Desalination.44 (1983)295-305 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
AND NICKEL ALLOYS IN ARTIFICIAL
SEA WATER M.
PUTZSCHKE and M.B. ROCKEL
MetallgesellschaftAG, P.O.Box 3724, 6000 Frankfurt (M) 1 (Germany)
ABSTRACT Stainless steels are increasingly used for sea water applications. Since stainless steels on the basis of AISI 316 are not sufficient in respect to their resistance against pitting, crevice and stress cracking corrosion higher in Cr and MO containing stainless steels as well as improved NiCrFeMo alloys have been developed in the past decade. In this research work representative stainless steels and nickel alloys of this group were testet in artificial sea water in order to determine their resistance against pitting corrosion the mos serious and important type of corrosion. Potential-currentdensity curves and therefrom the so called 'pitting potentials' were determined. Plots of the pitting potential versus the temperature show that the pitting resistance generally drops strongly in the temperature range of 60 OC. Only few alloys namely those with the highest Cr and MO contents are pitting resistant even at temperatures up to around 100 "C. Les aciers inoxydables s'utilisent de plus en plus dans des applications impliquant de l'eau de mer. Etant donne que les aciers inoxydables conformes a la norme AISI 316 ne sont pas suffisamment allies, au point de vue resistance a la corrosion par piqOre, b la corrosion en fissures et a la corrosion sous tension, des aciers inoxydables a plus grande teneur en Cr et MO ainsi que des alliages NiCrFeMo ont et6 developpes ces dix dernieres an&es. Pour le present travail, des aciers inoxydables et des alliages de nickel ont 6th testes dans de l'eau de mer artificielle pour determiner leur resistance P la corrosion par piq5re, l'un des types de corrosion les plus graves et les plus repandus. Les courbes potentiel-densitede courant et les potentiels de corrosion par piq5re en resultant ont [email protected]
determines. Les diagrammes opposant les potentiels de corrosion par piq^urea la temperature montrent que la resistance a la corrosion par piqfirebaisse en general fortement autour de 60 "C. 11 n'y a que peu d'alliages, et ce sent ceux aux plus grandes teneurs en Cr et MO, qui resistent a la corrosion par piqfirembme a des temperatures allant jusqu'a 100 OC environ.
INTRODUCTION The chemical and other industries increasingly need water for process cooling. Since natural low chloride containing (river and lake) waters get shortened water with higher chloride levels up to brackish and sea water has to be used. As promising and economic materials for heat exchanger application stainless steels as well as Nickel alloys are considered. The disadvantage of these materials is the possibility of pitting, crevice corrosion under especially stagnant conditions and stress corrosion cracking at higher OOll-9164/83/$03.00 0 1983ElsevierSciencePubIishersB.V.
temperatures. In recent years higher in Cr and MO alloyed stainless steels as AISI 304/316 have been developed. Some of them are resistant against hot sea water. This paper deals with the determination of the pitting potential in artificial sea water at temperatures up to 90 OC. Potentiodynamic polarisation (potential-currentdensity) curves were taken using the well known electrochemical testing methode. The analysis of tested materials is given in Table 1. While most of the steels are austenites the austenitic-ferriticand the superferritic stainless steels are briefly discussed, too. Some high nickel alloyed materials are also considered. In general the materials are tested as delivered from the semi producer.
ELECTROCHEMICAL TESTINGS As a measure of pitting corrosion resistance the pitting potential was determined in these experiments. While especially the potentiodynamicmethod was used by shifting the potential continuously and recording the current additionally potentiostaticmeasurements were made point by point. The solution was artificial sea water according to ASTM with a constant pH of 8.2, adjusted with 0.1 n NaOH. In all tests the solution was stirred by a magnetic stirrer and slightly bubbled by air. Material specimens were little flags upside down with a testing surface of 10 by 10 mm machined out of 2 - 6 mm thick sheets in the as delivered form. Surface and edges were ground by grit 600 paper. Before testing all specimens were briefly passivated in 10 % nitric acid at 60 "C for 20 minutes. Specimens were immersed into the solution and the open circuit potential determined for 60 minutes. There after, the potential was shifted to more potentials
mV/h. When the current density reached
about 1 mA/cm' potential was reversed into the negative (cathodic) direction. In order to determine the pitting potential more accurately, additional potentiostatic testings were done. For that reason potentials close to the steep rise in potential-currentdensity (E-i) curves were applied for longer times (24 h) at separate specimens and a) the current density was recorded as well as b) the surface of the specimens was checked for pits. Specimens were considered to be pitting susceptibel at a potential where the current increased constantly and/or pits occurred. Reverse curves were considered to give indications of the repassivation tendency.
TESTING RESULTS Only some representive examples of the electrochemical testings can be discussed here.
Fig. 1 shows E-i curves for higher alloyed stainless steels in artificial sea water at 30 OC.
stainless steels indicate good pitting resistance
because the steep rise of the current is only observed at around
i.e. far above the redox potential of the solution. The E-i curves of standard stainless steels of type 316 are shifted to more negative potentials and show additionally a big hysteresis meaning less repassivation tendency. An increase of temperatur to 50 OC leads to a relatively strorgshifting of the E-i curves to more negative potentials, see fig. 2, except for the highly alloyed steel 1.4529 with 21 Cr 25 Ni 6
stainless steels are considered to be pitting resistant as long as their pitting potential indicated here by the steep rise of the current is above the redox potential which lies at about 400 mVH. Fig. 3 presents an overview of more than only pitting data of stainless steels and selected Ni alloys: it shows additionally data on the open circuit and the repassivation potential determined by both potentiodynamicand potentiokineticmeasurements. The graph valid for 75 OC clearly shows what materials are considered to be resistant, mVH
namely those whose pitting potentials (full circles) are 'abovethe 400 redox line of sea water.
In Fig. 4 an example of the change of the E-i curve in dependance on the temperature is given for one selected stainless steels. While for other stainless steels the drop of the pitting potential starts already in the temperature range of 50 OC this highly alloyed stainless steel shows only a slight dropping at 75 OC. In any case, i.e. also.even at 90 OC the pitting potential is more noble than the redox potential of the solution, thus the steel can be considered resistant against pitting. Since the so called superferritic stainless steels show also excellent pitting resistance in chloride containing solutions E-i curves were determined for comparsion, see fig. 5. Superferrites are highly resistant as compared with standard stainless steels 304/316. However, these results indicate that at higher temperatures austenitic stainless steels with highes Cr and MO contents may
behave slightly better. A Summary of the pitting potentials tested herein is given in fig. 6.
FOR MATERIAL APPLICATION
A look at all received data and especially of those of fig. 6 can be interpreted as follows. Stainless steels on the basis of 304 and 316 show a too low pitting potential at all temperatures and should therefore not in sea
crevice corrosion is considered. A borderline
for the 317 (4.5 MO) steel,
well as for the Ni alloy (2.4858), especially at temperatures higher
as well and
as Ni alloys
NiCrMo alloy is
6 MO). Most excellent
so called gives
can be done
pitting resistance (artificial) sea
1.4541 1.4571 1.4462 1.4439 2.4858 1.4503 1.4539 1.4563 2.4641 1.4529 1.4575 2.4856
Cr and MO levels.
or Ni alloy,
20 Cr 25 Ni 6 MO stainless
work a and
Al: 0,lO Ti: 0,72
1 0,016 1 0.006
3.52 1 1.19 1
I O,OOl3; 0.006
Ctonilrr C,oai,cr C,“,Uk.l
I010 11 170 NCN ,915
Cronifcr 1925 hM0 C,rJ,,lkl 2328 C,or,,fc~ 2105 NCN
Fig. 1: Potentiodynamicpotential-currentdensity curve of selected stainless steels in artificial (ASTM) sea water, at 30 OC, air bubbled, stirred.
# __. ..::
/... /I ‘.“..’
.__.___, ._-._., , .._r -. .9w .mm dlml I a,, Cmni,,l vu”li I‘439cem,tw 1113 NI‘N 1‘519 Oonilr, w5 . . US29 Cronilrr ,925hU.a
lm”, NE ~~__L_--_I .I00 .O
Clon,~t *3*.¶ C#“,>,k 2205NW
'otentiodynamicpotential-currentdensity curve of selected kainless steels in artificial (ASTM) sea water, at 50 OC rir bubbled, stirred.
dyn +1200 -
I.LSOJ U&62 -tat d -tat
lllOO llOOO l900+800+700 +600 +soo+LOO-
I. I0 Ii I’ I
and potentiostatic and Ni alloys in artificial
= 1200 mV/h
, ............ JOOC * _.--gJo(J 375+T
+700 +800 +900 rwoo
Fig. 4: Potentiodynamic potential-currentdensity curve of a ZOCr25Ni6Mo stainless steel (1.4529) in artificial (ASTM) sea water, at 30, 50, 70 and 90 T, air bubbled, stirred
CmVl EH +1300 +1200 +llOO +lOOO + 900 ustenite
c 800 + 700 \QSuperferrites
+ 600 + 500 +400 + 300 + 200
+ 100 0
100 a IIOCI
Fig. 5: Pitting potential as a function of temperature for superferritic and auatenitic stainless steels in artificial (ASTM) sea water, air bubbled, stirred. Austenite: 20Cr 25Ni 6Mo
. .._ 1.u39
l. ..*............ l.&lj(J-j Cronifcr 2329
4221 h MO
Pitting potential and Ni alloys in
as a function of artificial (ASTM)
temperature sea water,
for stainless steels air bubbled, stirred.
Cr + 3,3 x ‘1. MO 1
Fig. 7: Relative Resistance to Pitting Corrosion
Pitting Index 32 36 36