Technical And Cost-Effect Study Of Using Monel400 Or Inconel 600
Alloys In The Venting System At Tobruk M.E.D Desalination Plant as
Substitutes of The Stress Corroded Stainless Steel 316L Alloy
Eng. Mohamed R. Abubaker
Dr. Belied S. Kuwairi
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
introduction

There are mainly two types of desalination technologies are implemented now days
in the world. They include thermal technologies, and membrane technologies.

Thermal technologies are those that heat water and accumulate condensed vapor
(distillation) to produce pure water, thermal desalination processes, has been known,
and used for as long as one century ago, with the early generation of present-day.

membrane technologies uses pressure as the driving force to separate the saline feed
into a product stream and a concentrate stream.

Membrane technologies use thin, semi permeable membranes to separate the feed
stream into two streams of differing concentration, a product and concentrate stream.
In desalination applications, the feed is either brackish or seawater.
S.S 316L stress corrosion
cracking – Mohamed Rezek
AbuBaker – Tobruk Desalination
Plant
introduction

The desalination process ,as in any other industrial processes, confronts technical
and operation problems, which may lead to the decrease of units efficiency due to
the scale formation and corrosion.

Corrosion can cause variety of undesirable consequences, including damage of
equipment, unplanned shutdowns, expensive repairs, leaks and contamination of
products as well as serious personal injuries.

the damage of units components such as heating exchange pipes, heat exchangers
and condensate pipes, due to the super heating phenomenon.

the failure of components due to corrosion constitutes a major concern and interest
for researchers and thus many study cases have been reported in literature.

Successful desalination requires metals, and alloys, that can resist the aggressive
S.S 316L stress corrosion
corrosion environment resulting heating seawater and brine.
cracking – Mohamed Rezek
AbuBaker – Tobruk Desalination
Plant
Description Of The Current Problem

The sea water entering the Desalination plant is normally chlorinated to control
marine growth.

The presence of ammonia resulting from dumped untreated sewage in the sea can
lead to the formation of bromamines, and chloramines by reaction with bromine
present naturally in sea water and with free residual chlorine from chlorination unit.

Bromamines and chloramines will be stripped in the partially vacuumed venting
system.

Decomposition of these components will eventually induce stress corrosion
cracking (SCC) on the outer shell of the venting system heat exchanger
(downstream condenser).

Corrosion of alloys under the influence of stresses in service could accelerate and
lead to environmentally assisted cracking known as stress corrosion cracking (SCC).
Catastrophic failures have occurred due to SCC, resulting in loss of life and
expensive repairs.
S.S 316L stress corrosion
cracking – Mohamed Rezek
AbuBaker – Tobruk Desalination
Plant
Description Of The Current Problem

Materials and corrosion are important factors in the capital cost, reliability and
maintenance of thermal desalination processes.

Experience gained from operation of the early distillation plants has led to a greater
understanding of the corrosion mechanisms that limited the life of the plant. This in
turn has resulted in the development of materials able to withstand better the
aggressive conditions that exist in the distillation of seawater.

Tobruk desalination plant; was established in 1978, with Multi Stage Flash (MSF);
4 units, design capacity (24,000 m3/d) with 120 MW.

In 2002 the plant increased by 3 units of Multi Effect Desalination (MED), with total
productivity of (40,000 m3/d).
S.S 316L stress corrosion
cracking – Mohamed Rezek
AbuBaker – Tobruk Desalination
Plant
Description Of The Current Problem

Corrosion was first observed in outer shell of the heat exchanger situated in the
venting system at Tobruk desalination plant made of stainless steel AISI 316L and
then officially reported in by Sidem company the construction company of the plant,
as early as 2003.

Sidem was contracted by the owner to inspect and investigate the incurred corrosion
on the heat exchanger. Sidem carried out an extensive field and laboratory
investigations in an attempt to determine the causes of failure.

The preliminary findings were submitted by the company in a form of brief report
(Sidem company, 2003) outlining probable causes of failure and proposed remedial
actions to prevent or at least mitigate the problem.
S.S 316L stress corrosion
cracking – Mohamed Rezek
AbuBaker – Tobruk Desalination
Plant
Description Of The Current Problem

Laboratory analysis carried-out by Sidem showed evidence that ammonia found as a
major pollutant in the sea water intake has reacted with chlorine gas used in the plant
to control marine growth and with the naturally dissolved bromine in seawater.

As a result chlorine and bromine compounds mainly as bromamines and chloramines
are formed. These compounds are stripped from sea water and then get concentrated
and decomposed in the venting system inducing stress corrosion crack (S.C.C) of the
outer shell of the heat exchanger.

Chloramines and Bromamines compounds are suspected to be the main initiators of
the observed corrosion.

This type of corrosion is termed as Bromine Stress Cracking, and has been reported
for the first time in the early seventies mainly in the Acid Treated M.S.F plants by(
J.W Oldfield and B. Todd , 1981). Now we report the similar case in Tobruk M.E.D
plant.
S.S 316L stress corrosion
cracking – Mohamed Rezek
AbuBaker – Tobruk Desalination
Plant
Stress corrosion cracking trilogy

SCC usually occurs in certain specific alloy-environment-stress combinations.

SCC is classified as a catastrophic form of corrosion, as the detection of such fine
cracks can be very difficult and the damage not easily predicted.
S.S 316L stress corrosion
cracking – Mohamed Rezek
AbuBaker – Tobruk Desalination
Plant
Historical Background Of The Problem
Venting system heat exchanger at Tobruk desalination plant suffering from intensive
corrosion attack, as shown in the following Figures. figures 1, 2, and 3, shows crack
initiation and propagation, figures 4, 5, and 6, shows the failure history or scenario of
the downstream condenser shell which made of S.S 316L .
S.S 316L stress corrosion
cracking – Mohamed Rezek
AbuBaker – Tobruk Desalination
Plant
Research goals

To experimentally assess the alternative solution of using Monel 400 and Inconel
600 alloys as substitutes of Stainless Steel Type AISI 316L.

To carry out an economical feasibility study to evaluate all alternative solutions of
the problem using an appropriate optimization technique.

To assess and investigate the proposed remedial action to solve the problem by
Sidem company.
S.S 316L stress corrosion
cracking – Mohamed Rezek
AbuBaker – Tobruk Desalination
Plant
Experimental work

corrosion monitoring was carried out by studying the corrosion behavior of
S.S316L alloy and the suggested alloys.

Corrosion coupons samples of two different materials, Monel 400 and Inconel 600
(see table 1) from Dragon Company have been ordered. An English company that
is specialized in corrosion field. At site, an additional third sample made of
S.S316L were fabricated (the currently used material in the heat exchanger shell).
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
Experimental work

Coupon holders and plugs were prepared at Tobruk plant workshop (see Figure 2) ,
using lathe machine, welding machine, and drilling machine. The metallic coupons
were fixed in venting system downstream condenser shell(test area) exposed for 30
days, 60 days and 90 days.
Fig 2. Corrosion coupon holders prepared with coupons samples fixed on them.
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
Experimental work:

Coupons were fixed in venting system downstream condenser shell (test area)
exposed for 30 days, 60 days and 90 days. Nine corrosion coupons were installed
for the three different alloys tested in this work (three coupons for each one), see
Figure (3). The test exposure time periods were; one month, two months, and
three months.

Tobruk desalination plant consists of three units named (81,82,83) the three alloys
were distributed on the three units (Inconel 600 fixed in unit 81, Monel 400 fixed
in unit 82, S.S316L fixed in unit 83) at the same working condition and at the
same time as follow:
Fig 3. Coupon holders fixed in the downstream condenser
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
Results



Deterioration of the samples have been determined by weight loss method.
After one month S.S316L shows initiation of crack and significant amount of
weight loss, after two months S.S316L shows a heavy corrosion and complete
failure due to SCC see Figure 5a and Figure 5b,whereas the other two alloys
Monel 400 (Figure 6a, Figure 6b) and Inconel 600 (Figure 7a, Figure 7b) were
in a very good condition.
After three months both alloys Monel 400 and Inconel 600 shows a very good
condition, see Figure 8a and Figure 8b.
Figure 5a. S.S 316L after one month
Figure 5b S.S 316L after two months
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
Results
Figure 6a. Monel 400 alloy after one month
Figure 6b. Monel 400 alloy after two months
Figure 7a. Inconel 600 alloy after one month
Figure 7b. Inconel 600 alloy after two months
FigureS.S
8a.316L
Monel
400 corrosion
after threecracking
months – Mohamed
Figure
8b. Inconel
600–alloy
after
three months
stress
Rezek
AbuBaker
Tobruk
Desalination
Plant
Results : Wight loss readings
After the predetermined intervals have been passed for each set, the coupons were
dismantled, for inspection and weight loss calculation. Each coupon from each set
was separately dismantled and weighted by digital balance immediately after
retrieving to determine corrosion deposit weight, and later on after it was cleaned to
determine corrosion weight loss for the three alloys i.e. Table 2
Table 2 Monel 400 Corrosion Coupon Sample Readings
Location
Evaporator 82
Coupon type
Stressed U-Bend
Stressed U-Bend
Stressed U-Bend
Size
Ref.no
Date installed
Date retrieved
Days exposed B
Initial wt.( grams) C
50×25×1.6mm
03
01.05.2013
30.05.2013
30 days
16.0105
50×25×1.6mm
04
01.05.2013
30.06.2013
60 days
15.7840
50×25×1.6mm
05
01.05.2013
30.07.2013
90 days
16.3190
Wt. After exposure (grams)
D
16.0025
15.7663
16.3130
Wt. After cleaning (grams)
E
15.9975
15.765
16.2930
Wt. of deposit ( grams) (D-E)
0.0050
0.0013
0.0200
Wt. Loss (grams) (C-E)
0.0130
0.0190
0.0260
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
Discussion Of Results
Experimental Results (Technical Discussion)
The corrosion behavior of the samples was assessed using the conventional weight
loss measurement at every thirty days interval for a total period of ninety days in
accordance to NACE Standard RP0775-2005.Table (8) gives a summary of weight
loss in grams for all coupons.
Table 8 Weight Loss (in grams) of the Tested Alloys with Relation to Time
Time
S.S 316L
Monel 400
Inconel
30 days
0.0500
0.0130
0.0042
60 days
Failed (cracked)
0.0190
0.0377
90 days
Failed
0.0260
0.0406
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
Discussion Of Results
Figure (9) shows schematic plots of weight loss in grams versus exposure time in
days for the three alloys tested in this work
Figure 9. Weight loss versus exposure time
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
Cost effectiveness

The experimental work has proved beyond doubt that Stainless Steel 316L is not
the proper material selection in this corrosive environment regardless of its
relatively low corrosion rate calculated after one month exposure .Cracks were
developed on surface of the tested Stainless Steel 316Lcoupons’ after two months of
continuous exposure causing complete failure due to SCC .

This observation coincides with what has been experienced in Tobruk Desalination
Plant for years as mentioned earlier , and it was attributed to the presence of
ammonia and dissolved bromine coming along with polluted seawater intake to the
plant reacting with residual chlorine forming Chloramines and with Bromine
forming Bromamines; the two substances responsible for the occurrence of SCC.

The experimental work also has proved that the two other alloys tested; i.e. Monel
400, and Inconel 600 may present good substitutes to S.S 316 L as construction
material for the shell of the venting system condenser without any need for extra
chemical dosing to de-chlorinate, and with added advantage of minimal stoppage of
operation for maintenance.
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
Despite the implementation of chemical dosing system to reduce the residual chlorine to minimum
level, proposed by Sidem Company, which began operation in 2005 in an attempt to mitigate the
SCC problem in the venting system heat exchanger. The cracking and consequently, failure of the
condenser shell continues.
Chemicals Dosing Cost 2005 - 2015
Operation hrs
Unit 81 Unit 82 Unit 83
running
hrs
Operation
days (days)
consumption/year
(tons)
price /Ton
(L.D)
cost/year
(L.D)
2005
7387
7954
7345
22686
945.25
70.89
1993.48
141325.27
2006
8417
8076
8374
24867
1036.2
77.71
1993.48
154912.08
2007
8489
8457
7905
24851
1035.5
77.66
1993.48
154812.41
2008
8445
8445
7957
24847
1035.9
77.65
1993.48
154787.49
2009
8418
8432
8166
25016
1042.3
78.17
1993.48
155840.3
2010
8631
8634
8457
25722
1071.7
80.38
1614
129735.34
2011
8188
8233
8170
24591
1024.2
76.85
1614
124030.86
2012
8517
8566
8377
25460
1060.3
79.56
1614
128413.88
2013
8484
8576
8554
25614
1067.2
80.04
1614
129190.61
2014
8270
8347
8173
24790
1032.91
54.163
1393.61
75482.1
2015
7654
8024
7760
23438
976.58
46.636
1393.61
64992.4
Total
90900
91744
89238
271882
11328.04
799.709
19210.62
1413522.74
Year
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
economical feasibility assessment

In order to determine which alterative is more cost effective in comparison to the
other, one requires price quotations from a reputable vendor specialized in the
construction of heat exchangers for the same mechanical design specification
differing by the material of construction.

Therefore a request for commercial price quotations was sent along with detailed
mechanical design specification of the existing and working condition in the plant,
to Shanghai Shenglin M&E Technology Co., Ltd, a known manufacturing company
for heat exchangers.

One heat exchanger shell is to be constructed from S.S 316 L, and the shells of the
two are to be constructed one from Monel 400 and the other from Inconel 600, and
the tubes for the three heat exchangers are to be constructed from Titanium.
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
economical feasibility assessment
Hereunder an economical feasibility assessment were carried out among the three
alternatives, i.e. the selection of one of the tested alloys as a material of
construction, based on; (i) cost effectiveness; and on (ii) the community continuous
demand for potable water in the absence of any other reliable good quality sources
of fresh water in the city of Toburk.
After negotiation and clarifications, the following quotations were received:

Quotation a) Heat Exchanger made of S.S 316L, Titanium Tubes (price for 1pc
= 20492.0 USD), For Exchange Currency Rate of 1.4168 as issued by Central
Bank of Libya [4], 1pc =29,033.066 L.D, 3pc = 87099.2 L.D.

Quotation b) Heat Exchanger made of Monel 400 or Inconel 600; Titanium
Tubes (price for 1pc = 36852.5USD), 1pc = 52,212.622 L.D, 3pc= 156637.866
L.D.
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
economical feasibility assessment

In order to select among alternatives, the present worth method is employed to
set the three alternatives of equal-service life of 6 years of identical capacities.
The following equation used to calculate the present worth for each
alternative:

Present Worth = First Cost – [Present Value of Annual Cost]
Where; the Present Value PV of The Annual Cost A is calculated at the
minimum attractive rate of return MARR ( i = 0.05 %) for number of years
n(6years) by the following equation;

Thus the present worth of each of the three alternatives are calculated for the
compound interest factor of 5.076, and the results are summarized in Table (4)
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
economical feasibility assessment
Table 4 Alternatives comparison
Comparison mode
Alternative A
S.S316L
Alternative B
Monel 400
Alternative C
Inconel 600
First Cost ( $)
87099.2
156637.866
156637.866
Annual cost ( $/year )
128502.09
0.0
0.0
Remaining Service Life , years
6
6
6
-156637.866
-156637.866
Present worth , PW ($)
-739375.8
It is concluded that alternative A; i.e. the selection of S.S 316 L has the least negative
or numerically largest, and thus more costly than the other alternatives B and C; i.e.
Monel 400 or Inconel 600 which have numerically the least cost since they do not
require chemicals for de-chlorination.
As a result of the cost effectiveness analysis, it would more feasible economically
to immediately replace the existing Stainless Steel condensers in the venting system of
the three evaporators in units 81, 82, and83, and install newly fabricated condensers
whose shells are constructed of Monel 400 or Inconel 600 since the two alloys have
shown superior resistance to SCC, and have proven excellent mechanical properties
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
Community Continuous Demand for Potable
Water

the community continuous demand for potable water in the absence of any other
reliable good quality sources of fresh water in the city of Toburk, represents
another motivation toward taking a decision to replace the existing Stainless
Steel condensers in the venting system of the three units in the plant, with ones
their shells fabricated from Monel 400 or Inconel 600.

Tobruk city population now is around 250,000 to 300,000. According to the
world health organization (WHO), every inhabitant needs about 250liters/day
potable water daily, thus Tobruk demand for potable water is around 62,500 to
75,000 m3/day. The overall daily average produced water by the plant and
transmitted to the city for municipal consumption was 32,420m3/day as
illustrated in Table (5) .Therefore the deficit between demand and transmitted
produced water is around 30,080 to 42,580m3/day.

The absence of any other reliable source of water in Tobruk, results in high and
continuous demand for potable water from the plant, and thus an uninterrupted
full scale operation of the three units of the plant is a necessity, and that cannot
be realized if one or two of the three evaporators are shut down for maintenance
because of leaky condenser shells due to SCC.
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
Community Continuous Demand for Potable Water
Table 5
Water Production-Distribution Plan Year - 2015
Month
Production
To Tobruk city
To Power
Station
Domestic
1
857,361
847,071
6,385
1,177
853,456
27,657
2
839,478
829,541
8,258
338
837,799
29,981
3
1,108,640
1,102,760
7,377
355
1,110,137
35,763
4
1,051,024
1,046,601
6,894
230
1,053,495
35,034
5
1,089,746
1,077,549
8,713
1,162
1,086,262
35,153
6
1,019,947
1,019,390
4,321
456
1,023,711
33,998
7
1,078,186
1,058,465
5,557
283
1,064,022
34,780
8
973,568
978,467
7,281
490
985,748
31,405
9
873,990
864,144
7,551
528
871,695
29,133
10
983,428
982,351
1,204
181
983,555
31,723
11
1,098,943
1,089,426
2,097
154
1,091,523
36,631
12
1,009,498
1,002,450
8,920
570
1,011,370
32,564
Total
11,983,809
11,898,215
74,558
5,924
11,972,773
Average = 32818.66
Total Transmitted Average Production/day
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
Conclusion
An experimental and an economical feasibility study was carried out in this work to
investigate the frequent failure of condenser shells of the venting system of the three
evaporators in Tobruk MED desalination plant due to accelerated corrosion of the outside
surface of the existing shells which were fabricated of S.S 316L .
An attempt was made to experimentally determine the suitability of alternative alloys to
substitute S.S 316 L as construction materials in the heat exchangers in the venting
system of the three evaporators i.e. Monel 400 and Inconel 600 which are known for their
superior corrosion resistance and excellent mechanical properties.
Therefore 9 different coupons prepared in accordance to NACE standards ,and were
tested and their corrosion rates were measured by a direct ER probe ,and by weight loss
method , after being exposed continuously to the corrosive media inside the shells while
the units in the plant are running for periods of 1,2,and 3months.
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
Conclusion
After one month exposure to non-condensable gases in the venting system
downstream of the condenser, the coupons made of S.S 316 L showed initiation of
cracks accompanied by significant weight loss, and complete failure was observed
after two months which is typical of stress crack corrosion (SCC). On contrast the
remaining coupons made of Monel 400, and Inconel 600 showed excellent
resistance to SCC, with nil weight loss after 3 months exposure to the same
corrosive environment.
An economic feasibility assessment was conducted among three alternatives
proposed in this work to mitigate the problem based on cost effectiveness and
continuous and increasing community demand for good quality potable water
produced by the plant.
The present worth method was employed .It has showed that replacing the existing
three S.S 316L condensers in the venting system with chemical dosing for dechlorination ,with Monel 400 or Inconel 600 condensers ,and without chemical
dosing ,is cost effective, and can guarantee uninterrupted production in the plant.
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
Recommendation
As a result of the some recommendations may be drawn some of which may
implemented immediately to ease the corrosive nature of seawater ,and the rest
are long range and require further budgetary considerations from plant
management office.
1- Some technical measures must be considered to ensure that chlorination levels
are kept as low as possible just enough to eliminate marine fouling in the
seawater intake leaving a barely detectable residual chlorine level. This will
drastically reduce the formation of chloroamines responsible for SCC.
2- The plant seawater intake feed can be de-chlorinated with sodium bisulphite
before acidification, by increasing dosing until acceptable residual chlorine
level is maintained. This measure was proposed by Sidem and was considered
expensive.
3- Coordinated efforts with the authority of Tobruk municipality, should be initiated
to solve the sewage and septic drainage to the open sea .This will drastically
minimize harmful pollutants such as ammonia from entering with the seawater
plant intake.
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
Recommendation cont
4- In case a decision is taken to erect another desalination plant to meet urgent
community demand for potable water .It is highly recommended to consider a
new design which can overcome the existing problems especially in the feed
water and in the venting system.
5- Corrosion monitoring systems may be installed to keep track of the performance
and behavior of materials consequently improves the efficiency of the plant.
6- Thus in order to keep the already existing deficit in water supply to the
municipality of Tobruk as low as possible and in the absence of plans in the
near future for an erection of a new desalination plant, the replacement of the
existing three condensers proposed earlier is economically feasible and may be
considered as a necessity to keep the three evaporators running without frequent
stoppage for maintenance and without costly chemical dosing for dechlorination.
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant
S.S 316L stress corrosion cracking – Mohamed Rezek AbuBaker – Tobruk Desalination Plant