NEW
MATERIALS
AND
CORROSION
CONTROL
CORROSION RESISTANCE OF STAINLESS STEELS AND ALLOYS IN NITROPHOSKA PRODUCTION OPERATION MEDIA Yu. S. Sidorkina and G. P. Bekoeva
UDC 620.193.4 + 669.14.018.8 661.635.213
The production of Nitrophoska, one of the complex mineral fertilizers, is based on the decomposition of apatite concentrate by nitric acid with subsequent final decomposition with sulfuric acid and ammoniation of the slurry in the presence of sulfuric acid. The process is accomplished in 20 successively operating reactors at temperatures of 55-I15~ (Table i). An investigation made in the New Moscow V. Io Lenin "Azot" Production Association showed that the decomposition, final decomposition, and first ammoniation reactors (fifth to the eighth) are subject to the greatest corrosion failure. Decomposition reactors made of 12KhI8NIOT steel, as for the final decomposition and ammoniation (from the fifth to the eighth) reactors made of 10Khl7Nl3M2Tsteel, were subject to significant corrosion in the weld joints, as a result of which the reactors were replaced by similar ones after three to six years. Metallographic analysis of a weld sample from the shell of the fifth ammoniation reactor showed the presence of 2800-~m-deep knife-edge corrosion (Fig. i). The ammoniation reactors (from the 9th to the 16th) made of 10KhI7NI3M2T steel were in service for i0 years. The reactors for mixing the slurry with potassium chloride (from the 17th to the 20th) made of 10KhI7MI3M2T steel are subject to deep point and pitting corrosion and fail after four to six years of service. At the Ionava Nitrogen Fertilizer Plant all of the first-phase reactors were made of 06KhN28MDT alloy. The welding was done with OZLI7U electrodes. The second-phase reactors (from the first to the eighth and from 17th to the 20th) are made of 06Kh28MDT alloy and from the 9th to the 16th, of 10KhI7NI3M2T steel~ In inspecting the inner surfaces of these TABLE i.
Reactor Operating Conditions
o
o
~ 1 2
Production operation
Medium
Decomposition of apatite withnitric acid
4
Final decomposition of apatite with sulfuric acid
a
Ammoniation of the slurry
7
(partialneutralization)
3
6
a 9 10 11 12 13 14
Ammoniation of the slurry in the presence of sulfuric acid
] Apatite + 55% nitric acid LI Slurry + 92.5% sulfuric acid
60-90
60-90
Slurry + gaseous ammonia
Slaty +gaseous ammonia + sulfuric acid
of neutraliza-
Mixing with potassium chloride
Slurry +gaseous ammonia
Shm:y +gaseous ammonia + potassium chloride; pH ->4.5; available phosphoric anhydride !09
Translated from Khimicheskoe i Neftyanoe Mashinostroenie,
432
55- s0 55-80
90-- 115 95--100 95 - - l 15 100 --II5 I00--115 I00--I15 190 --I]5 100--115 I I 0 0 --115 100 --115
I
18116175I tionC~176
19 20
Temperature, ~
0009-2355/81/0708-0432507.50
100 - - 1 1 5 I00 --115 95 - - 1 1 5 95-115 95--115 95 --I15
No. 8, pp. 26, August, 1981o
9 1982 Plenum Publishing Corporation
Fig. i. Knife-edge corrosion of a weld joint in 10KhI7NI3M2T steel made with an NZh-13 electrode.
TABLE 2,
Chemical Composition of the Steels and Alloys I
Material
II Eiectrode
Chemical corn~sition C
Si
Mn [ Cr t Ni [ Cu
Mo
S
P
Ti/Nb
I
83KhlSNII 12KhlSNIOT 08Kh22NGT
10EILtTN!gM2T 0SKh21NOM2T
03Kh21N21M4GB 0~?KIuN2 8 M D T
03KI~28MDT
0,03 0.0~ 6,06 EA400/1OU' 0,07 ~ A 4 0 0 / 1 0 U 0,051 OZL-17U 0,01~ OZL-17U 0,04~ OZL-i ?U 0,02(
OZL-22 TsL-II TsL-I!
0,8 2,0 18,0 [11,5 I -0,61 1,22 ~17,35[I0,371 -0,64 0,40 12o,89l 5,341 - 0,31
1,40 16,75 12,741 - 0,33 21,08 6,21 -0,88 21 81 20,25} - 0,30 22,40127,88 2,87
0,48 0,52 0,69 0,39 0,20 22,43 27,30 3,38
--
~-,34 1,87
2,9I 2,60 3,06
0,020 0,010 0.011 0,010 0,010 0,008 0,011 0,017
0,035 0,023 0,023
0,035 0,027 0,017 0,027 0,021
~G 0,42 0,49 0,31 0,72 0,76 0,47
reactors, visible corrosion of the base metal and the weld joints was not observed as a result of the heavy deposits. Therefore, to determine the conditions of the base metal and the weld joint of the shell of the fourth reactor made of 06KhN28MDT alloy (in use for 2~ years) a weld sample was cut. Metallographic investigation of it showed the presence of 990-~m-deep knife-edge corrosion in the weld joint. Investigation of a similar sample of the shell of the 19th reactor revealed 157-~m-deep intergranular corrosion in the joint and insignificant intergranular corrosion in the heat-affected zone (less than 30 ~m deep). In visual inspection of the inner surface of this reactor after washing of it, point and pitting corrosion was observed over the whole surface. The depth of the areas of corrosion was as high as 2 mmo The All-Union Scientific-Research and Design Institute for Chemical Machinery Building has made investigations of the corrosion resistance of steels and their weld joints under the operating conditions of the 20th reactor of the Nitrophoska department of the Ionava Nitrogen Fertilizer Plant (Table 2). Tests of the steels and alloys according to All-Union State Standard 6032-75 did not reveal intergranular corrosion. After 2200 h of operation in the second decomposition reactor and in the 7th and the 9th to the 18th ammoniation reactor in the liquid phase the rate of general corrosion of all of the steels and alloys did not exceed 0~ mm/yro In visual inspection insignificant intergranular corrosion from the ends was observed only on samples of 08Kh22N6T steel. Under the operating conditions of the final decomposition reactors all of the steels had reduced resistance and general corrosion of the base metal and the weld joint is observed. The corrosion rates of 03KhI8NIIT,12KhI8NIOT, 08Kh22N6T, 10KhI7NI3M2T, 08Kh21N6M2T, and 03Kh21N21M4GB steels are 0.3mm/yr in the third reactor and 0.8mm/yr in the fourth and the rates of type 23-28 alloys are 0.13 mm/yr in the third reactor and 0~ ~ / y r in the fourth. In visual inspection and metallographic analysis of samples from the third and fourth reactors made of 12KhI8NIOT, 08Kh22N6T, 10KhI7NI3M2T~ and 08Kh21N6M2T steels and 06KhN28MDT and 03KhN28MDT alloys knife-edge corrosion with a depth of 40-155 Dm was observed. It should be noted that the depth of the knife-edge corrosion of 03KhN28MDT alloy is significantly less than of 06Kh28MDT alloy~ In the third reactor made of 06KhN28MDT and 03KhN28MDT alloys knife-edge corrosion was not observed~ Knife-edge corrosion is absent in these reactors only for 03KhI8N!I and 03Kh21N21M4GB steels and 03KhN26MDB alloy. The character of knife-edge corrosion of 08Kh22N6T and 08Kh21N6M2T steels is structurally selective with failure occurring in the austenite, while in 12KhI8NIOT and 10KhI7NI3M2T steels and 06KhN28MDT and 03KhN28MDT alloys it is uniform over the line of fusion.
433
Under the operating conditions of the reactors for mixing slurry with potassium chloride 03KhI8NII, 12KhI8NIOT, 08Kh22N6T, 10KhI7NI3M2T, and 08Kh21N6M2T steels are not corrosion resistant, but are subject to deep point and pitting corrosion over the whole surface. In contrast to the above steels, on samples of 03Kh21N21M4GB steel and type 23-28 alloys only individual points of point and pitting corrosion were observed, and the corrosion rate did not exceed 0.08 mm/yr. On the basis of the investigation data and production test results, 03KhN28MDT alloy is recommended for production of the first to the eighth reactors and after mastering of it 03KhN26MDB alloy, for the 9th to the 18th ammoniation reactors 08Kh21N6M2T steel, and for the reactors for mixing slurry with potassium cloride 03Kh21N21M4GB steel and 06KhN28MDT, 03KhN28M])T, and 03KhN26MDB alloys.
FAILURE OF TECHNICAL-ALUMINUM WELDED JOINTS IN NITRIC ACID L. V. Zaitseva, A. N. Kuzyukov, E. K. Malakhova, and No S. Mashchenko
UDC 621.791.052.620.197 461.7 + 669.71
From the results of checking equipment made of type A5, A7, or A85 aluminum and the information from questionaires from chemical combines, it has been found that the most vulnerable places are the welded joints. The fused metal in the seam and the thermal effect zone are subjected to intense corrosion. However, the service life of equipment up to the first repair is different. It should be noted that in the very same piece of equipment there are seam sections with only weak traces of corrosion and also with intensified corrosion which passes into perforative failure. In a number of cases of operating equipment at 30-40~ (storage vessels or cisterns for concentrated nitric acid), the depth of corrosion of the weld seams after a year of operation is 2-3 mm in the liquid phase, but in the gas phase during the same period perforative failure occurs. Operation at 80-I00~ (decolorizing columns, reaction vessels of autoclaves, etc.) leads to perforative failure in most cases after 200-3000 h of operation. In [1-3], an intercrystalline character of the failure of welded joints in nitric acid is indicated. Studies of sections of welded joints having extensive and slight corrosion, cut from the reaction vessel of an autoclave (Fig. i), confirmed the presence of intensive corrosion in seams which are prone to intercrystalline corrosion (ICC). In seams which were not prone to ICC, the corrosion was considerably less. To increase the service life of equipment, it is necessary to find out the reasons for intercrystal!ine failure of welded joints in 98% nitric acid, since up till now aluminum has been the basic constructional material for apparatus design in this field. There are various opinions about the reasons for ICC in aluminum [3, 4]; however, all the suggestions can be provisionally divided into two general areas; the effect of the disorientation angle, and the effect of impurities.
Fig~ !o Character of failure in seams of reaction vessel of an autoclave made of A5 aluminum i00 •
Translated from Khimicheskoe i Neftyanoe Mashinostroenie, No. 8, pp. 27-29, August, 1981.
434
0009-2355/81/0708-0434507~
9 1982 Plenum Publishing Corporation
