PLATE

Abstract

The present invention relates to a plate of a plate heat exchanger, said plate comprising a material which constitutes a part of said plate with a surface made of this material positioned to be in direct contact with a chloride-containing cooling liquid, such as sea water, in which the material is a duplex stainless steel alloy containing in weight-%: C max 0.06%, Si max 1.5%, Mn 0-3.0%, Cr 23.0-32.0%, Ni 4.9-10.0%, Mo 3.0-8.0%, N 0.15-0.5%, B 0.-0.010%, S max 0.030%, Co 0-3.5%, W 0-3.0%, Cu 0-2.0%, Ru 0-0.3%, Al 0-0.2%, Ca 0-0.010% balance Fe and normal occurring impurities, wherein the ferrite content is 35-70 volume-%.

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a plate of a plate heat exchanger, said plate comprising a material which constitutes a part of said plate with a surface made of this material positioned to be in direct contact with a chloride-containing cooling liquid, such as sea water, and a plate heat exchanger adapted to utilize a chloride-containing liquid, such as sea water, as cooling medium.

BACKGROUND OF THE INVENTION AND PRIOR ART

In different plants, such as different types of machines, electric converter stations and the like, located close to the sea or on off-shore platforms it is necessary to use chloride-containing liquid in the form of sea water as cooling medium in heat exchangers used for cooling purposes in these plants. The use of sea water in these heat exchangers puts high demands on the material used due to the tough high corrosive environment created by the sea water.

The crevice corrosion phenomena constitutes the main problem in plate heat exchangers utilizing sea water as cooling liquid, since it may not be avoided that connecting interfaces between adjacent plates of the heat exchanger are located so that the sea water will reach the crevices or joint thus formed between adjacent plates, and they have also to be located where the cooling liquid has a comparatively high temperature, which is also critical (see below). This problem is much smaller for tube heat exchangers, where such crevices or joints are less severe and may be located where the risk of crevice corrosion is much lower. The problems of crevice corrosion in plate heat exchangers may be reduced by welding the plates to each other and connect them to each other by sealings, but the problem will by that not disappear. Appended FIG. 1 schematically shows a plate heat exchanger PHE of this type having a number of plates P joined to each other in a stack for the flow of a cooling medium in the form of sea water SW in channels formed in every second gap between adjacent plates P₁, P₂ and a medium to be cooled in adjacent channels in every second gap between such plates. The crevice or joint sensitive to crevice corrosion is indicated at C.

Crevice corrosion destroying the material is temperature dependent, and when the material for a given cooling liquid, in this case sea water, has a temperature below a critical crevice temperature (CCT) substantially nothing will happen, but when the temperature of the material is raised above this temperature the corrosion of the material at said crevice will be very strong and in a short time destroy the connection, so that temperatures above said critical crevice temperature may not be accepted. This crevice corrosion temperature should in a plate heat exchanger using a chloride-containing cooling liquid, such as sea water, as cooling medium be at least 50° C., preferably at least 60°, for providing an acceptable cooling capacity of the heat exchanger. Sea water used in plate heat exchangers may be chlorinated for the purpose of killing micro organisms. If these microorganisms are not killed by e.g. chlorination, their presence will cause an increase in the corrosivity of the environment. At low temperatures, i.e. below approximately 40° C., the chlorination in itself does not result in any increased corrosivity towards e.g. stainless steels. At temperatures above 40° C., the increase in redox potential caused by the chlorination severely increases the corrosivity of the water with respect to pitting and crevice corrosion, thus limiting the choice of available construction materials for heat exchangers. Plate heat exchangers of the type defined in the introduction, i.e. utilizing chloride-containing cooling liquid, such as sea water, as cooling medium, for use at higher temperatures have for that sake so far almost exclusively been provided with plates made of titanium, which has a critical crevice temperature above 80° C. in sea water. However, titanium is a very expensive material and it is also not easy available, so that it may sometimes be impossible to avoid waiting times for deliveries thereof in the order of a year or more irrespectively of the financial resources of the buyer.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a plate of a plate heat exchanger being less costly and easier available than such plates of titanium while still having sufficiently high corrosion resistance for making it attractive to be used in a plate heat exchanger using a chloride-containing cooling liquid, such as sea water, as cooling medium.

This object is according to the invention obtained by providing such a plate in which said plate material is a duplex stainless steel alloy containing in weight %: C max 0.06%, Si max 1.5%, Mn 0-3.0%, Cr 23.0-32.0%, Ni 4.9-10.0%, Mo 3.0-8.0%, N 0.15-0.5%, B 0.-0.010%, S max 0.030%, Co 0-3.5%, W 0-3.0%, Cu 0-2.0%, Ru 0-0.3%, Al 0-0.2%, Ca 0-0.010% balance Fe and normal occurring impurities, wherein the ferrite content is 35-70 volume-%. The steel may also contain impurities resulting from the raw material used and/or the manufacturing process, and the average Eq1-value of the two phases of the alloy exceeds 40.5, whereby Eq1=% Cr+3.3% Mo, wherein % is weight-%. Examples of impurities from the manufacturing process are Al and Mg. However, the content of impurities are preferably kept at such a level that the properties of the produced material is substantially unaffected thereof.

The inventors have realized that for materials used for plate heat exchanger using a chloride-containing cooling liquid as cooling medium the critical crevice temperature is the main issue and that the composition of the material may be determined with the aim to raise this temperature to a level acceptable for a plate heat exchanger without spending any particular efforts on other resistance properties of the material. It has been found that duplex stainless steel alloys with a composition within these ranges has a critical crevice temperature in sea water well exceeding 50° C. and in fact exceeding 60° C. Thus, it has been found that not the PRE-value, also containing a factor of 16% N, decides the crevice corrosion behaviour of the duplex stainless steel alloy, but it is the content of Cr and Mo, that is the determining factor for the critical crevice temperature of the material. Furthermore, such a Eq1-value above 40.5 has turned out to result in a critical crevice temperature of the steel alloy exceeding 60° C. For some compositions within these ranges the critical crevice temperature may even be raised to the region of 80° C. This means that this material will constitute an attractive substitution to titanium in a plate heat exchanger. The cost thereof will be only a small fraction of the cost for titanium, such as about 10-20% thereof, and it will be manufactured at any time avoiding the long waiting times that may occur for titanium.

According to another embodiment of the invention said average Eq1-value of the two phases of the alloy is higher than 41, preferably higher than 42. It has been found that such high levels of the Eq1-value are influencing the critical crevice temperature of the material in chloride-containing environment towards higher levels.

According to another embodiment of the invention the Eq1-value for both the ferrite and the austenite phase is higher than 35, preferably higher than 36, which in combination with an average Eq1-value exceeding 40.5 is preferred for keeping the critical crevice temperature of the material at a required high level. Mo and Cr will primarily choose the ferrite phase, so that the Eq1-value of the austenite phase may be close to 35, although said average Eq1-value is above 40.5.

According to another embodiment of the invention, the content of Mo is 4.5-6.5 weight-%. It has turned out that for a given value of Eq1 it is preferred to have a content of Mo being high, namely within this range, since the content of Mo has turned out to be the most important factor for the critical crevice temperature of the material. For obtaining a given Eq1-value it is also preferred to increase the content of Mo rather than that of Cr, even though an increased content of Cr increases the workability of the material when producing the plate, but it increases at the same time the risk of formation of CrN. According to another embodiment of the invention the content of Mo is 4.5-5.5 weight-%.

According to another embodiment of the invention the content of Cr is 23.0-30.0 weight-%. It has been found that a content of Cr within this range is suitable for obtaining a crevice corrosion behaviour of the alloy in a chloride-containing environment aimed at.

According to another embodiment of the invention the average PRE-value of the two phases of the alloy is higher than 46, preferably higher than 47, whereby PRE=% Cr+3.3% Mo+16% N, wherein % is weight-%. It has turned out to be advantageous to have such a high PRE-value of the material, although the Eq1-value is more important for the crevice corrosion resistance of the material.

According to another embodiment of the invention the content of Al is 0-0.1 weight-%.

The invention also relates to a plate heat exchanger adapted to utilize the chloride-containing liquid, such as sea water, as cooling medium, which is characterized in that it is made of plates according to the invention. The advantageous features and advantages of such a plate heat exchanger appear clearly from the above discussion of the plate according to the invention. The invention also relates to use of a plate heat exchanger according to the invention for cooling a medium to be cooled by a chloride-containing cooling liquid, such as sea water, and such a use in which the temperature of said cooling liquid is allowed to reach a temperature of at least 50° C., preferably at least 60° C. It is pointed out that the medium to be cooled by the heat exchanger may be of any type, and it may be a gas or gas mixture, such as air, just as well as for example a liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a very simplified view showing the general structure of a plate heat exchanger with a portion thereof enlarged for explaining the problems to be solved by the present invention,

FIG. 2 is a graph of critical crevice temperatures versus PRENW-value for alloys according to the invention and reference alloys,

FIG. 3 is a graph of critical crevice temperatures versus Eq1-value for alloys according to the invention and reference alloys,

FIG. 4 is a simplified view illustrating how a material test has been carried out for alloys according to the invention and reference alloys, and

FIG. 5 is a graph corresponding to the graph of FIG. 3 based on another testing method.

DETAILED DESCRIPTION OF THE INVENTION

A high critical crevice temperature in chloride-containing environment is obtained by the combination of elements in a duplex stainless steel alloy according to the invention. The alloy according to the invention contains (in weight-%):

	C	max	0.06%
	Si	max	1.5%
	Mn		0-3.0%
	Cr		23.0-32.0%
	Ni		4.9-10.0%
	Mo		3.0-8.0%
	N		0.15-0.5%
	B		0.-0.010%
	S	max	0.030%
	Co		0-3.5%
	W		0-3.0%
	Cu		0-2.0%
	Ru		0-0.3%
	Al		0-0.2%
	Ca		0-0.010%

balance Fe and normal occurring impurities, wherein the ferrite content is 35-70 volume-%.

Carbon (C) has limited solubility in both ferrite and austenite. The limited solubility implies a risk of precipitation of chromium carbides and the content should therefore not exceed 0.06 weight-%, preferably not exceed 0.02 weight-%.

Silicon (Si) is utilized as desoxidation agent in the steel production and it increases the flowability during production and welding. However, too high contents of Si lead to precipitation of unwanted intermetallic phase, and the content thereof is limited to 1.5 weight-%.

Manganese (Mn) is added in order to increase the N-solubility in the material. However, it has shown that Mn only has a limited influence on the N-solubility in the type of alloy in question. Instead there are found other elements with higher influence on the solubility. Besides, Mn in combination with high contents of sulfur can give rise to formation of manganese sulfides, which act as initiation-points for pitting corrosion. The content of Mn should therefore be limited to between 0-3.0 weight-%.

Sulfur (S) influences the corrosion resistance negatively by forming soluble sulfides. Furthermore, the hotworkability deteriorates, for what reason the content of sulfur is limited to max 0.030 weight-%, preferably max 0.010 weight-%.

Chromium (Cr) is an active element in order to improve the crevice corrosion resistance. Furthermore, a high content of chromium implies that one gets a very good N-solubility in the material. Thus, it is desirable to keep the Cr-content as high as possible in order to improve the corrosion resistance. For good crevice corrosion resistance the content of chromium should be at least 23 weight-%. However, high contents of Cr increase the risk for intermetallic precipitations and the formation of CrN, for what reason the content of chromium should not exceed 32 weight-%, preferably not 30 weight-%.

Nickel (Ni) is used as austenite stabilizing element and is added in suitable contents in order to obtain the desired content of ferrite. In order to obtain the desired relationship between the austenitic and the ferritic phase with between 40-65 volume-% ferrite, an addition of 4.9-10.0 weight-% nickel is required.

Molybdenum (Mo) is an active element which improves the crevice corrosion resistance in chloride environments. The Mo-content in the present invention should lie in the range of 0-8.0 weight-%, preferably above 4.5 weight-%. The content of Mo in combination with the content of Cr is the determining factors for obtaining a high critical crevice temperature of the alloy.

Tungsten (W) increases mainly the resistance to pitting corrosion. But the addition of too high contents of tungsten in combination with that the Cr-contents as well as Mo-contents are high, means that the risk for intermetallic precipitations increases. The W-content in the present invention should lie in the range of 0-3.0 weight-%.

Copper (Cu) may be added in order to improve the general corrosion resistance in acid environments such as sulfuric acid. At the same time Cu influences the structural stability. However, thigh contents of Cu imply that the solid solubility will be exceeded. Therefore the Cu-content should be limited to max 2.0 weight-%.

Cobalt (Co) has properties that are intermediate between those of iron and nickel. Therefore, a minor replacement of these elements with Co, or the use of Co-containing raw materials (Ni scrap metal usually contains some Co, in some cases in quantities greater than 10%) will not result in any major change in properties. Co can be used to replace some Ni as an austenite-stabilizing element. Co is a relatively expensive element, so the addition of Co is limited to be within the range of 0-3.5 weight-%.

Aluminium (Al) and Calcium (Ca) are used as desoxidation agents at the steel production. The content of Al should be limited to max 0.2 weight-%, preferably max 0.1 weight-%, in order to limit the forming of nitrides. Ca has a favourable effect on the hotductility. However, the Ca-content should be limited to max 0.010 weight-% in order to avoid an unwanted amount of slag.

Boron (B) may be added in order to increase the hotworkability of the material. At a too high content of Boron the weldability as well as the corrosion resistance could deteriorate. Therefore, the content of boron should be limited to max 0.010 weight-%.

Nitrogen (N) is a very active element, which increases the corrosion resistance, the structural stability as well as the strength of the material. Furthermore, a high N-content improves the recovering of the austenite after welding, which gives good properties within the welded joint. In order to obtain a good effect of N, at least 0.15 weight-% N should be added. At high contents of N the risk for precipitation of chromium nitrides increases, especially when simultaneously the chromium content is high. Furthermore, a high N-content implies that the risk for porosity increases because of the exceeded solubility of N in the smelt. For these reasons the N-content should be limited to max 0.50 weight-%.

The content of ferrite is important in order to obtain good mechanical properties and corrosion properties as well as good weldability. From a corrosion point of view and a point of view of weldability a content of ferrite between 35-70% is desirable in order to obtain good properties.

DESCRIPTION OF PREFERRED EMBODIMENTS

Table 1 below shows the composition of alloys 1-25 according to the invention and reference alloys being not according to the invention as well as results of a testing, Testing 1.

Testing 1

The crevice corrosion resistance of 25 alloys according to the invention and 7 reference alloys was tested according to MTI-2. The critical crevice temperature (CCT) was determined for all the 32 alloys for two different samples. The average value of CCT for each alloy is indicated in Table 1. Furthermore, the conventional expression for “pitting resistance equivalent” in the alloys is given in Table 1 (PRENW=% Cr+3.3% Mo+0.5% W)+16% N, as well as the Eq1-value defined as Eq1=% Cr+3.3% Mo.

All the alloys were produced by melting, hot working and annealing followed by water quenching.

TABLE 1
			%
			Cr +
	CCT/		3.3%
	° C.	PRENW	Mo	C	Si	Mn	Cu	Co
Alloy 1	77.5	49.9	42.8	0.015	0.16	0.86	0.01	1.5
Alloy 2	65	49.0	42.7	0.017	0.18	1.06	0.01	1.5
Alloy 3	77.5	50.5	42.8	0.017	0.23	0.99	0.01	1.5
Alloy 4	82.5	50.8	42.8	0.019	0.23	1.12	0.03	0.6
Alloy 5	65	48.2	42.1	0.017	0.22	1.01	0.03	1.5
Alloy 6	70	48.2	42.3	0.018	0.2	1.1	0.03	0.5
Alloy 7	75	48.4	42.5	0.012	0.15	1.04	0.01	1.4
Alloy 8	80	50.4	42.7	0.016	0.2	1.07	0.03	1.0
Alloy 9	65	46.9	41.9	0.017	0.21	1.02	0.04	3
Alloy 10	67.5	50.3	43.0	0.02	0.25	1.1	0.14	1.5
Alloy 11	85	50.2	43.0	0.02	0.23	1.1	0.14	0.6
Alloy 12	70	48.5	41.4	0.018	0.25	1.1	0.13	1.5
Alloy 13	77.5	48.6	41.7	0.019	0.23	1	0.13	0.6
Alloy 14	82.5	47.9	40.5	0.017	0.21	2.7	0.12	1
Alloy 15	75	50.0	41.9	0.018	0.2	1	0.13	1
Alloy 16	77.5	49.3	43.3	0.02	0.25	1.1	0.14	<0.1
Alloy 17	90	50.5	41.6	0.019	0.25	1	0.18	1
Alloy 18	85	49.9	41.4	0.02	0.28	1.1	1	1
Alloy 19	80	49.2	42.7	0.021	0.24	1	0.14	1.5
Alloy 20	72.5	49.3	43.3	0.019	0.23	1.1	1.5	<0.1
Alloy 21	65	47.8	41.4	0.019	0.22	0.5	0.02	<0.1
Alloy 22	62.5	49.8	43.1	0.019	0.62	0.47	0.02	<0.1
Alloy 23	67.5	49.8	43.2	0.017	0.21	0.49	<0.01	<0.1
Alloy 24	62.5	49.3	43.2	0.021	0.61	0.48	0.01	1.0
Alloy 25	62.5	48.6	42.0	0.019	0.24	0.51	<0.01	1.0
Ref 1	50	50.1	38.3	0.034	0.42	0.86	1.0	1.0
Ref 2	35	47.4	34.2	0.055	0.89	0.93	2.0	<0.1
Ref 3	45	50.3	38.0	0.035	0.48	0.93	1.0	1.0
Ref 4	40	49.4	36.5	0.007	0.12	0.9	2.0	0.1
Ref 5	30	49.9	34.5	0.006	0.12	0.95	2.0	2.0
Ref 6	35	51.9	36.8	0.06	0.11	1.09	<0.01	<0.1
Ref 7	40	47.3	40.3	0.008	0.14	1.07	<0.01	<0.1
			%
			Cr +
	CCT/		3.3%
	° C.	PRENW	Mo	Cr	Ni	Mo	W	N
Alloy 1	77.5	49.9	42.8	28.9	6.6	4.2	0.01	0.44
Alloy 2	65	49.0	42.7	28.8	6.5	4.2	0.01	0.39
Alloy 3	77.5	50.5	42.8	28.8	7.0	4.2	1	0.38
Alloy 4	82.5	50.8	42.8	28.8	7.6	4.2	0.99	0.4
Alloy 5	65	48.2	42.1	28.1	6.5	4.2	0.01	0.38
Alloy 6	70	48.2	42.3	28.4	6.9	4.2	<0.01	0.37
Alloy 7	75	48.4	42.5	28.8	7.0	4.2	<0.01	0.37
Alloy 8	80	50.4	42.7	26.9	6.5	4.8	1.01	0.38
Alloy 9	65	46.9	41.9	28.6	6.5	4.0	0.01	0.31
Alloy 10	67.5	50.3	43.0	29.0	6.5	4.2	<0.01	0.46
Alloy 11	85	50.2	43.0	29.0	6.8	4.2	<0.01	0.45
Alloy 12	70	48.5	41.4	27.5	5.9	4.2	<0.01	0.44
Alloy 13	77.5	48.6	41.7	27.8	6.1	4.2	<0.01	0.43
Alloy 14	82.5	47.9	40.5	27.6	6.9	3.9	1	0.36
Alloy 15	75	50.0	41.9	28.7	6.6	4.0	1	0.4
Alloy 16	77.5	49.3	43.3	30.0	7.1	4.0	<0.01	0.38
Alloy 17	90	50.5	41.6	28.5	7.0	4.0	1	0.45
Alloy 18	85	49.9	41.4	28.2	6.6	4.0	1	0.43
Alloy 19	80	49.2	42.7	28.8	7.0	4.2	<0.01	0.41
Alloy 20	72.5	49.3	43.3	29.3	6.5	4.2	<0.01	0.38
Alloy 21	65	47.8	41.4	25.8	7.1	4.7	<0.01	0.4
Alloy 22	62.5	49.8	43.1	26.1	7.0	5.2	<0.01	0.42
Alloy 23	67.5	49.8	43.2	26.1	7.1	5.2	<0.01	0.41
Alloy 24	62.5	49.3	43.2	26.3	7.0	5.1	<0.01	0.38
Alloy 25	62.5	48.6	42.0	26.2	6.5	4.8	<0.01	0.41
Reference	50	50.1	38.3	30.8	7.5	2.2	2.7	0.46
1
Reference	35	47.4	34.2	29.2	7.6	1.5	3.72	0.44
2
Reference	45	50.3	38.0	30.6	7.7	2.2	2.88	0.47
3
Reference	40	49.4	36.5	31.6	9.4	1.5	3.86	0.41
4
Reference	30	49.9	34.5	29.3	6.2	1.6	3.94	0.56
5
Reference	35	51.9	36.8	31.9	6.3	1.5	3.85	0.55
6
Reference	40	47.3	40.3	28.7	7.4	3.5	<0.01	0.44
7

Appended FIG. 2 shows the relationship between PRENW and CCT and FIG. 3 the relationship between Eq1 and CCT for Testing 1. FIG. 2 shows that the CCT is for all the alloys according to the invention above 60° C., whereas it is not above 50° C. for any of the reference alloys in spite of high PRENW-values thereof. It is shown that the PRENW-value as long as it is in a region above 46 has no real influence upon the critical crevice temperature of the alloy. FIG. 3 shows that Eq1 should be higher than 40.5 for obtaining a CCT above 60° C. It should be noted that the experimental spread in determining the CCT-value is great (about ±10° C.), and that it is not only the total value of Eq1 that is determining but also how it is distributed between the two phases of the material (ferrite and austenite).

Testing 2

An electrochemical test according to a modified version of ASTM G-150, modified in the way that the samples examined were provided with a crevice former in PVDF mounted in approximately the same way as in MTI-2 in Testing 1. A constant tightening moment of 3 Nm was used for mounting crevice formers. A constant crevice pressure was maintained by means of four spring washers 1 mounted according to FIG. 4, in which 2 shows the sample and 3 and 4 the crevice formers.

The test was carried out for four different constant potentials: 0, +200, +400 and +700 mV_SCE. The start temperature was 20° C., and the temperature increased during the experiment 1° C. per minute. The CCT for each sample has been defined as the temperature resulting in a corrosion current density of at least 0.1 mA/cm² in 60 seconds.

CCT was determined for 4-6 samples per material and potential. Table 2 below shows the compositions and the results in the form of min values of CCT for all potentials over 0 mV versus SCE. The variations in CCT between different potentials were small. In most cases no crevice corrosion was noted at 0 mV.

TABLE 2
Alloy	Cr	Ni	Mo	W	N	PRENW	Eq1	CCT
Alloy 26	31.71	7	3.45	<0.01	0.50	51.095	43.1	82.7
Alloy 27	26.62	7	4.73	<0.01	0.38	48.309	42.2	86.7
Alloy 28	26.02	9	5.06	<0.01	0.4	49.118	42.7	87.1
Reference 6	31.9	6	1.47	3.85	0.55	51.9035	36.8	67.1
Reference 7	28.71	7	3.51	<0.01	0.44	47.333	40.3	76.3
Reference 8	29.88	7	1.5	1.98	0.44	45.137	34.8	58.4
Reference 9	29.36	8	2.51	2.1	0.44	48.148	37.6	73.1
Reference 10	25	7	4.00	<0.01	0.28	42.68	38.2	70.6
Reference 11	29.21	7	2.32	<0.01	0.37	42.786	36.9	72.3

FIG. 5 shows the relation between Eq1 and CCT according to this modified ASTM G-150. Some of the alloys are present both in Testing 1 and Testing 2. The two different testing methods give somewhat different values of CCT, which is considered to be normal as a consequence of the differences of the methods.

However, the relationship between the composition and the CCT-value is mainly the same in the two Testings.

This Testing 2 measures primarily the tendency to initiate crevice corrosion, while the Testing 1 primarily measures the tendency to propagation of crevice corrosion. This means that the CCT-values will in the Testing 2 be somewhat higher than in the testing 1. This is the reason while the lowest CCT-value allowed for use in a plate heat exchanger according to the Testing 2 should be set to 80° C.

Testing 3

This testing was an electrochemical testing with constant potential and temperature in 24 hours. The same type of crevice former and the same crevice pressure as in the Testing 2 were used. New samples were used for each temperature-/potential-combination. The potential was: +200 mV versus SCE.

The CCT-value for each sample was defined as the temperature resulted in a corrosion current density of at least 0.01 mA/cm² in 60 s. This testing is extremely tough, since the material is tested in an active state, whereas it in the preceding testings was passivated before testing, when the temperature was low. The ability of the material to form a passivating oxide layer at certain temperature was measured in this Testing 3. The passivating temperature is for most of the materials considerably lower than the CCT.

A comparison of the alloy 26 and 27 in the testing 2 shows that the alloy 26 was not passivated at 45° C., whereas the alloy 27 was passivated at 45° C. as well as at 55° C. However, it may not be passivated at 65°. This means that if a corrosion attack has been started as a consequence of for example an over temperature, the alloy 27 will be able to be passivated already when the temperature falls down below 55° C., which remarkably reduces the risk for propagating attacks upon disturbances in the operation of the plate heat exchanger.

Thus, the alloy 27 having a higher content of Mo has a better ability to be passivated than the alloy 26 in spite of the fact that the alloy 26 has a higher Eq1-value. The conclusion is that the Eq1-value well describes the initiation and the propagation of crevice corrosion, while passivation against crevice corrosion is mainly controlled by the material content of Mo.

An alloy according to another embodiment of the invention has the following approximate composition: C 0.017%, Si 0.2%, Mn 0.5%, P 0.005%, S 0.006%, Cr 26%, Ni 7%, Mo 5.2%, W<0.01%, Cu<0.01%, Co<0.010%, Ti<0.005%, Al 0.004%, B 24 ppm, Ca 22 ppm, N 0.41%. Testing of critical crevice temperature (CCT) according to MTI-2 for two samples resulted in 65° C. and 70° C.

Claims

1. Plate of a plate heat exchanger, said plate comprising a material which constitutes a part of said plate with a surface made of this material positioned to be in direct contact with a chloride-containing cooling liquid, such as sea water, wherein said plate material is a duplex stainless steel alloy containing in weight-%: C max 0.06% Si max 1.5% Mn 0-3.0% Cr 23.0-32.0% Ni 4.9-10.0% Mo 3.0-8.0% N 0.15-0.5% B 0[[.]]-0.010% S max 0.030% Co 0-3.5% W 0-3.0% Cu 0-2.0% Ru 0-0.3% Al 0-0.2% Ca 0-0.010%

balance Fe and normal occurring impurities,

wherein the ferrite content is 35-70 volume-%, and that the average Eq1-value of the two phases of the alloy exceeds 40.5, whereby Eq1=% Cr+3.3% Mo, wherein % is weight-%.

2. A plate according to claim 1, wherein said average Eq1-value of the two phases of the alloy is higher than 41.

3. A plate according to claim 1, wherein the Eq1-value for both the ferrite and the austenite phase is higher than 35.

4. A plate according to claim 1, wherein the content of Mo is 4.5-6.5 weight-%.

5. A plate according to claim 4, wherein the content of Mo is 4.5-5.5 weight-%.

6. A plate according to claim 1, wherein the content of Cr is 23.0-30.0 weight-%.

7. A plate according to any of the preceding claim 1, wherein the average PRE-value of the two phases of the alloy is higher than 46, whereby PRE=% Cr+3.3% Mo 1-16% N, wherein % is weight-%.

8. A plate according to claim 1, wherein the content of Al is 0-0.1 weight-%.

9. Plate heat exchanger adapted to utilise a chloride-containing liquid, such as sea water, as cooling medium, wherein it is made of plates according to claim 1.

10. Plate heat exchanger according to claim 9, wherein said plates are joined to each other in a stack for forming channels between adjacent plates and that said channels formed in every second gap between adjacent plates are configured to have a chloride-containing cooling liquid flowing therein and said channels formed in every second gap between adjacent plates are configured to have a medium to be cooled flowing therein.

11. Use of a plate heat exchanger according to claim 9 for cooling a medium to be cooled by a chloride-containing cooling liquid, such as sea water.

12. Use according to claim 11, in which the temperature of said cooling liquid is allowed to reach a temperature of at least 50° C.

13. Use according to claim 12, wherein said cooling liquid is allowed to reach a temperature of at least 60° C.

14. A plate according to claim 2, wherein said average Eq1-value of the two phases of the alloy is higher than 42.

15. A plate according to claim 3, wherein the Eq1-value for both the ferrite and the austenite phase is higher than 36.

16. A plate according to claim 7, wherein the average PRE-value of the two phases of the alloy is higher than 47.