Document Type

Thesis - Open Access

Award Date

2015

Degree Name

Master of Science (MS)

Department / School

Dairy Science

First Advisor

Sanjeev Anand

Abstract

Pitting corrosion of stainless steel (SS) is observed in many different industries including the dairy industry. It is of concern, because it weakens the steel, and can cause cracking. Any replacement or repair of SS equipment is very costly and also causes delays in product manufacture. Microbial Induced Corrosion (MIC) is a possible accelerator of natural corrosion seen in galvanized steel pipes. Studies have shown a correlation between surface roughness and the ability of bacteria to colonize and form biofilms. Dairy industry utilizes higher food grade SS 304 and 316. However, even these relatively corrosion resistant stainless steel grades may experience some pitting corrosion. These two grades have compositional differences that lead to differences in their resistance to corrosion. This study compares the ability of thermoduric bacterial biofilms to cause microbiologically induced corrosion on SS 304 and SS 316 coupons. High Heat Resistant Spore forming (HHRS) bacteria such as Bacillus sporothermodurans, and Geobacillus stearothermophilus were used to study the development of pitting corrosion on polished and unpolished coupon surfaces of SS 304 and SS 316. Studies also investigated the ability of mixed species biofilms to induce pitting corrosion. Additionally, potential of leaching of any heavy metals from corroded surfaces of the coupons that were exposed to mixed species biofilms under milk environment was studied. Experiments were xxii conducted to evaluate the effects of Clean-In-Place (CIP) chemicals on enhancement of pre-formed pitting corrosions due to microbial biofilms. It can be concluded from this study that both SS 304 and 316 support the development of thermoduric biofilms. Both polished and unpolished surfaces supported the biofilm formation. Biofilm formation, as evaluated by viable cell counts technique, showed the highest embedded cells in the case of B. sporothermodurans, as compared to G. stearothermophilus or a mixed species biofilm of the two species. Mono species thermoduric bacterial biofilms of B. sporothermodurans, G. stearothermophilus, and the mixed species of both could induce pitting corrosion on SS 304 and 316 coupons under lab conditions. Both unpolished and polished surfaces proved susceptible to pitting corrosion. Induction of pitting was found to occur within just one week on SS 304, when exposed to multispecies biofilm environment. On the other hand, pitting corrosion occurred after 2 weeks on both grades of SS exposed to thermophilic biofilm of G. stearothermophilus. Both grades of SS exposed to biofilm of B. sporothermodurans had first signs of pitting after 4 weeks. This proves that mixed species biofilm had most detrimental effect on SS in terms of induction of pitting corrosion, and SS 304 proved to be less corrosion resistant as compared to SS 316. Pits observed on unpolished surfaces were in the form of large craters, which were very widely distributed. These pits also had pinhole like structures inside the craters. Pit morphology observed on unpolished surfaces of either grade was found to be comparable. On the other hand, pits seen on polished coupons were deep and scattered, and had corrosion products deposited as debris either outside or inside the pits. The pit morphology observed on polished surfaces of either grade appeared alike. More numbers xxiii of pits were always observed on unpolished surfaces as compared to polished surfaces of both grades of SS. This proved that unpolished surfaces were more vulnerable to microbial pitting attack as compared to polished surfaces of either grade. An increase in elemental Oxygen and Sulfur, and a decrease in Iron in the pits, was demonstrated by Elemental Dispersive Spectroscopy (EDS), which confirmed the presence of corrosion products, and hence MIC. The presence of large amounts of Iron in the used up milk suspension indicated the leaching out of Iron from microbially induced pit surfaces into the spent milk during the course of pitting or its induction. No leaching of Nickel and Chromium was observed. The CIP treatments of SS 304 coupons, with preformed pitting corrosion, resulted in some enhancement of pitting corrosion after 5 cleaning cycles. Under similar cleaning treatments, SS 316 coupons proved resistant to any enhancement of corrosion of preformed microbially induced pits by cleaning chemicals up to five cleaning cycles. This study validates the currently held opinion of higher corrosion resistance of SS 316 as compared to SS 304. It is recommended to conduct further plant scale studies or studies under continuous systems such as a bioreactor to ascertain if the induction of pitting corrosion or heavy metal leaching would occur at the same scale as exhibited by the lab scale system. Studies for optimizing CIP chemical concentration, contact time, and temperature need to be conducted, to avoid any adverse effects of CIP cycles on SS surfaces.

Library of Congress Subject Headings

Microbiologically influenced corrosion
Stainless steel -- Corrosion

Description

Includes bibliographical references (pages 181-184)

Format

application/pdf

Number of Pages

215

Publisher

South Dakota State University

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Rights Statement

In Copyright