Materials for mid furnace environments of kraft recovery boilers
Kraft recovery boilers experience localized accelerated corrosion in mid furnace areas. Corrosion patterns may vary from one boiler to other depending upon the operation parameters and general design of the boiler. Local gaseous environments were characterized in areas with corrosion and equivalent areas without accelerated corrosion in a selected boiler over the period of two years. Results from this study indicated that in the areas with accelerated corrosion there was a large variation in the local gas composition at the waterwall surface. Reducing and sulfidizing gases were found in the areas with accelerated corrosion whereas the gas compositions were more oxidizing in the low corrosion areas. Observations during field tests have indicated that the local waterwall temperature may also vary in the mid furnace due to localized combustion and air flow patterns. Mid-furnace environments were simulated in laboratory to evaluate the effect of gas composition and fluctuations on corrosion kinetics of carbon steel tube material. Laboratory tests with SA-210 carbon steel samples with gas composition cycling between oxidizing and reducing/sulfidizing led to an unstable scale formation on the surface. Extent of accelerated corrosion was found to depend upon the extreme partial pressure values of sulfur and oxygen. Carbon steel samples were coated with chromized or aluminized diffusion coatings in the laboratory and were tested under sulfidizing conditions. Both type of diffusion coatings had superior performance compared to the carbon steel samples under similar conditions. Midfurnace corrosion can be controlled by making gaseous environments in the mid furnace more stable. However, more practical solution seems to be to use waterwall tubes with protective outer layer. This may be done by using composite tubes (stainless steel outer layer), thermal spray coatings of resistant alloys or diffusion barrier coatings like chromizing or aluminizing treatment.
The effect of temperature excursion on sulfidation of carbon steel in 1% H2S environment
isgenerallyattributedto thesmeltspallingin the localareas. Water wall tubes in the lower furnace areas of Generally, the temperature excursions in the lower furnace area are below 500°C [6]. Previous work done by Colwell and recovery boilers are known to experience thermal excursions. Sulfidationtestswith thermalexcursionscanresult in higher Fonder [7] on SA-210carbon steel alloy in a 1% H2S environment at 320°C for 20 days reported that 10 spikes to corrosion rates compared to equivalent tests without thermal 400°C increased the weight loss 19 times compared with excursions. Scale damage depends upon scale thickness, history of exposure and spike location. Results from the weight loss from 320°C isothermal test. The same result present study show that even a couple of infrequent thermal represented a 374% increase when compared with weight loss from a 400°C isothermal test. Weight gain of 9-Cr steel excursions, which lead to scale damage, may lead to a higher decreased to 15 % of the weight gain of 2-Cr steel and mild corrosion rate than the isothermal tests at the higher temperatureof the thermal excursion, steel in oxidizing environment containing 0.3% SO2when thermally cycled between 200 and 430°C [8]. Results from INTRODUCTION 1.25Cr-0.SMosteel in oxidizingenvironmentof 0.2% SO2 with thermal cycling between a high temperature and room Water wall tube surfaces in the kraft recovery boilers experience temperature excursions that can increase the temperature showed a strong dependence of corrosion rates on corrosion rate in the localized areas of the recovery boiler, the higher temperature, where the corrosion rates increased Occasional temperature excursions can influence the corrosion abruptly by 5 times around higher temperature of 700°C [9]. Thermal cycling doubled the corrosion rate of carbon and 304 of the boiler tubes on fireside by one or a combination of the followingmechanisms; stainlesssteelsat 371°Cand482°C,respectively,whencycled to room temperature in 0.5% H2S-containing environment 1 increase in diffusion-controlled reaction rates and overallsulfidationreactionrate, [10]. However,the magnitudeof the metal loss for carbon steel in the1% H2S environment due to occasional thermal 2 microcracking of the semiprotective sulfide scale due to thermal stresses and increased reaction kinetics, excursions is not completely understood. Most of the previous 3 molten salt corrosion (if the molten smelt is in touch published work to study the effects of thermal excursion on withthefiresidetube), gaseouscorrosionwas done where continuouscyclingwas introduced during a given test period [7-t0]. These studies When the temperature excursions are more than the melting point of smelt then the molten salt corrosion will clearly demonstrate that the corrosion attack is more severe increasemetal wastagesignificantly, due to the thermal cycling. However, in the lower furnace of Diffusion in most common sulfides (FeS, Cr283, and recovery boiler, thermal excursions are not continuous. The NiS) is generally much faster than that in corresponding present study was aimed at establishing an effect of infrequent thermal excursions on the water wall tube corrosion from oxides due to higher concentration or mobility of defects in fireside. To do so, test parameters in the present work were sulfide scales [1]. As a result, sulfide scales have a greater tendency to deform plastically than the corresponding oxides, changed to study the effects of scale damage and post Relative coefficients of thermal expansion of the metal and the excursion exposure on the overall corrosion rate. corrosion product scales are of great importance in the generation of stresses in the scale. Other important factors in EXPERIMENTAL WORK developing stresses in the scale are scale thickness and SA-210 carbon steel specimens were cut from a 5cm-OD tube and burnished according to the standard geometry of the metal surface. Stresses in the scale generally increase with increase in scale thickness. Scales formed on procedure described in AF&PA/95 report [7,11]. The chemical composition of SA-210 carbon steel alloy is given in curved surfaces have higher stresses than the scale formed on a flat surface [2]. For scales that grow at the scale/gas Table I. The specimen dimensions were 2.5 cm x 1.8 cm x 0.5 cm. Coupons were cleaned with acetone and weighed before interface, the metal surface geometry will contribute to either compressive or tensile stresses depending on whether the they were hung on a glass tree and placed in the reaction tube. curved surface is convex or concave [3]. Data on oxides show that NiO and CoO (with metal/oxide thermal expansion ratio [Table I. Chemical Composition of SA-210 Carbon Steel of 1.03 and 0.93, respectively)seldomcrack or spalIon _ Alloy. cooling to room temp rature, while FeO and Cr203 (with 1.25 lement C Mn Si P S Fe and 1.30 metal/oxide thermal expansion ratio, respectively) IWeight % 0.21 0.65 0.22 0.01 0.016 Bal. 1 The present experiments were designed to study the Results from 320°C isothermal tests plotted as log effects of individual temperature excursions at different time time vs. log weight loss indicate that the reaction follows a periods during the tests on the corrosion rate of SA-210 classical sulfidation behavior where initial corrosion rate and carbon steel in a 1% H2S environment. Dimensions of scale growth depend on reaction kinetics which is followed by a thermal excursion are shown schematically in Figure 1. Tests diffusion-controlled corrosion reaction, at a relatively lower were carried out for 72, 162 or 312 hours with temperature rate. The present set of results follow diffusion-controlled scale spikes at different times, as listed in Tables II to IV. growth until about 162 hours at 320°C. Corrosion rate increases significantly between 162 hours and 312 hours, which may be attributed to some other mechanisms, e.g. sulfide scale cracking. 480oC ....... Results from 480 °C isothermal tests give higher corrosion rates compared to 320°C.
Failure analysis for poor solderability of Au surface of PCB
Nonwetting is one of the common problems and is always rejected in electronic assemblies. ENIG pads of a PCBA exhibited nonwetting after lead-free reflow soldering. With the aid of stereomicroscope, SEM&EDS and XRF, a series of analysis was performed. And after visual inspection, surface and cross section analysis, thickness measurement and solderability test, it can be inferred that oxidation of plating surface was the main reason for nonwetting, and corrosion in Ni plating caused an adverse impact on solderalibity.