UREA PLANT CORROSION T. A. Lees

Sherritt Gordon

Fort Saskatchewan,

At our Fort Saskatchewan plant we use the Starnicarbon process. We attribute our success with this process to the fact that they have a patented method of air or oxygen injection. In Figure 1 you will note the stream from the top comes into the first-stage rectifier. The solution goes through the rectifier, down into the bottom. and then up into the heater. This is heated to 325~F. It then goes into the separator, the gas leaving out the top and the liquid out the bottom. A form of level glass is indicated there,

The worst corrosion is at the top. on the gas line. The line we have is 316L and we have had to re-place it about every 9 to 10 months. We are going to change this to 317L very definitely. We did toy with Hastelloy F, however, we are going to try the 317L. As you can note the corrosion is marked at this end of the elbow and where it goes into the rectifier.

First-stage sepa rotor

The first-stage separator is there on the right. It showed bad corrosion inside from the top down, this was probably due to water and gases, We had a purge water system going in there for level indication and it

G3S_

Corrosion

-160 Ib steam

-

CorrOSion 7

Solution

I ~2~:;;;:===::::=J 35 Ib_ condensate ... Solution

Figure 1. Corrosion sitl!S in the first-stage separator and rectifier.

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Mines Limited

Alberta, Canada

seemed to trickle down along the side where the corro-sion occurred. The gas, flowing upward against it, caused the etching as there was little below the gas inlet nozzle, We thought we had eliminated this by putting a titanium tube down below the gas inlet but the corrosion did continue. As a result, in two years time we had to replace this vessel. The vessel was 316L 3/8-in. thick and is now 316L l/2,-in. thick.

We have had the new vessel in operation about 7 months. We examined it in July, 1965, and itls in excellent condition. There is no corrosion whatsoever.

Rectifier inlet

The next place that is a sore spot is where the gas enters the rectifier. The nozzle at this point did give us a lot of trouble, and by the way, while lim talking about the rectifier and your mentioning 304, we went for two years and noticed a leak on a thermowell for a good many months and as it started getting bad we had to take the plant down in a hurry. When we had it down we examined and found the thermowell was 304 and it had been in there fOl' two full years. It was re-placed with 316L.

The inlet gas nozzle on this first rectifier that I mentioned started giving us trouble at the weld. We overlaid this weld in July, 1963. (The unit was put on-stream October 1, 1962.) Later, the nozzle corroded underneath the weld, and through, outside to the re-inforcing plate. We found it leaking out the weepho1e, so we had to shut the plant down again in January, 1964, and reinforce this nozzle.

Dish weld corrosion

During our July 1964 regular shutdown, we re-moved the stub completely and we overlaid the weld with Hastelloy C. This did not do too badly but if you will notice, there are two dots indicating corrosion at the bottom dish of the vessel where the weld had started to corrode. Metal thickness measurements continued to worsen. In January of this year we had to shut down for other reasons and upon examining this, we did find that the dish weld was getting bad, however, we figured it would last until July. We then decided to replace the bottom half in July, 1965, and did so. We have now replaced the bottom half of this vessel with 317L 1/2-in. thick stainless. It was 316L 3/8-in. thick,

Solution short circuit

One other thing that we found on one occasion was a solution short circuit. We have a second-stage

rectifier similar to this first stage. We found a short circuit due to a blockage necessitating the dumping of the product to the sewer. It wasn't getting over to our storage at all. This forced us to shut down. Examina-tion revealed that our second stage heater, which is comparable to the one shown in Figure I (a little smaller), was solidly plugged. We got nothing through it. The reason was that it was full of iron throughout the Raschig rings. We further found that there was a terrific amount of corrosion in that first stage rectifier. The Raschig rings had become paper thin in this first stage. We have taken steps to eliminate this problem and 1 think we have it licked.

The only other corrosion we have is a slight bit of etching in the bottom of our reactor. We have always ground this and then filled with weld. On the next opening we go in again to find the same thing is occurring. So this time we tried something new. I don't know whether it is going to work 01' not. We ground where the etchings were and did not fill them all. About half the etchings were refilled by welding and we are going to see how these compare.

Check valves

Another problem occurs with our check valves, which are on three lines (3,000 lb.) leading into the reactor; carbamate, carbon dioxide, and ammonia. They get little etchings on them, which I assume to be corrosion. They offer us a problem because when we shutdown, we rely on these valves, and if the operators are not quick enough in shutting the manual isolations, it gives us a problem by allowing solution to backup and plug the lines. Every time we get a chance, we lap these check valves. We go to great lengths on this even though it is very time consuming.

One other thing I might mention here is that we use aluminum gaskets on top of our reactor and they stand up very well.

We apply Arocoat, which is made by the British American Paint Co., to our tower walls at the bottom, to any exposed steel, and all around our pillars in the building, It has pretty well eliminated corrosion of steel. If you don't get a good bond, of course, solution

is going to get underneath, but it does a commendable job and we are pleased with it.

On the top of our wash column we had a 316 gasket, which we couldn't stop from leaking. We finally resorted to Durabla. We have used Durabl a on 250 lb. systems and at 200°F with no problems.

Suction valves

In connection with the Teflon suction valves, we use a Hills McCanna with stainless steel ball and Teflon seat. We previously had two stainless steel wedge valves that gave us nothing but problems, and we couldn't shutdown to repair one pump or the other. We got this Hills McCanna plug-type ball valve and it has been in service for two years. It has always held and it has never given us the least bit of trouble.

Another excellent valve that we have found for small jobs like sample places, is made by Western Associated Engineering Co. W-c call it the WACO valve, and I heard you mentioning the other day Nort about the stems and things going. We find the only way to success-and I'm sure you must know this-is that you must have a valve where the packing is between the plug and the rising stem thread. WACO has this.

We have a Dutch State Mine valve with which we are very pleased. We tried to get this valve for use in our plant expansion, but delivery was too slow, but we are pleased with the Dutch State Mine valve. If you have the time to order, is an excellent valve.

Safety valves

In connection with safeties, we have no prob-lems with them. We have stearn jackets on all our urea safeties, and then on the discharge of the safety we have live steam continuously directed into the dis-charge. This keeps the safety hot and urea will not solidify in the discharge line. We did find one safety valve that would eventually have given us problems. We noticed that the pressure was high and the safety did not lift. Subsequent investigation revealed that the discharge side was full of carbamate and this is why the safety didn't lift.

DISCUSSION

Pertaining to papers by N. H. Walton and T. A. Lees.

WALTON-SunOlin Chemical: This is certainly in-teresting data. Looking at my own experience, I visited a Toyo plant at Aruba. I think there are several being built in this country now but none actually in service yet. They had as I remember two corrosion problems there. One was the reactor. It was a lead-lined re-actor. and every three to six months they had to shut-down and go into the reactor to make repairs.

When they shutdown they found lead downstream of the reactor in various places in the system. The lead was in the shape of balls and they varied in size from the size of shot to the size of baseballs.

Another problem that they had at that plant was they used titanium for their let-down line stem, and again they had th,e same type of results that we had where erosion seems to be a problem.

BRESS-Foster Wheeler: With reference to the data you presented on autoclave corrosion, I think it's important that in a historical reference the intro-duction of oxygen or air permitted the use of stainless

steel in autoclave. The eadier processes were based on the complete exclusion of air. Lead, silver, and, in the downstream parts of the plant, Ampco are success-ful. Lead will react quantitatively with any air that enters the system, however, if you exclude ail', it worked perfectly. I believe that zero corrosion rates were found.

The important thing about lead is its successful application to the autoclave. We learned it and our clients learned by bitter experience how to do this. However, once it was done, corrosion rates went to substantially nothing. The introduction of the stainless steels, in the exclusion-type processes, where there was no ail' in the autoclave, were not successful.

Since the conditions are not so severe in the back end of the plant, this may permit the use of stainless steels. However, you do not have the same application of oxygen 01' air to the back end of the plant. In places like valves or valve stems where you cannot get ail' in, these conditions may give the same type of corrosion that was experienced in the autoclave when you excluded oxygen and tried to use stainless steel.

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WALTON: I think the point's well taken here. You have to have passivated conditions to start with and maintain passivation of your stainless steel in order to keep corrosion rates down. We have had a couple of occa-sions where we've gotten a couple of parts per million of hydrogen sulfide in the carbon dioxide and you make red prills in a hurry. In our e:l-.-perience, the only way to stop it is to shutdown the reactor, fill up with water, and bubble a few cylinders of oxygen up through it to repassivate the liner again, then you're okay.

BRESS: In those areas where you experienced fairly rapid and repetitive corrosion such as the vapor line, do you resort or did you resort to on-stream inspection for metal thinning or did you rely upon visual inspec-tion during shutdown.

LEES: No, we definitely did not use on-stream in-spection. We took these pieces apart every time we had a shutdown. We have an audio gauging method with which we can read through the metal but we don't rely on it too much when we are on-stream. We don't think it is that good while on-stream.

Usually, if we have a leak it develops slowly, we know pretty well whether we are going to be able to last or what we have to do about it. It is not too hard to shut the plant down to fix it up with a temporary repair or something of this nature.

WALTON: This is certainly one good thing about urea plants. You can shut them down and start them up in a big hurry. It's not like a natural gas reformer where it takes days to shut it down and start it up again.

REED-Girdler Corp.: I'd like to confil'm a few of the points that were made earlier. With respect to nickel, I saw an instance in which a nickel screen was used in a centrifuge for separating urea crystals from mother liquor in a solution that contained substantially pure urea in water at a relatively low temperature of say l40°F. This nickel screen dissolved in approximately one week, which was quite surprising. It was a thin screen with fine openings and it encountered high velocity so that you had erosion.

Concerning your mention of crevice corrosion, I think that the comment that was made here just a moment ago is pertinent to that point. Protection, or corrosion resistance, of the austenitic stainless steels depends upon an oxide film on the surface and, in the operation of the urea plant, it depends upon the con-tinuous addition of oxygen to the reaction stream. If you have areas in the plant where the oxygen can be absorbed and there is no fresh oxygen present, then the stainless in the contact with carbamate and urea at these high temperatures will corrode. It is a point that needs to be watched very carefully in the design of the plants to avoid such areas wherever possible. In some instances it is advisable to make arrangements to in-ject a small amount of air into such areas. For example, liquid level control of bottles, etc., can be protected by the deliberate injection of a small, very small quantity of air. Perhaps in the Stamicarbon plant injection of a small stream of ail' into the separator might protect that gas line by providing additional oxygen. It may be that in the initial rectifier that the oxygen that's present in the solution coming from the reactor has flashed out overhead and the liquor stream going through the heater does not carry enough oxygen to protect the equipment.

Mr. Walton already mentioned the erosion of the titanium sterns on the let-down valve. I was going to mention that titanium is very definitely a comparatively soft metal. It again depends on oxide film, and under conditions of high velocity where the oxide film is re-

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moved, erosion-type corrosion will occur very rapidly. It's easy to see visually because the color of the ma-terial is completely differ.ent where the film is absent compared to where it is present.

Another point I would confirm is that Hastelloy F has proved to be a satisfactory material as was in-dicated by these tests. Zirconium has also proved to be satisfactory but again difficult to obtain. Also, ex-perience in locations in the hot end of the system, on the high pressure end of the system where 304 was inadvertently put in instead of 316L, it ordinarily did not stand up very well.

SPEED-International Nickel: The data Mr. Walton presented defines both the question and the answer on materials of construction for urea plants. E:l-.-perience has shown that corrosion is proportional to the avail-able free oxygen in the system. The presence of air permits the use of the austenitic stainless steels which depend on a passive film for maximum corrosion re-sistance.

Let us review some of the data presented, As an aid to this review, I have listed the nominal chemical analyses of the alloys under discussion in Table 1.

TABLE 1. NOMINAL CHEMICAL ANALYSIS OF ALLOYS.

Alloys Fe er Ni Mo Cu Mn S. S. 202 67 18 5 9 S. S. 304 74 18 8 S. s. 316 69 18 10 2.5 S. S. 317 63 19 12 3.5 2 S. s. 329 70 25 4 1 S. s. 309 62 23 13 2 S. S. 310 53 25 20 17-4 PH 75 17 4 4 Carpenter 20 Cb 3 38 21 35 3 3 Incoloy 804 25 30 42 Incoloy 825 32 21 42 3 2 Hastelloy F 17 22 47 6.5 Hastelloy B 5 61 28 Hastelloy C 5 15.5 54 16 Inconel X-750 7 15 73 2.5 Ti Inc one I 600 7 16 76 Nickel 99.5 Monel 400 66 31.5 Illium G 6 22 56 6 6

The successful use of the austenitic stainless steels in this service requires careful selection of the alloy's iron, chromium, nickel, and molybdenum con-tent.

For example, Inconel alloy 600, Inconel alloy X-750, Inconel alloy 700, Nickel, Nimonic alloy 75 showed poor resistance to the urea-ammonium carbon-ate environment. The reason: too high a nickel content, too low in chromium, iron, and molybdenum.

Hastelloy alloys Band C also did poorly. The reason: too high a nickel and molybdenum content and too low in chromium and iron.

The high iron alloys, such as 405 stainless steel and 18% nickel mar aging steel performed poorly. The reason: insufficient chromium, nickel, and molybdenum.

The copper-bearing alloys (Monel alloy 400 and Ampco 8), as to be expected, were severely corroded.

In general, the best resistance to corrosion was demorlstrated by the austenitic stainless steels ranging from 304 stainless steel to Hastelloy alloy F.

Note, however, that within this nickel-chromium-iron range are certain alloys designed for high tem-perature service and were not necessarily intended for aqueous corrosion service. These alloys are:

309, 310, 330 stainless steels, Incoloy alloy 804, and Incoloy alloy 90 I.

With the remaining alloys, 304, 316, 317 stain-less steels. Carpenter 20 Cb 3, Incoloy alloy 825, and Hastelloy alloy F, the resistance to corrosion increases in the order as listed.

In my opinion, 316 and 316L stainless steels should be considered the minimum standard alloy for urea plant service. In areas where chloride ion stress corrosion cracking and intergranular corrosion lTIay be a problem or where the alloy lacks general corro-sion resistance, then Carpenter 20 Cb 3, Incoloy alloy 825, or Ha.stelloy alloy F should be considered.

The reason for SOlTIe of the failures exhibited by Mr. Walton are clear when we exalTIine the failure on the basis of the previous discussion.

For example, on the valve stern problem the 316 stainless steel failed from erosion-corrosion. Type 17-4 PH steel was substituted in an effort to ob-tain better erosion resistance. Unfortunately, the heat treatment used for hardening also sensitized the alloy and failure occurred, not by erosion-corrosion, but by intergranular attack.

We were also shown a section of 316 stainless steel pipe with a carbon steel jacket used for stealTI heating or water cooling. Failure occurred when the 316 stainless steel collapsed in an area; where the carbon steel was welded circumferentially to the 316.

=========:::::..--~ Carbon steel Jacket A;; "A" J3l6 5.5. pipe

Figure L Section of 316 stainless steel pipe with carbon steel jacket lIsed [or steam healing or water cooling.

If you sectioned the pipe at Area A, Figure 1, and examined the failure, you would probably find one or both of the following:

(I) During welding, the 316 stainless steel could have become sensitized. Intergranulal' corrosion caused a loss of strength in the pipe which resulted in a collapsing-type failure.

{2} A circumferential weld of this type results in a high residual stress. This is aggravated by the difference in thermal coefficients of e::o..-pansion between the carbon steel jacket and the 316 stainless steel pipe. Chloride ions in the steam could have caused the failure by stress corrosion cracking. All of the ingre-dients for this type of failure were present; i.e., chloride ions, stress, oxygen, and temperatures above 140 0 F.

WALTON: What is the answ:er on this steam-jacketed piping?

SPEED: Steam tracing will help. That would be the simplest and the most econolTIical solution. The only other thing you can do is to go to an alloy that resists stress cracking.

Here again is where your nickel comes into play. The least a!loy we would recommend for this is Carpenter 20 Cb which has the 34% nicl~ell then up to 825 which has 42% nickel, and then up to the F which is 46% nickel.

STOCKBRIDGE-Southern Nitrogen: I can corroborate the collapse too. We had a similar case. It is our

philosophy that if thin pipe fails, put in a thicker one. We had a Schedule 10 and replaced it with Schedule 40. So far, we haven't had a collapse.

WALTON: Have you considered Unitrace?

STOCKBRIDGE: No.

WALTON: We had some aluminum Unitrace in our draining system. It was a grand and glorious failure in our experience. I wasn't there the first year that the plant was in operation, and I don't know what the prob-lems were but I know that when I came in they had some pieces of it sitting around and they said don't ever ask us to use it.

STOCKBRIDGE: I want to ask Mr, Walton about the globe valve problem you had. You said that you did some welding. It wasn't clear to me whether welding was done inside the valve and you retained the same body.

WALTON: Yes, inside the valve. You build up a mass of metal where the seat ring would be screwed in, then machined out a seat which then is integral with the body of the valve.

STOCKBRIDGE: I've recently seen a new product that Dupont's coming out with for gaskets. It's a material that is Teflon felt. They dip this into a solution of Teflon. It looked very good to me. I haven't tried it yet, but I intend to try it some.

WALTON: Talking about crevice corrosion of ring-type joints, I'm still uncertain in my own mind what the solution is because these riug protectors, that we're going to try, will only prevent turbulence from occurring. They're not a positive pressure seal and carbamate will intrude behind the ring protector right up to the ring. So I don't feel as though this is the answer.

Injecting air in between the flanges of the ring-type joints, while this would certainly do it, is a complicated thing to do and maintain, therefore, it doesn't seem practical.

MASON-Dow Chemical: You were mentioning the difficulty with pitting in the tubes on the water side. Have you considered the possibility of using copper clad on the water side? We have had quite a lot of corrosion problem that have been almost com-pletel y eliminated by copper cladding the material on the water side.

WALTON: Copper cladding stainless?

MASON: This was just copper cladding on ordinary steel pipe.

WALTON: Yes, duplex tubes would help here although you get a lot of different opinions on duplex tubes. It can be a fighting word in some places. I think I was probably partly responsible for introducing duplex tubes to the Atlantic Refining Co., and they had some pretty sad results with collapsed tubes for a number of years. I think probably the science of duplex tubes has improved quite a bit today.

CHRISTIAN- United States Steel: I would Hke to add a few additional comments to Mr. Speed's discussion of the failure of the steam-jacketed type 316 stainless steel line carrying carbamate solution. E::

steel is not very common. I think it would be desirable to establish just what type of failure has occurred at the welded connection. If failure was due to intergranu-lar corrosion, I believe it's more likely to have originated from the process {carbamate solution} side, in which case going to a low carbon grade of type 316 stainless steel might prevent a recurrence of the prob-lem. Certainly, the use of type 316L stainless steel should be investigated before going to some of the more exotic constructional materials that were suggested.

A question I also wanted to ask Mr. Walton con-cerns the cooling water in his urea plant which pro-duced pitting of type 316L stainless steel tubes in a high pressure carbamate cooler. Mr. Walton, are you using a corrosion inhibitor in the cooling water?

WALTON: Yes.

CHRISTIAN: Do you happen to know which type it is?

WALTON: It's a Nalco treatment. I don't remember right now what the prescription is.

CHRISTIAN: Is it a chromate base inhibitor and are you chlorinating your cooling tower water for algae control?

WALTON: Yes. Whenever you get into metallurgy you get into disagreements. Each of us who either is a metallurgist or takes a part time approach to it, as I do, thinks that we have evidence to support our stand. For instance, the remarks that were made about lead lining in a reactor, there is a reactor that doesn't have oxygen in it, it is lead lined and it is in trouble. Un-doubtedly there are other lead-lined reactors without oxygen that are not in trouble.

ROSENBLOOM-Mobil Chemical: I think it's a very good idea to inject the oxygen as Mr. Reed suggests, but would this be infringing on Stamicarbon's patent?

WALTON: Well, we could get into a great big argument about this. Stamicarbon is not the only one that uses oxygen in their process. What the patent situation is, I don't know. There are a number of processes which use oxygen injection into the reaction system. by some means or another. In the Toyo plant that I saw, the oxygen injection was downstream of the reactor. I guess the other processes that I know or have seen have, by some means, oxygen going into the reactor.

REED: Injection of oxygen at any point downstream of the reactor should fall outside the claims of the Stamicarbon patent.

CRISTO-Esso Research: We're relatively new to the fertiliz.er field and we have had our share of difficulties. It follows that we are very interested in all the discus-sion here. One thing we have learned is that perhaps the delta ferrite content is important in 316 and 316L and that maintaining a low ferrite content in the welds of this material is important. I was just wondering whether this group has had any e}.."pel'ience with this. Do you control the ferrite content in 316 and 316L? Mr. Walton, In these corrosion tests, that you ran, do you know what the delta ferrite content was both in the material and in the welds?

WALTON: No. I would have said that any of the 300 series are completely austenitic and have no ferrite structure.

On the 329, I just had one piece of information, and that was a double one which of course is very good but it's impossible to depend on one reading.

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Anonymous: You can go back to Mr. Swan's answer too. He had the 440 which is a high chromium 400 series stainless, and the chromium gives you resistance to this type of environment. This is why his success was good.

Anonymous: This would be another control that you would have to specify. We don't deliberately try to get ferrite. This defeats the whole purpose of the austen-itic structure.

JONES-Canadian Industries, Ltd.: I noted eaJ:lier in your test data, that, in general, corrosion rates were slightly lower with the regular carbon grades of stain-less steel than with the low carbon grades. The higher chromium content of type 309 did not improve corrosion behaviour, neither did the higher molybdenum content of type 317 or 317L. It would seem that there is a pattern here suggesting preferred corrosion of the delta ferrite phase, since the low carbon grades are more likely to have grain boundary delta ferrite than the regular carbon stainless steels. The high chro-mium-nickel ratio of type 309 and the higher molyb-denum content of type 317 are also likely to result in increa~ed delta ferrite formation, and this appears to be reflected in their disappointing corrosion rates.

We did have occasion to require wholly austen-itic type 316L material, two or three years ago, for a specific strong nitric acid environment, in which we found this alloy was considerably mOl'e resistant than type 304L. It was possible to obtain satisfactory Huey ratings on this special type 316L, and microscopic examination of welded and sensitized specimens re-vealed very clean grain boundaries, free from delta ferrite. We bought the material in heats and paid a premium price for the special control of composition necessary. It is likely that stainless steel to this chemistry would do well in urea environments.

A completely austenitic 316L will generally have a lower chromium-nickel ratio than a 316L containing delta ferrite, i.e., a 17% Cr-14% Ni will be more wholly austenitic than an 18% Cr-lO% Ni stainless steel. American made 316 is generally in the former category, whereas European equivalents tend to be in the latter category.

The practical difference is that the latter category will tend to corrode more rapidly in urea environments. The delta ferrite present at the grain boundaries will probably be the anode in a galvanic corrosion cell, with the austenite grains cathodic. If there is a large amount of delta ferrite, say, over 10%, then it is likely that there will be a fairly continuous path through the grain structure along delta ferrite enabling extensive corrosion to develop. Similarly, if there is only a small amount of delta ferrite present, it is unlikely that there is a continuous route through the structure, and the material will resist corrosion.

Delta ferrite will tend to form in the heat af-fected zone during welding, and corrosion, is likely to be more rapid at this location. Since ferrite is magnetic, it is possible that welded samples of stain-less steels to be used for urea plants could be evalu-ated this way.

MARCH-Atlas Chemical: We have a Stam.icarbon urea plant, the same as Mr. Lees. We pay a great deal of attention to ferrite content in our 316. This is true not only in original materials, as Mr. Lees will vouch for, but in any welding repairs on the 316. We ferrite check right as we weld, and if we find a ferrite content that is higher than we consider permissible we grind it out and do it again. The big problem that we find in holding the ferrite content down is cleanliness. We've had to

educate our welders 50 that we get ferrite contents that we think will resist corrosion.

SPEED: We have had the same experience on paper and pulp digesters where the ferrite content is coated preferentially and on fatty acid reactors where the 316 is coated preferentially. We're repairing with Incoloy alloy 825 coated electrode in most cases. Here it's just a matter of economics. You get a fully austenitic structure and this coated electrode is about half the price of Carpenter 20 coated electrode. So you might face a big jump on alloy costs. Certainly I would think this would offset the expense of going back and check-

ing and chipping out. This will give you the assurance of a full austenitic structure.

PRESCOTT-C. F. Braun: The Stamicarbon people do specify a maximum ferrite content. The problem is to find the supplier who can meet these requirements. It's very difficult to get somebody to guarantee the very low ferrite content in a large piece of stainless steel. The other problem is to measure accurately the amount of ferrite in stainless steel. If you get ten different people, or ten different laboratories to make this measurement, you'll get ten different answers. It's a little bit troublesome to resolve this point.

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