beenexpected . After 3500 hours of operation the coating was replaced by 250 pm ofthe baked epoxy resin type "Metallogen DV" . This coating, regarded to be morediffusion-proof, turned out to be insatisfactorily, too . It was replaced by a 200 pmbaked coating of phenol formaldehyde resin, type "Metallogen ATV, whichcoating so far gave no rise to any complaint and is still in operation .

The heater of the vacuum degassing vessel, a steel block equipped withinternal electric heaters, was coated by "Metallogen DT-hell" which coating stoodthe test.

Several steel thermometer wells protruding into the circulating brine (125 °C,concentration factor 3) have by way of trial been covered by various bakedcoatings, but without satisfactory results .

The MEWAK I corrosion test program, whose second part is now underway, could not have been carried out without enormous efforts of all involved inplanning, construction, operation, analysis, evaluation and interpretation . Thecontribution of Miss L. Stahlenbrecher and numerous members of the GKSSstaff (TK, WK, WX) is gratefully acknowledged .

REFERENCES

1. F. BULDA, K. R. FR6uNER AND F. J. ScHMirr, Fourth Intern. Symp. on Fresh Water From theSea, Heidelberg, 2 (1973) 229-237.

2. H. PANARANMEH, Verfahrenstechnik, $(1974) 113.3. G. BQISDE, H. CoRtou, L. GRALL, R. MAHUT AND M. PELRAS, Third Intern.

Water From the Sea, Dubrovnik, 1 (1970) 579-590 .4. H. CoRsou, L. GRALL, C. MAHIEU, R. MAHuT AND R. YvE'r, Fourth Intern. Sym

Fresh Water From the Sea, Heidelberg, Sept. 9-13, 1973, 2 (1973) 239--247.5. F. J. KiEvrrs AND F. P. IJsmx-wa, Werkstoffe and Korrosion, 23 (1972) 1084-1096.6. H. Slcu, Werkstoffe and Korrosion, 23 (1972) 12-18.7. N. G. FEIGE, Titanium in Brine Applications, ASM-Session of the WESTEC-Conference, Los

Angeles, Calif., 1968 .8. K. RUDINGER AND J. LINDEMANN, Chemie-Ing: Techn., 46 (1974) 511-515.

a Fresh