SE530847C2 - Plate for plate heat exchangers, plate heat exchangers made up of such plates and use of this plate heat exchanger - Google Patents

Description

530 B47 with gap corrosion in plate heat exchangers can be reduced by welding the plates together and connecting them to each other by seals, but the problem will not disappear as a result. Attached Fig. 1 schematically shows a plate heat exchanger PHE of this type with a number of plates P joined to each other in a stack for the flow of a cooling medium in the form of seawater SW in channels formed in every other gap between adjacent plates P1, P2 and a medium to be cooled in adjacent channels in every other gap between such plates. The gap or joint susceptible to crack corrosion is indicated at C.

Crevice corrosion that destroys the material is temperature dependent, and when the material for a given coolant, in this case seawater, has a temperature below a critical cleft temperature (CCT), essentially nothing will happen, but when the temperature of the material rises above this temperature, the corrosion of the material at said gap to be very strong and destroy the connection in a short time, so that temperatures above said critical gap temperature cannot be accepted. This crevice corrosion temperature for a plate heat exchanger using a chloride-containing coolant, such as seawater, as cooling medium should be at least 50 ° C, preferably at least 60 ° C, in order to provide an acceptable cooling capacity of the heat exchanger. Seawater used in plate heat exchangers can be chlorinated with the intention of killing microorganisms. If these microorganisms are not killed by, for example, chlorination, their presence will cause an increase in the corrosivity of the environment. At low temperatures, i.e. below approximately 40 ° C, the chlorination itself does not result in any increased corrosivity to, for example, stainless steels. At temperatures above 40 ° C, the increase in redox potential caused by chlorination seriously increases the corrosivity of the water with respect to point and crevice corrosion, thus limiting the choice of available construction materials for heat exchangers. Plate heat exchangers of the initially defined type, i.e. which use chloride-containing coolant, such as seawater, as the cooling medium, for use at higher temperatures have for this reason been hitherto almost exclusively provided with 10 15 20 “25 30 35 5313 34? tiles made of titanium, which has a critical gap temperature above 80 ° C in seawater. However, titanium is a very expensive material and it is also not readily available, so that it can sometimes be impossible to avoid waiting times for delivery thereof in the order of a year or more regardless of the buyer's financial resources.

SUMMARY OF THE INVENTION The object of the present invention is to provide a plate material for a plate heat exchanger, which is less expensive and more readily available than titanium while still having sufficiently high corrosion resistance to make it attractive to use in a plate heat exchanger. which uses a chloride-containing coolant, such as seawater, as the coolant.

The object is achieved according to the invention by providing such a material for such plates in the form of a duplex stainless steel alloy containing in% by 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%, the residue is Fe and normally occurring impurities, the ferrite content being 35-70% by volume. The steel may also contain impurities that result from the raw material used and / or the manufacturing process.

Examples of contaminants from the manufacturing process are Al and Mg. However, the content of impurities is kept at such a level that the properties of the material produced are substantially unaffected thereby.

The inventors have realized that for materials used for plate heat exchangers using a chlorine-containing coolant as the coolant, the critical column temperature is the main thing and that the composition of the material can be determined with the aim of raising this temperature to a level acceptable for a plate heat exchanger. to make some special effort on other resistance properties of the material. It has been found that duplex stainless steel alloys having a composition in these areas have a critical cleft temperature in seawater, which well exceeds 50 ° C and actually exceeds 60 ° C. For some compositions in these areas, the critical gap temperature can even be raised to the range of 80 ° C. This means that this material will be an attractive substitute for titanium ice plate heat exchangers. This cost will be only a small fraction of the cost of titanium, such as about 10-20% of it, and it can be manufactured at any time, which avoids long waiting times that can occur for titanium.

According to one embodiment of the invention, the average Eq1 value of the two phases of the alloy exceeds 40.5, where Eq1 =% Cr + 3.3% Mo, where% is% by weight. It has been found that the PRE value, which also contains a factor of 16% N, does not determine the crevice corrosion behavior of the duplex stainless steel alloy, but it is the content of Cr and Mo which is the decisive factor for the critical the gap temperature of the material.

In addition, such an Eq1 value above 40.5 has been found to result in a critical gap temperature of the steel alloy exceeding 60 ° C.

According to another embodiment of the invention, said average Eq1 value of the two phases of the alloy is higher than 41. chloride-containing environment towards higher levels.

According to another embodiment of the invention, the Eq1 value for both the ferrite and austenite phases is higher than 35, preferably higher than 36, which in combination with an average Eq1 value exceeding 40.5 is preferred to maintain the critical gap temperature of the material. at, a required high level. Mo and Cr will mainly select the ferrite phase, so that the Eq1 value of the austenite phase can be close to 35, although said average Eq1 value is above 40.5. 10 15 20 '25 30 35 530 84? According to another embodiment of the invention, the content of Mo is 4.5-6.5% by weight. It has been found that for a given value of Eqi it is preferable to have a content of Mo which is high, namely in this range, since the content of Mo has been found to be the most significant factor for the critical gap temperature of the material. To achieve a given Eqt value, it is also preferable to increase the content of Mo rather than that of Cr, although an increased content of Cr increases the machinability of the material in the production of the plate, but it also increases the risk of forming CrN. .

According to another embodiment of the invention, the content of Mo is 4.5-5.5% by weight.

According to another embodiment of the invention, the content of Cr is 23.0-30.0% by weight. It has been found that a content of Cr in this range is suitable for achieving the intended crevice corrosion behavior of the alloy in a chloride-containing environment.

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, where PRE =% Cr + 3.3% Mo + 16% N, where% is weight. %. It has been found to be advantageous to have such a high PRE value of the material, although the Eqt value is more important for the crevice corrosion resistance of this material.

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

The invention also relates to a plate of a plate heat exchanger designed to use a chloride-containing liquid, such as seawater, as cooling medium, which is characterized in that its surfaces positioned to be in direct contact with the chloride-containing liquid are made of the material according to the invention, and a plate heat exchanger designed to utilize the chloride-containing liquid, such as seawater, as a cooling medium, which is characterized in that the surfaces of its plates are positioned to be in direct contact with said chloride-containing liquid are formed by the material according to the invention. The advantageous features and the advantages of such a plate and such a plate heat exchanger are clear from the discussion above and from the material according to the invention. The invention also relates to the use of a plate heat exchanger according to the invention for cooling a medium to be cooled by a chloride-containing coolant, such as seawater, and such use in which the temperature of said coolant 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 can be of any type, and it can be a gas or gas mixture, such as air, as well as, for example, a liquid. The invention also relates to the use of a material according to: the invention in a chloride-containing environment in a plate heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a very simplified view showing the general construction of a plate heat exchanger with a portion thereof enlarged to explain the problems to be solved by the present invention; Fig. 2 is a diagram of critical column temperatures in relation to the PRENW value for alloys according to the invention and the reference position Fig. 3 is a diagram of critical column temperature ratios in relation to the Eqi value for alloys according to the invention and reference alloys, Fig. 4 is a simplified view illustrating how a material tests have been performed for alloys according to the invention and reference alloys, and Fig. 5 is a diagram corresponding to the diagram according to Fig. 3 based on another test method. DETAILED DESCRIPTION OF THE INVENTION A high critical gap temperature in a chloride-containing environment is achieved by the combination of elements in a duplex stainless steel alloy according to the invention. The alloy according to the invention contains (in% by weight): C max 0.06% Si max 1.5% Mn 0-3.0% Cr 23.0-32.0% Ni 4i'9-10.0% Mo 3 .0-8.0% N 0.15-0.5% BO, -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% with the remainder being Fe and normally occurring impurities, the ferrite content being 35-70% by volume.

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

Silicon (Si) is used as a deoxidizing agent during steel production and it increases the flowability during production and welding.

However, too high levels of Si lead to precipitation of undesired intermetallic phase, and the content thereof is limited to 1.5% by weight.

Manganese (Mn) is added to increase the N solubility of the material. However, it has been found that Mn has only a limited effect on the N solubility of the alloy type in question. Instead, other elements with a greater impact on solubility have been found. In addition, Mn in combination with high levels of sulfur can give rise to the formation of manganese sulphides, which act as initiation points for point corrosion. The content of Mn should thus be limited to between 0 ~ 3.0% by weight.

Sulfur (S) has a negative effect on the corrosion resistance by the formation of soluble sulphides. In addition, the hot workability deteriorates, for which reason the sulfur content is limited to a maximum of 0.030% by weight, preferably a maximum of 0.010% by weight.

Chromium (Cr) is an active element in improving crevice corrosion resistance. In addition, a high content of chromium means that a very good N-solubility in the material is achieved. Thus, it is desirable to keep the Cr content as high as possible to improve the corrosion resistance. For good crevice corrosion resistance, the content of chromium should be at least 23% by weight. However, high levels of Cr increase the risk of intermetallic precipitation and the formation of CrN, for which reason the content of chromium should be limited to a maximum of 32% by weight, preferably not higher than 30% by weight.

Nickel (Ni) is used as an austenite stabilizing element and is added at appropriate levels to achieve the desired level of ferrite.

In order to achieve the desired ratio between the austenite and ferrite phases with between 40-65% by volume of ferrite, an addition of 4.9-10.0% by weight of nickel is required.

Molybdenum (Mo) is an active element that improves crevice corrosion resistance in chloride environments. The mo content of the present invention should be in the range of 0-8.0% by weight, preferably above 4.5% by weight. The content of Mo in combination with the content of Cr is the determining factor for achieving a high critical gap temperature in the alloy.

Tungsten (W) mainly increases the resistance to point corrosion. But the addition of excessive levels of tungsten in 10 15 20 '25 30 35 530 84? combination with the fact that the Cr levels and the Mo levels are high means that the risk of intermetallic precipitation increases. The W content of the present invention should be in the range of 0-3.0% by weight.

Copper (Cu) can be added to improve the overall corrosion resistance in acidic environments, such as sulfuric acid.

At the same time, Cu affects the structural stability. However, high levels of Cu cause the solids solubility to be exceeded. The Cu content should thus be limited to a maximum of 2.0% by weight.

Cobalt (Co) has properties that lie between those of iron and nickel. 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 higher than 10%) will not result in any decisive change in the properties. Co can be used to replace some of Ni as an austenite stabilizing element. Co is a relatively expensive element, so that the addition of Co is limited to being in the range of 0-3.5% by weight.

Aluminum (Al) and Calcium (Ca) are used as deoxidizing agents in steel production. The content of Al should be limited to a maximum of 0.2% by weight, preferably a maximum of 0.1% by weight, in order to limit the formation of nitrides. Ca has a beneficial effect on hot ductility. However, the Ca content should be limited to a maximum of 0.010% by weight to avoid an undesirable amount of slag.

Boron (B) can be added to increase the hot workability of the material. At too high levels of boron, weldability and corrosion resistance could deteriorate. Thus, the content of boron should be limited to a maximum of 0.010% by weight.

Nitrogen (N) is a very, active element that increases the corrosion resistance, the structural stability and the strength of the material. In addition, a high N content improves the regeneration of austenite after welding, which provides good properties within the weld. To achieve a good effect of N 10 15 20 '25 30 530 84? N should be added at least 0.15% by weight. At high levels of N, the risk of precipitation of chromium nitrides increases, especially when the chromium content is high at the same time. A high N content also means that the risk of porosity increases due to the exceeded solubility of N in the melt. For these reasons, the N content should be limited to a maximum of 0.50% by weight.

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

DESCRIPTION OF PREFERRED EMBODIMENTS Table 1 below shows compositions of alloys 1-25 according to the invention and reference alloys which are not according to the invention and results of a test, Testing 1.

Testing 1 The crack corrosion resistance of 25 alloys according to the invention and 7 reference alloys was tested according to MT1-2. The critical gap temperature (CCT) was determined for all 32 alloys for two different samples. The average value of CCT for each alloy is shown in Table 1. In addition, the conventional expression for the "point corrosion equivalent" in the alloys is given in Table 1 (PRENW =% Cr + 3.3% Mo + 0.5% W) + 16% N , and the Eq1 value defined as Eq1 =% Cr + 3.3% Mo.

All alloys were produced by melting, hot working and annealing followed by rapid water cooling.

Table 1 CCT / ° C PRENW 77.5 65 Alloy 1 Alloy 2 Alloy 3 77.5 Alloy 4 32.5 Alloy 5 55 Alloy 6 70 Alloy 7 75 Alloy 8 80 Alloy 9 65 Alloy 10 57.5 Alloy 11 55 Alloy 12 70 Alloy 13 77.5 Alloy 14 32.5 Alloy 15 75 Alloy 16 77.5 Alloy 17 90 Alloy 18 85 Alloy 19 80 Alloy 20 72.5 Alloy 21 65 Alloy 22 62.5 Alloy 23 67.5 Alloy 24 52, 5 ëeaefms 25 62.5 Ref 1 50 Ref 2 35 Ref 3 45 Ref 4 40 Ref 5 30 Ref 6 Ref 7 49.9 49.0 50.5 50.8 48.2 48.2 48.4 50.4 46 .9 50.3 50.2 48.5 48.6 47.9 50.0 49.3 50.5 49.9 49.2 49.3 47.8 49.8 49.8 49.3 48.6 50.1 47.4 50.3 49.4 49.9 51.9 47.3 5313 84? 11% Cr + 3.3% Mo 42.8 42.7 42.8 42.8 42.1 42.3 42.5 42.7 41.9 43.0 43.0 41.4 41.7 40.5 41.9 43.3 41, 6 41.4 .42.7 43.3 41.4 43.1 43.2 43.2 42.0 38.3 34.2 38.0 36.5 34.5 36 .8 40.3 C 0.015 0.017 0.017 0.019 0.017 0.018 0.012 0.016 0.017 0.02 0.02 0.02 0.018 0.019 0.017 0.018 0.02 0.019 0.02 0.021 0.019 0.019 0.019 0.017 0.017 0.021 0.019 0.034 0.055 0.035 0.007 0.006 0.06 0.008 Si 0.16 0.18 0.23 0.23 0.22 0.2 0.15 0.2 0.21 0.25 0.23 0.25 0.23 0.21 0.2 0.25 0.25 0, 28 0.24 0.23 0.22 0.62 0.21 0.61 0.24 0.42 0.89 0.48 0.12 0.12 0.11 0.14 Cu 0.01 0.01 0.01 0.03 0.03 0.03 0.01 0.01 0.03 0.04 0.14 0.14 0.13 0.13 0.12 0.13 0.14 0.18 0.14 1, 5 0.02 0.02 <0.01 0.01 <0.01 1.0 2.0 1.0 2.0 2.0 <0.01 <0.01 Co ... x ... so ..xo ... x..a_; 1 _0320 * AA 9: * .O »cn- * AA-» Jovmmcnwoa-cnmoacncncn <0.1 <0.1 <0.1 1.0 1.0 1.0 <0.1 1.0 0.1 2.0 <0.1 <0.1 55301 847 12 CCT / ° C PRENW% Cr + Cr Ni Mo WN 3.3% Mo Legefing1 77.5 49.9 42.8 28.9 6.6 4.2 0 .01 0.44 Alloy2 65 49.0 42.7 28.8 6.5 4.2 0.01 0.39 Alloy3 77.5 50.5 42.8 28.8 7.0 4.2 1 0.33 Alloy4 82.5 50.8 42.8 28.8 7.6 4.2 0.99 04 Alloy 65 48.2 42.1 28.1 6.5 4.2 0.01 0.38 Alloy 70 48.2 42.3 28.4 6, 9 4.2 <0.01 0.37 1429611097 75 48.4 42.5 28.8 7.0 4.2 <0.01 0.37 Alloys 80 50.4 42.7 26.9 6.5 4 .8 1.01 0.38 Alloy9 65 46.9 41.9 28.6 6.5 4.0 0.01 0.31 Alloy1ng10 57.5 50.3 43.0 29.0 5.5 4.2 <0.01 0.45 Leger1ng11 35 50.2 43.0 29.0 5.3 4.2 <0.01 0.45 159900912 70 43.5 41.4 27.5 5.9 4.2 <0 .01 0.44 Le9ef1 fl9 13 77.5 43.5 41.7 27.3 5.1 4.2 <0.01 0.43 Alloy 14 32.5 47.9 40.5 27.5 5.9 3 .9 1 0.35 Alloy 15 75 50.0 41.9 28.7 6.6 4.0 1 0.4 Alloy1ng15 77.5 49.3 43.3 30.0 7.1 4.0 <0, 01 0.33 Alloy 17 90 50.5 41.5 23.5 7.0 4.0 1 0.45 Le9erin918 35 49.9 41.4 23.2 5.5 4.0 1 0.4 3 1-999110919 30 49.2 42.7 23.3 7.0 4.2 <0.01 0.41 1-9991111920 72.5 49.3 43.3 29.3 5.5 4.2 <0 .01 0.33 Reading 21 55 47.3 41.4 25.3 7.1 4.7 <0.01 0.4 19991109 22 52.5 49.3 43.1 25.1 7.0 5.2 <0.01 0.42 Le9ef109 23 57.5 49.3 43.2 25.1 7.1 5.2 <0.01 0.41 1-999009 24 52.5 49.3 43.2, _g5, 3 7.0 5.1 <0.01 0.33 0595011925 52.5 43.5 42.0 25.2 5.5 4.3 <0.01 0.41 Ref 1 50 50.1 33.3 30 , 3 7.5 2.2 2.7 0.45 Ref 2 35 47.4 34.2 "29.27.5 1.5 3.72 0.44 Ref 3 45 50.3 33.0 30.5 7.7 2.2 2.33 0.47 Ref 4 40 49.4 35.5 31.3 9.4 1.5 3.35 0.41 Ref 5 30 49.9 34.5 29.3 5, 2 1.5 3.94 0.55 Ref 6 35 51.9 35.3 31.9 5.3 1.5 3.35 0.55 Ref 7 40 47.3 40.3 23.7 7.4 3 , 5 <0.01 0.44 10 15 20 '25 30 35 530 8-4? 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 CCT is for all the alloys according to the invention above 60 ° C, while it is not above 50 ° C for any of the reference alloys despite their high PRENW values. It has been shown that the PRENW value, as long as it is in a range above 46, has no real effect on the critical gap temperature of the alloy. Fig. 3 shows that Eq1 should be higher than 40.5 to achieve a CCT above (50 ° C. It should be noted that the experimental spread when determining the CCT value is large (about i10 ° C), and that it is the / total value of Eq1 that is decisive 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 the ASTM G-150, modified by fitting the examined samples with a column former in PVDF, mounted in approximately the same way as in MTI-2 in Testing 1. A constant tightening torque of 3 Nm was used for assembly of gap formers. A constant gap pressure was maintained by means of four spring washers 1 mounted according to Fig. 4, 2 showing the sample and 3 and 4 gap formers.

The test was performed for four different constant potentials: 0, +200, +400 and + 70O mVSCE. The initial 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 that results 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 results in the form of minimum values of CCT for all-a potentials above 0 mv in relation to SCE. 10 15 20 25 30 35 5313 34? 14 The variations in CCT between different potentials were small. In most cases, no crevice corrosion was noted at 0 mV.

Table 2 _ Legermg cr NI M0 w N PRENW Eq1 CCT Alloy 25 cm 7 3.45 <° -01 o, so 51,095 43.1 sz. 7 Lager-ing 27 25.62 7 4.73 <0.01 033 48,309 42.2 86, 7 Lager-ing 28 26.02 9 5.06 <0.0l 0.4 49.113 42] 57_1 Ref 5 31.9 6 1.47 3.85 055 5l, 9035 36.6 67.1 Ref 7 28.71 7 3.51 <0.01 0.44 47.333 40.3 76.3 Ref 8 29.88 7 1.5 1.98 0 , 44 45_137 343 53,; Ref 9 29/35 8 2,51 2f1 0,44 48,148 31,6 73,1 Ref 10 25 :: -, - m, <0,01 0,28 42,68 38.2 me Ref 11 292] -, - m2 Fig. 5 shows the relationship between Eq1 and CCT according to this modified ASTM 6150, some of the alloys are present in both Testing 1 and Testing 2. The two different testing methods give slightly different values of CCT, which is judged to be normal as a result of the differences between the methods. However, the relationship between the composition of the CCT value is essentially the same in the two Testings.

This Test 2 primarily measures the tendency to initiate crevice corrosion, while Testing 1 primarily measures the tendency for crevice corrosion to progress. This means that the CCT values in Testing 2 will be slightly higher than in Testing 1. This is the reason why the lowest permissible CCT value for use in a plate heat exchanger according to Testing 2 should be set to 80 ° C .

Testing 3 This testing has an electrochemical testing with constant potential and temperature for 24 hours. The same type of column former and the same column pressure as in Testing 2 were used. New samples were used for each temperature / potential combination. The potential was: +200 mV in relation to SCE. 10 15 20 '25 30 EBÜ 34? The CCT value for each sample was defined as the temperature that resulted in a corrosion current density of at least 0.01 mA / cm 2 in 60 seconds. This testing is extremely tough, as the material is tested in an active state, whereas in the previous tests it was passivated before testing, when the temperature was low. The ability of the material to form a passivating oxide layer at a certain temperature was measured in this test 3. The passivation temperature is for most of the materials considerably lower than CCT.

A comparison of alloys 26 and 27 in test 2 shows that alloy 26 was not passivated at 45 ° C, while alloy 27 was passivated at 45 °° and at 55 ° C. However, it can not be passivated at 65 ° C. This means that if a corrosion attack has been started as a consequence of, for example, an overtemperature, the alloy 27 will be able to be passivated already when the temperature drops below 55 ° C, which significantly reduces the risk of propagation of attacks in the event of operational disturbances. of the plate heat exchanger.

The alloy 27 with a higher content of Mo thus has a better ability to be passivated than alloy 26 despite the fact that alloy 26 has a higher Eqt value. The conclusion is that the Eqt value well describes the initiation and 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 24ppm, Ca 22ppm, N 0.41%. Critical gap temperature (CCT) testing according to MT1-2 for two samples resulted in 65 ° C and 70 ° C.

Claims (10)

10 15 20 '25 30 35 5310! 84? 16 Patent claims A plate of a plate heat exchanger, the plate comprising a material forming part of the plate with a surface made of this material positioned to be in direct contact with a chloride-containing coolant, such as seawater, characterized in that said plate material is a duplex stainless steel steel alloy containing by 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% with the remainder being Fe and normally occurring impurities, the ferrite content being 35-70% by volume, and the average Eq1 value of the two phases of the alloy exceeding 40.5, whereby Eq1 =% Cr + 3.3% Mo, where% is% by weight. Plate according to Claim 1, characterized in that the average Eq1 value of the two phases of the alloy is higher than 41, preferably higher than 42. Plate according to Claim 1 or characterized in that the Eq1 value for both the ferrite and austenite phases is higher than 35, preferably higher than 36. 20 15 20 '25 5313 84? 17 Plate according to one of the preceding claims, characterized in that the content of Mo is 4.5-6.5% by weight. Plate according to Claim 4, characterized in that the content of Mo is 4.5-5.5% by weight. Plate according to one of the preceding claims, characterized in that the content of Cr is 23.0-30.0% by weight. Plate according to any one of the preceding claims, characterized in that the average PRE value of the two phases of the alloy is higher than 46, preferably higher than 47, wherein PRE =% Cr + 3.3% Mo + 16% N, where% is% by weight. Plate according to one of the preceding claims, characterized in that the content of Al is O-0.1% by weight. Plate heat exchanger designed to utilize a chloride-containing liquid, such as seawater, in that it is constructed of drainage plates according to any one of claims 1-8. Use of a plate heat exchanger according to claim 9 for cooling a medium to be cooled by a chloride-containing coolant, such as seawater. 11%. Use according to claim 10, in which the temperature of the coolant is allowed to reach a temperature of at least 50 ° C, preferably at least 60 ° C. as a refrigerant, characterized