CORROSION BEHAVIOUR OF STEEL IN AQUEOUS ENVIRONMENT OF BLEACH SECTION IN PAPER INDUSTRY

Abstract

The work presented in this thesis pertains to the corrosion behavior observed by commercial grades of mild steel and stain-less steels in liquors of bleach section of paper industry. Other related aspects e.g...analysis of corrosion products formed in these metal—environment systems, cost—effective aspects of corrosion protection measures etc. have also been dealt with . The studies presented here involved laboratory tests and those performed. in actual mill environment in two mills namely MOS Seshasayee Paper and Boards Ltd., Erode (Tamilnadu) (.111 A) and M/B. Ballarpur Industries:Ltd., Yamunanagar (Haryana) (mill B). Chapterwise summary of the thesis is presented in the following paragraphs. CHAPTER-1: This chapter introduces the paper manufacturing process in brief. The process comprises of pulping, bleaching, recovery and paper making steps. In all integrated Indian paper mills Sulfate (Kraft) pulping process is used. Paper Industry in general is prone to corrosion attack which results in untimely loss or failure of machinery/equipment. A literature surVey is given on .corrosion in pulp and paper industry. . The frView- includes Calculation of annual cost of general corrosion and cost involved in adopting different corrosion control methods. On the basis of literature survey it was .observed that bleach plant is responsible for approximately 50% of. the total corrosion cost of (i) paper mill. Accordingly, the bleach plant was considered as an area for investigations. Based on the above literature review, it is established that the washer section of bleach plant is one of the heavy corrosion areas and a little information is available on actual plant per-formances with respect to corrosion rates, the nature of corro-sion and corrosion products etc. Accordingly, the following plan of work was proposed to be taken up. i. To obtain well formulated data on corrosion in mill environ- ment using different types of materials. The data generation will include weight—loss measurements, pitting and crevice corro-sion assessment, analyses of corrosion products ustag modern techniques. The performance in the vats and in the gaseous phases is to be studied individually. In order to correlate the corro-sion data to environmental parameters, analyses of various mill process parameters will be made. ii. generate data on corrosion reactions in the aqueous media, laboratory based experiments will be conducted. The electrochemical tests using bleach liquor of varying'canpositions will be used to evaluate the electrochemical behavior. iii. The results from mill and laboratory based experiments will be analyzed to understand the nature of corrosion, the mechanism of corrosion and the influence of the various parameters on the nature and rate of corrosion. iv. Above results will be used to draw inferences for possible methods of minimizing losses due to corrosion either by using a more suitable material or by control of process parameters, using possible protection methods etc. CHAPTER-2: This chapter deals with the various experiments performed in the present study. The experiments were carried out at two mill locations and in the laboratory. The materials used in corrosion studies for both mill and laboratory experiments included mild steel and five different types of stainless steels. The composi-tion of materials used in the studies was determined by chemical and optical emission spectroscopic methods. Rectangular coupons were prepared from plane sheets for weight—loss measurements and cylindrical samples were made from rods for electrochemical studies. These were suitably ground, polished, degreased and finally cleaned ultrasonically before mounting them. For mill .studies the coupons were fitted in polymer racks while they were hung by a polymer thread in the flasks for laboratory studies.. Mill tests were conducted at two mills namely NIS Seshasayee Paper 01, Boards Ltd. Erode (Tamilnadu) and M/S Ballarpur Indus- tries Ltd., Yamunanagar (Haryana). The studies were conducted by putting the coupons in the-vats for aqueous media and near sprays for gaseous environment. The conditions maintained in the mill, the characteristics of liquors used and time of exposure were carefully noted. It is interesting to mention that in one af the studied mills, the liquid environment had sulfamic acid addi- tions. This required some experimentations for finding the effect of sulfamic acid on corrosion behavior. For the laboratory tests, liquor was preparecr:'fram the bleach liquor available in a nearby paper mill to.give varying concentration of free available chlorine from 150-600 he pH was maintained z 8-9 and in some experiments sulfamic acid( was added to the tune of 10 — 30 ppm. The corroded coupons 1ml* from the mill and the laboratory exposure for varying durations were subjected to weight—loss determination. The rusts.scraped from mill corroded coupons were analyzed for the product i_dentifica-tion by X—ray diffraction and Mossbauer spectroscopy both at room and low temperature.