Corrosion Prevention during Acid Cleaning of Pulping Equipment Jorge J. Perdomo , Pablo R. Conde * , Preet M. Singh , Moses Wekesa * Engineering Services Division, Smurfit-Stone Container Co., Carol Stream, IL 60188-2130 * Institute of Paper Science and Technology at Georgia Tech, Atlanta, GA 30332-0620 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245 ABSTRACT Kraft digesters and evaporators are typically acid cleaned to remove scales that precipitate from chemical species present in liquors. Their removal by chemical methods can detrimentally affect the base metal where they grow on. Scale removal may in turn affect the condition and integrity of the equipment by causing general or localized corrosion, which may lead to unexpected failures. Previous work has addressed acid cleaning methods by looking at a number of variables that affect scale removal as well as corrosion. However, no published work is available in terms of the removal of actual scales. The objective of this work is to show how optimal cleaning practices developed in the laboratory for Kraft digesters, evaporators and other pulping equipment work on simulated scales grown on different base metals. The results of this study will help minimizing cleaning costs, maximizing its effectiveness, and to ultimately minimizing corrosion (metal loss) during scale removal. BACKGROUND Most Kraft digesters and evaporators in North American paper mills are periodically acid cleaned to remove insoluble scales like burkiete (Na 2 CO 3 .2N 2 SO 4 ) and carbonate scales on heat exchanger surfaces [1]. Despite the importance of such maintenance procedures, many mistakes have occurred causing significant damage (i.e., corrosion) of digester and evaporators vessel walls, screens and piping [2-4]. Scale characteristics (e.g., composition) may be different in evaporators compared to those observed in digesters or liquor heater surfaces, but have to be regularly removed for efficient heat exchange. Scales are considered generally non-corrosive because a protective carbonate film (carbonaceous) is formed on the surface of the metal. The presence of carbon dioxide in waters favors the formation of bicarbonate ions as pH is increased [5]. The bicarbonate ion forms insoluble calcium carbonate that may also precipitate on the surface of the equipment. Carbonate compounds have reverse solubility and precipitate out as temperature rises. In terms of corrosion the presence of carbonate layers (some times referred as scaling or fouling) are considered beneficial. However, throughputs are reduced, while pumping costs and heating or cooling requirements are increased because of the lowering flow rates and heat transfer coefficients and by the increased pressure drops [6], hence, the need for acid cleaning to remove those scales. Although most cleaning for pulping equipment is performed with inhibited muriatic acid (HCl), other acids such as sulfamic, formic and nitric have been studied [2]. Acid concentration, inhibitor, temperature, time and flow are important parameters that will establish well-defined procedures to minimize damage to the equipment. Despite the relevance of such procedures to the pulp and paper industry, very little published work has addressed the problems faced during acid cleaning. In the past, only the work conducted by Crowe in the early nineties [2] and Henrikson in the early eighties [7] looked at the importance of acid cleaning procedures on the integrity of structural equipment. The topic had been long forgotten until recently when information of common acid cleaning practices was made available in the literature [8]. In general, corrosion rates increase with acid concentration, and temperature. In the case of carbon steel, most acids will dissolve base metal at corrosion rates beyond 2.5 mm/y (100 mpy) [2]. Despite the use of corrosion inhibitors, the metallurgical damage associated with acid cleaning is reflected in the form of pitting corrosion regardless of steel type, end-grain corrosion of stainless steels (SS) and ferrite attack in welds [2-4]. The use of newer metallurgies in Kraft Digesters such as weld overlay clad, metal spray and duplex stainless steels have not been reported to suffer from damage during acid cleaning [8]. Two corrosion resistant weld overlay stainless steels commonly used are: type 309L and 312 stainless steel alloys, for continuous and batch digesters, respectively [3]. In regards to thermal spray coatings, there is very little information published on their corrosion behavior in cleaning environments at high temperatures [9-10]. Also, there is a new trend in North America of
Transcript
Corrosion Prevention during Acid Cleaning of Pulping Equipment Jorge J. Perdomo , Pablo R. Conde*, Preet M. Singh , Moses Wekesa*
Engineering Services Division, Smurfit-Stone Container Co., Carol Stream, IL 60188-2130 * Institute of Paper Science and Technology at Georgia Tech, Atlanta, GA 30332-0620
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245 ABSTRACT Kraft digesters and evaporators are typically acid cleaned to remove scales that precipitate from chemical species present in liquors. Their removal by chemical methods can detrimentally affect the base metal where they grow on. Scale removal may in turn affect the condition and integrity of the equipment by causing general or localized corrosion, which may lead to unexpected failures. Previous work has addressed acid cleaning methods by looking at a number of variables that affect scale removal as well as corrosion. However, no published work is available in terms of the removal of actual scales. The objective of this work is to show how optimal cleaning practices developed in the laboratory for Kraft digesters, evaporators and other pulping equipment work on simulated scales grown on different base metals. The results of this study will help minimizing cleaning costs, maximizing its effectiveness, and to ultimately minimizing corrosion (metal loss) during scale removal. BACKGROUND Most Kraft digesters and evaporators in North American paper mills are periodically acid cleaned to remove insoluble scales like burkiete (Na2CO3.2N2SO4) and carbonate scales on heat exchanger surfaces [1]. Despite the importance of such maintenance procedures, many mistakes have occurred causing significant damage (i.e., corrosion) of digester and evaporators vessel walls, screens and piping [2-4]. Scale characteristics (e.g., composition) may be different in evaporators compared to those observed in digesters or liquor heater surfaces, but have to be regularly removed for efficient heat exchange. Scales are considered generally non-corrosive because a protective carbonate film (carbonaceous) is formed on the surface of the metal. The presence of carbon dioxide in waters favors the formation of bicarbonate ions as pH is increased [5]. The bicarbonate ion forms insoluble calcium carbonate that may also precipitate on the surface of the equipment. Carbonate compounds have reverse solubility and precipitate out as temperature rises. In terms of corrosion the presence of carbonate layers (some times referred as scaling or fouling) are considered beneficial. However, throughputs are reduced, while pumping costs and heating or cooling requirements are increased because of the lowering flow rates and heat transfer coefficients and by the increased pressure drops [6], hence, the need for acid cleaning to remove those scales. Although most cleaning for pulping equipment is performed with inhibited muriatic acid (HCl), other acids such as sulfamic, formic and nitric have been studied [2]. Acid concentration, inhibitor, temperature, time and flow are important parameters that will establish well-defined procedures to minimize damage to the equipment. Despite the relevance of such procedures to the pulp and paper industry, very little published work has addressed the problems faced during acid cleaning. In the past, only the work conducted by Crowe in the early nineties [2] and Henrikson in the early eighties [7] looked at the importance of acid cleaning procedures on the integrity of structural equipment. The topic had been long forgotten until recently when information of common acid cleaning practices was made available in the literature [8]. In general, corrosion rates increase with acid concentration, and temperature. In the case of carbon steel, most acids will dissolve base metal at corrosion rates beyond 2.5 mm/y (100 mpy) [2]. Despite the use of corrosion inhibitors, the metallurgical damage associated with acid cleaning is reflected in the form of pitting corrosion regardless of steel type, end-grain corrosion of stainless steels (SS) and ferrite attack in welds [2-4]. The use of newer metallurgies in Kraft Digesters such as weld overlay clad, metal spray and duplex stainless steels have not been reported to suffer from damage during acid cleaning [8]. Two corrosion resistant weld overlay stainless steels commonly used are: type 309L and 312 stainless steel alloys, for continuous and batch digesters, respectively [3]. In regards to thermal spray coatings, there is very little information published on their corrosion behavior in cleaning environments at high temperatures [9-10]. Also, there is a new trend in North America of
building digesters –and other equipment- from duplex stainless steels as service experience has been satisfactory [11-15]. In the case of evaporators, only 304L and 316 stainless steels have been used as construction materials. In general, the rates at which the scales form are affected by process variables (i.e., liquor composition, flow, time, temperature, etc.) and those will differ significantly in digesters and evaporators. Then, removing a specific scale should also take into account the base metal to minimize corrosion damage during acid cleaning. Inhibitors used to prevent corrosion in one alloy may not work to stop corrosion due to cleaning acids of other alloys. This has in fact occurred in the past and has occasionally led to unexpected corrosion failures of heat exchanger surfaces. Results from a recent survey [8] show that in spite of the use of newer metallurgies most problems are still associated with austenitic stainless steels and carbon steel materials. The improved performance of newer generation corrosion inhibitors for acid cleaning needs to be investigated. An inhibitor will form a protective coating in situ by reaction of the solution with the corroding surface. Corrosion inhibition is reversible, and a minimum amount is required to maintain the surface film. Because of the complexity and proprietary nature of today’s inhibitor technology, it is costly to characterize active components. The selection of an organic inhibitor depends on the acid to inhibit. For sulfuric acid sulfur-containing compounds have been found useful, whereas for muriatic acid (HCl), nitrogen-containing compounds (amines) will do [5]. Acid solution inhibitors (organic) are typically chemisorbed on the metal surface. Since chemisorption processes are specific to a metal substrate, organic inhibitors vary in effectiveness for different alloy systems. A wide variety of new organic inhibiting compounds are available, structures are complex, and quantitative predictions of inhibiting capabilities are difficult. Thus, most organic inhibiting formulations are derived empirically and are proprietary in nature. Acid cleaning procedures in terms of acid/inhibitor/alloy system are developed based on the performance of the alloys after exposure to those environments. Optimal cleaning solutions are recommended based on their effectiveness, efficiency, and minimal corrosion damage. The objective of the present study is to describe a number of acid cleaning systems on both laboratory screening tests and on simulated scales grown in the laboratory. EXPERIMENTAL TECHNIQUES Experimental variables for the present study were obtained through a nation wide survey [8]. This helped to narrow down the number of variables to conduct the experimental work. Due to the significant number of variables identified in the survey, a decision making tree (Figure 1) was envisioned to identify the most important factors to be studied. Having collected information in regards to the current acid cleaning methods required the addition of less popular ones for comparison purposes which in turn increased the experimental matrix even more. So the first step was to determine, for a particular metal, the corrosion rates, crevice corrosion, and pitting behavior at different times, temperatures and inhibitor concentrations. The least aggressive condition was then selected for further testing by means of stress corrosion cracking tests (SCC), flow velocity testing, to simulate drainage or flow conditions, and determine scale removal efficiency. If the acid solution didn’t perform well under the second stage corrosion evaluation, then the next best weight loss results from another acid cleaning system were used. Based on these results, an economical analysis can be made which can in turn be compared to other scenarios that take into account redesign, repairs with more corrosion resistant alloys, etc.. Details of the experimental procedures are as follows: Weight Loss Measurements For weight loss corrosion measurements, carbon A516 grade 70 (UNS K02700) and stainless 304L (UNS S30403) steel standard flat coupons, 19.5x51x3.1 mm with a 9.5 mm mounting hole, were used to measure corrosion rate in accordance with ASTM G1 recommended practice [16]. The hole in the samples was used to evaluate crevice corrosion by fastening two TFE-fluorocarbon blocks held together with a bolt and a nut with a maximum torque of 0.28 Nm as described elsewhere [17]. The test matrix is shown in Figure 2. The inhibitors and their recommended concentrations used are listed in Table I. The selection of exposure times, temperatures, acid concentrations and inhibitors was based on the survey data obtained with the exception of sulfuric acid. The chemicals used for preparing the acid solutions were ACS grade whereas the inhibitors where commercial products. Though most of the mills surveyed use HCl in the range of 5% to 35%, other acids at their recommended concentrations were included for comparison purposes.
The coupon surface to cleaning solution volume ratio, S/V, was 0.092 cm-1. Though it is desired to simulate the actual S/V ratio of the equipment being cleaned, e.g., ~0.01 cm-1 for a digester, this value was not used because it was prohibitively large. The volume used is more typical of evaporators. Active components of the inhibitors used in this experimental work are as follows: Inhibitor A An organic liquid, cationic corrosion inhibitor specially designed to inhibit the attack of hydrochloric acid on iron and steel during industrial cleaning operations. It does not contain arsenic, chlorinated hydrocarbons or lead based compounds (Table II). Inhibitor B A liquid cationic acid inhibitor designed for use in chemical cleaning operations where chloride-free solvents, such as citric, hydroxyacetic, and formic acids are used [18]. It has been claimed to be an effective inhibitor for phosphoric, sulfuric, sulfamic, acetic, citric, hydroxyacetic, formic, hydroxyacetic/formic mixture, tartaric, oxalic and sodium bisulfate (Table III). Inhibitor C A solid developed for use in removing both iron and copper oxides. It’s main constituent is thiourea (Figure 3). It has been claimed to be an effective inhibitor for most mineral acids at moderate concentrations including hydrochloric acid [18]. However, its use at higher concentrations may accelerate corrosion. It may also cause hydrogen absorption on the steel surface. Thiourea derivatives are more common as in the case of Inhibitor B shown above. SCC Weld Testing To test the resistance of welds to stress corrosion cracking by varying the heat input, standard flat coupons of A516/A516, A516/SS304L, and SS304L/SS304L for U-bend tests were gas tungsten arc welded (GTAW) across the width using AWS certified electrodes ER70S-2, 309L, and 309L wire, respectively following ASTM G58 [19]. Stress corrosion cracking tests were performed according to ASTM G30 [20]. A welding procedure was devised to test the effect of heat input on the weld. Heat input was estimated in accordance with the following formula [21]:
S60EIHE = (1)
Where E is the potential in [V], I is the current in [A] and S is the travel speed of weld metal deposition rate in [ms-
1]. The parameters shown on Table IV formed part of the welding procedure. The weld join dimensions are shown in Figure 4. Heat inputs were varied as per Table V. Effect of Flow Velocity Despite the fact that acid cleaning is regularly carried out in stagnant conditions for a number of hours, drainage of the cleaning solution may take hours as it has been reported in the survey. In order to show the effect of flow, a number of tests were conducted using a rotating cylinder electrode. The test chosen reflect optimal conditions during stagnant acid cleaning scale removal. Rotating cylinder electrodes have been extensively used to simulate turbulent flow conditions in corroding systems under mass transfer control [22-25]. Steel electrodes were polished to 600-grit paper, cleaned with acetone and rinsed with deionized water prior to the tests. The exposed surface area of each cylinder electrode was approximately 10 cm2 (3 cm in length and 1 cm in diameter). The reference electrode used in flow tests was a saturated calomel electrode (SCE). Corrosion rates were
calculated at various rotating speeds using linear polarization resistance measurements [26] at a scan rate of 0.125 mVs-1. One of the important parameters is how to relate liquid velocity on the cylinder geometry to that of any other geometry, e.g., a pipe, to yield meaningful corrosion rates. Silverman [23] has shown that by equating the shear stresses on the cylinder to those of a pipe, the following relationship may be obtained:
452
8570
2852
731
41
1 vDdd118450v /
.
/
//
.
=
−
ρµ
µρ
(2) Where V1 and V2 are the linear velocities of the rotating cylinder and the pipe, respectively, given in [ms-1], ρ is the density of the fluid in [gm-3], µ is the viscosity in [gcm-1s-1], d1 and d2 are the diameters of the rotating cylinder and the pipe, respectively, given in [m], and D is the diffusion coefficient of hydrogen protons in [m2s-1]. Scale Cleaning Efficiency using CaCO3 powder Determining the amount of calcium carbonate dissolved by the cleaning solution was performed as follows: 10-gram samples of CaCO3 ACS grade were weighed and placed in 250 mL beakers with 100 mL of the acid solution. Mixing was provided by continuously stirring the solution. The entire heated sample was then filtered through Whatman filter paper No.1. The solids retained were dried at 105ºC, cooled and weighed again to determine the amount of solids not dissolved with respect to the original amount. Values were expressed in weight percent of effectiveness. These measurements were made as a function of time. Scale Cleaning Efficiency using Grown Scales in the Laboratory The objective of this test was to make a scale which is similar in physical and chemical composition to those found on evaporators. The deposition of the scale on the carbon steel coupons was achieved by the experimental design outlined in Figure 5. The heat exchanger was built in house to simulate evaporator conditions. A 3.5 L autoclave reactor was used. A special heat exchanger was designed with a flat section in it where carbon steel A516 grade 70 coupons could be attached. Each coupon roughly measured 2 by 2 by 0.3 cm and weighed approximately 9 g. The samples were polished to 600 girt, degreased with methanol and pre-weighed prior to each test. A total of 16 coupons were securely fastened with a bolted metal strip to the stainless steel sample holder (block) shown in Figure 5 (4 coupons per face) exposing only one face per coupon (~2 by 2 cm). Hot distilled water at 100 C was circulated through the closed hot heat exchanger submerged in the solution saturated with the scaling chemicals for 72 hrs. This exposure time provided about 13 g. of scale on the surface of the coupons. The scaling chemicals were prepared from commercially available ACS grade salts (Na2SO4 and Na2CO3) in 2.5 L distilled water to produce a solution with 43% mass by weight. The Na2CO3 to Na2SO4 mole ratio was fixed at 2:6 to achieve burkiete scale composition which is commonly found in evaporator tubes. At the end of each test, the carbon steel coupons with the deposited scale were retrieved, air-dried overnight in a dissicator and re-weighed. The difference in weight of the carbon steel coupons with and without the scale was recorded to determine the scale weight. The scale composition was confirmed by means of X-ray diffraction analysis. Two acid cleaning systems were selected to test their efficiency on actual burkiete scales simulated in the laboratory, namely 5% HCl and 2%v of Inhibitor “A” and 20% sulfamic acid with 0.5%v of inhibitor “B” at 25 and 50 C. A hot plate connected to a thermocouple was used for heating purposes. The electric stirrer was used to continuously mix the solution and consisted of a shaft of 1.5 cm in diameter. The stirrer was rotated at either 1,000 or 2,000 rpm. The coupon surface to cleaning solution ratio, S/V, was ~0.01 cm-1. Samples were suspended in the solution by the magnetic rod for a specified period of time after which they were removed, dried and weighed. The difference in weight between the initial weight of the coupon with scale and the final weight of the coupon with scale was taken as the loss in weight of the scale.
RESULTS AND DISCUSSION Weight Loss Measurements Figures 6 to 15 show the results of the weight loss tests at different times and temperatures for the conditions shown in Table I. In the case of acid cleaning with HCl for carbon steel (Figure 6), there is a marked effect of temperature and inhibitor concentration used. At room temperature, uninhibited 5% HCl solutions exhibit a corrosion rate around 1 mm/y (~40 mpy). Typically, longer exposures than 4 hours to this environment make the corrosion rate decrease probably due to the large amount of ferrous ions produced which in turn precipitate out in the form of some oxide-hydroxide scale on the surface of the coupons. Increasing additions of Inhibitor “A” cause corrosion rates to decrease; the best performance or efficiency is obtained at 2.0%v of inhibitor “A”. Increments in temperature dramatically increase the corrosion rate of uninhibited 5% HCl. Inhibitor additions lower the corrosion rates at higher temperatures. However, the effect of temperature is so marked that for instance the corrosion rates of inhibited 5% HCl at 57ºC show similar results to those of uninhibited tests at room temperature. Another important observation is that in general, higher temperatures provide corrosion rates larger than 0.1 mm/y on carbon steels even when they are inhibited. The 0.1 mm/y is an unwritten rule of thumb to estimate a reasonable wall thickness loss. So this information can be used to estimate the wall thickness loss per acid cleaning. In regards to crevice corrosion, all tests conducted at temperatures higher than 25ºC showed susceptibility to this type of degradation. Creviced areas showed heavy attack that can be higher than the general corrosion rates reported. One particular case stands out where stress corrosion cracking (SCC) was observed in 5% HCl at 24 h and 93ºC. In this case the sample was not subject to externally applied stresses but rather the residual stresses produced during cold work and machining. The sample literally came a part into fragments. This is an important finding as according to the survey at least one mill carried out acid cleaning under similar conditions. Figure 7 shows the results for carbon steel acid cleaning in 35% HCl solutions at room temperature. Uninhibited 35% HCl tests show extremely high corrosion rates similar to those obtained for uninhibited 5% HCl at 93ºC (Figure 6). However, the addition of inhibitor “A” substantially reduces the corrosion rate to more acceptable levels. The best results are obtained for longer time exposures. No crevice corrosion was evident in 35% HCl tests. Figure 8 shows the results of acid cleanings in 5% HCl solutions for stainless steel. The results at room temperature are somewhat similar to those of carbon steel under equivalent conditions (Figure 6). However, the effect of temperature is not as pronounced as in the case of carbon steel results and yet there is an increase of corrosion rate with respect to temperature. Inhibitor additions do reduce the corrosion rates when compared to the blank tests but corrosion rates for temperatures above room temperature are relatively large compared to the 0.1 mm/y target. No pitting was observed on any of the weight loss tests. Nonetheless, crevice corrosion was evident in all inhibited tests at 57ºC (Figure 8). The evident concern with stainless steel acid cleaning with HCl is related to pitting and stress corrosion cracking near welds. This will be addressed in this study as part of the experimental work proposed. Figure 9 shows the results for stainless steel acid cleaning in 35% HCl solutions at room temperature. Uninhibited 35% HCl tests show extremely high corrosion rates, higher than those obtained for uninhibited 5% HCl at 93ºC (Figure 8). However, the addition of inhibitor “A” substantially reduces the corrosion rate to more acceptable levels. The best results are obtained for longer time exposures. No crevice corrosion was evidenced in 35% HCl tests. Figure 10 shows the results for carbon steel in 10% HNO3 solutions at room temperature. Even though nitric acid is one of the strongest oxidizers available, several attempts were made to inhibit its effect on corrosion rates. There are no known inhibitors for this acid and it is not normally used in carbon steels. However, the focus of these particular tests was aimed at addressing acid cleaning where mixed metallurgies, e.g., carbon and stainless steels, are used. Figure 10 shows the results when inhibitor “A” was added. The addition of inhibitor increases the corrosion rate (regardless of the amount) dramatically. Even the blank tests in uninhibited solutions are extremely high ~100 mm/y. The coupons thinned down so much that they couldn’t be reused. Neither crevice nor pitting corrosion was observed. Though not reported here, other inhibitors, i.e., “B” and “C” in Table I, were also used for this acid.
Because of the poor performance shown reflected in the greatly increased corrosion rates with respect to uninhibited systems they have been omitted. Similarly, Figure 11 shows the results of corrosion rates for stainless steel in 10% HNO3 solutions at room temperature. Corrosion rates without inhibitor additions at room temperature are generally lower than 0.1 mm/y. It isn’t clear whether the inhibitor had an effect on the acid. However, temperature did affect the corrosion rate when it was increased from 25 to 57ºC. There was at least a 10-fold increase in corrosion rates in uninhibited solutions. No crevice or pitting corrosion was observed. Sulfamic acid (NH2SO3H) at 20% at room temperature seems to be a viable option for acid cleaning in both carbon and stainless steels as shown in Figures 12 and 13, respectively. In the case of uninhibited carbon steel (Figure 12), corrosion rates at room temperature range from 15 to 50 mm/y, increasing with acid cleaning time. Inhibitor “B” additions substantially mitigate corrosion rates to around 0.20 mm/y regardless of amount of inhibitor added and time of exposure. However, increasing the temperature from 25 to 57ºC affects both uninhibited and inhibited corrosion rates. They increase substantially, up to 100 times as in the case of 4h tests without inhibitor. The temperature was increased to emulate conditions where solubility of sulfamic acid is improved since it comes in powder form and has limited solubility at room temperature. No pitting was observed whereas crevice corrosion (CC) only occurred in a blank test at 24 h at room temperature. Comparatively speaking, stainless steel acid cleaning with sulfamic acid showed a much better response (Figure 13). The corrosion rates with and without inhibitors at all times and temperatures are below 0.1 mm/y. There is no clear effect of inhibitor efficiency. No pitting or crevice corrosion was evident in any of the tests. Sulfuric acid solutions were also tested as possible acid cleaning agents as shown in Figures 14 and 15. Carbon steel results (Figure 14) show that uninhibited corrosion rates are relatively large regardless of temperature. Only one condition showed somewhat acceptable results when the acid cleaning solution was inhibited, i.e., 4h, 0.4% Inhibitor “C” at room temperature. The corrosion rate was in the vicinity of the 0.1 mm/y target. Crevice corrosion (CC) was observed for uninhibited acid tests at 25°C. In the case of stainless steel tests (Figure 15) only inhibited conditions longer than 8h at room temperature showed promising results. Higher temperatures are detrimental even when inhibited to the extent that crevice corrosion (CC) was evident in sulfuric acid with 0.4% inhibitor at 57C at all exposure times. Based on the best performance (lowest corrosion rates) of acid cleaning systems described above, further tests were conducted to assess welding behavior, effect of flow velocity, and scale removal conditions. SCC Weld Testing Based on the stagnant condition coupon test results shown above, and using a 0.125 mm/y (5 mpy) as maximum allowable corrosion condition during acid cleaning, the solutions shown in Table VI were used for SCC tests with U-bend specimens. Table VI describes the best acid cleaning conditions found for general corrosion rates without the presence of localized corrosion. All U-bends for carbon to carbon, carbon to stainless and stainless to carbon steel welds with lower heat input welding cracked during bending before they were exposed to the environments of Table VI. Larger heat inputs seemed to work better since they did not undergo cracking while making the U-bends. After exposing them at the conditions of Table VI, no stress corrosion cracking was observed for any of the conditions. Effect of Flow Velocity Figure 16 presents the corrosion rate as a function of linear velocity calculated from the cylindrical specimen for several of conditions. From the data shown for carbon, it can be seen that the system most affected by fluid velocity was sulfamic acid cleaning. Corrosion rates rose dramatically with the velocity of the fluid. The shear stresses generated during turbulent flow conditions easily remove the inhibitor film on the surface of the substrate affecting film persistency. Even if some corrosion scale is formed, it is likely that the shear forces will also remove it providing a new surface for corrosion to occur. This may cause some flow enhanced corrosion due to erosion. Better results were obtained when hydrochloric acid was used on carbon steel with a very minor effect of velocity on
the corrosion rate. On the other hand, hydrochloric acid solution performed very poorly for stainless steel flow tests as shown in Figure 17. Uninhibited nitric acid seems to out perform other acid cleaning solutions as far as stainless steel is concerned regardless of flow velocity, followed by sulfamic acid and sulfuric acid solutions (Figure 17). Scale Cleaning Efficiency Efficiency tests were conducted for all the conditions were corrosion rates were below 0.1 mm/y. Calcium carbonate scale removal efficiency was 100% for nitric acid as shown in Figure 18. However, the corrosion rate for carbon steel (Figure 10) is 1000 times higher than that obtained for stainless steel under the same conditions ~ 0.1 mm/y (Figure 11). Therefore, this acid cleaning method may be used with the cautionary measure of not exposing carbon steel. The cleaning efficiency of hydrochloric acid (Figure 18) increases over time. All the corrosion rates relative to this environment are well below 0.1 mm/y for carbon steel (Figures 6) and slightly above 0.1 mm/y for stainless steel (Figure 7). Sulfamic acid has consistently showed stability not only in the corrosion rate measurements (Figure 12) but also in the scale removal efficiency as it seems to be independent of the amount of inhibitor used (Figure 18). Lastly but not surprisingly, sulfuric acid showed a negative efficiency as far as scale removal is concerned. This is not unexpected since in fact calcium from carbonate is dissolved and precipitated again in the form of calcium sulfate. X-ray diffraction analysis on the powder obtained showed that no calcium carbonate was present whereas calcium sulfate and other unidentified solids were observed. Scale Cleaning Efficiency using Grown Scales in the Laboratory Figures 19 and 20 show the weight change as a function of time for the laboratory grown scales at 25 and 50 C, respectively, exposed to inhibited hydrochloric and inhibited sulfamic acid stirred at 1,000 rpm. The scale dissolution in water was also included as a base line. The complete dissolution of burkiete in water at room temperature takes about 45 minutes (Figure 19). Of the two inhibitors used, inhibited sulfamic acid performed better taking about 12 minutes to dissolve the entire scale, whereas inhibited hydrochloric acid took 18 minutes. Higher temperature results (Figure 20) show that the kinetics of the reactions is thermally enhanced providing shorter times to achieve complete dissolution. Inhibited sulfamic acid outperformed again the inhibited hydrochloric solution. In all tests, surface inspection of the underlying metal after scale removal was free of pits and retained its weight. Burkiete is likely less tenacious and less adherent than more compact scales as CaCO3. So the porosity of the scale plays an important role on the efficiency of the inhibited acid to remove it. Porous scales provide sites for the solution to wet and penetrate increasing the surface area upon which the acid reacts. Figure 21 shows a simplified example of this behavior where burkiete was subjected to removal in water by mechanical means, i.e., flow impinging on the scale, at two different stirring velocities at room temperature. For comparison purposes, scale powder that had been scraped off the surface of a coupon, and weighed was dissolved in water in the shortest time. In this case, the surface area of every particle of powder exposed to water is large. As expected, higher speeds provide higher shear stresses that aid in the dissolution of the scale. CONCLUSIONS A number of acid cleaning solutions seem to perform well under stagnant conditions as shown in Table VI of this report. Under these conditions minimal corrosion rates occur with no localized corrosion. The welded samples made using welding procedure developed for this study, in the environments of minimal corrosion rate, show no evidence of stress corrosion cracking (SCC). Therefore, the environments tested seem not only to minimize general corrosion rates, but also, avoid SCC. In the case of carbon steels, inhibited sulfamic acid solutions at room temperature seem to work well under stagnant conditions, and not so well under flowing ones. Fluid effect has a detrimental effect by markedly increasing corrosion rates. Both inhibited hydrochloric acid and sulfuric acid work fairly well under flowing conditions (at least in moderate flow in case of sulfuric). For stainless steel, hydrochloric acid performed poorly under flowing conditions.
Acid cleaning efficiencies are 80% or better for 10% uninhibited nitric, inhibited 20% sulfamic and inhibited hydrochloric acid (of room temperatures and at least 4h exposures). Inhibited sulfuric acid has a negative efficiency because sulfuric acid reacts with calcium carbonate to form calcium sulfate and some other compound that precipitate out from the solution. Laboratory grown scales emulating those observed in evaporators, i.e., burkiete, proved successful. The porosity of the scales plays an important role on the dissolution kinetics. Times obtained for scale removal are relatively short for either inhibited sulfamic or hydrochloric acids. Sulfamic acid seems to out perform hydrochloric at both temperatures. REFERENCES
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of the TAPPI Engineering Conference, Atlanta, GA, pp. 379-398 (1999).
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TABLE I. Acid cleaning solutions and inhibiting concentrations used during testing.
Acid Concentration Inhibitor Inhibitor Concentration 15% and 35%* HCl Inhibitor A 0.5%, 2%
20% NH2SO3H Inhibitor B 0.5%, 2% 10% HNO3 Inhibitor A 0.5%, 2% 15% H2SO4 Inhibitor C 0.1%, 0.4%
* 35% HCl tests were only conducted at room temperature. TABLE II. Inhibitor A composition.
Components Chemical Formula 50 to 60% Complex keto-amines R-C=O NH2
1 to 10% Isopropyl alcohol (CH3)2CH-OH 1 to 10% Propargyl alcohol (CH2)3-(OH)3 0 to 3% Hydrochloric acid HCl
TABLE III. Inhibitor B Composition.
Components Chemical Formula 30 to 60% alkyl pyridines mixture
10 to 30% Sulfuric Acid H2SO4
10 to 30% 1,3-Diethylthiourea
TABLE IV. General welding Procedure Parameters
Base Metal* A516/A516 A516/304L 304L/304L Filler Metal ER70S-2 309L 309L
Welding sequence On pass on one side, grind second side, one pass on second side.
Polarity Straight Straight Straight Number of passes one/side one/side one/side
*sheet metal from Metal Samples For each set of base metal welds, heat inputs were varied as shown in Table 3. TABLE V. Heat input conditions for welding of U-bend specimens.
FIGURE 19. Burkiete scale removal as a function of time exposed to acid cleaning solutions as indicated at 25 C and 1,000 rpm.
0
2
4
6
8
10
12
14
0 5 10 15 20 25 30 35 40 45 50Time/[m]
Wei
ght C
hang
e/[g
]20% NH2SO3H + 0.5% Inh. "B"
5% HCl + 2% Inh. "A'
H2O
FIGURE 20. Burkiete scale removal as a function of time exposed to acid cleaning solutions as indicated at 50 C and 1,000 rpm.
0
2
4
6
8
10
12
14
0 5 10 15 20 25 30 35 40 45 50Time/[m]
Wei
ght C
hang
e/[g
]
1,000 rpm
2,000 rpm
Burkiete scale powder @ 1,000 rpm
FIGURE 21. Burkiete scale removal as a function of time exposed to water at 25 C and two stirring speeds, namely 1,000 and 2,000 rpm. Dissolution of burkiete powder scrapped off a coupon has also been included.
