Engineering : Chemical / M. Tech. (Chemical Engineering)

The electrochemical corrosion behavior of mild steel in apple, grape, mango, orange and the mixture of these juices was investigated in this study. Open circuit potential and weight loss analytical techniques were employed to establish the comparative corrosion rates of this material in the agro media over the interval of five days for a sixty-day immersion period at an ambient temperature. The chemical compositions of both mild steel and the agro media were determined to ascertain corrosion mechanism for the reaction. Polarization behavior of mild steel in the agro media were determined by Tafel extrapolation curves. The analysis showed that cathodic polarization curves were almost identical irrespective of the concentration of the various media while the anodic polarization curves exhibited varying active and passive corrosion behaviour due to passivating oxide films. The analysis of the results further showed that the corrosion rate of the metallic sample decreased with longer immersion periods which could be attributed to a gradual decline in acidity of these media as revealed by the pH results. Hence, the evolution of hydrogen gas and reduction of dissolved oxygen molecules from the reacting system were presumed to be major factors retarding corrosion of the solution involved. Similarly, the presence of suspended particles on the surface of the test steel sample could have also led to the impediment to corrosion rates from the surrounding atmosphere. Microscopic analysis of the corroded mild steel specimens revealed uniform and localized corrosion with Fe2O3 spotted as the main corrosion product. The result obtained from the electrochemical study showed that the corrosion rate of the mild steel sample was highest in the orange medium (1.53mm/yr), followed by grape medium (1.40mm/yr), mixture of these media (0.67mm/yr), mango medium (0.40mm/yr) while the metal corroded least in the apple medium (0.30mm/yr) over the duration of immersion. / Engineering : Chemical / M. Tech. (Chemical Engineering)

Additive manufacturing (AM) has the potential to overtake the conventional production of pipe systems of stainless steel grade 316L intended for bioprocessing applications by utilising powder bed fusion laser beam (PBF-LB) technologies. However, challenges such as high surface roughness need to be overcome and corrosion properties improved to meet industry standards. This can be done by applying surface post-treatments to the printed parts. Therefore, the first aim of this thesis was to investigate the ability of different post-treatments of PBF-LB 316L samples, including tumbling, electropolishing, chemical milling, Hirtisation®, and DryLyte®, to reduce the surface roughness. One set of samples was left untreated, as-built set, and used as reference. The second aim was to try to correlate the surface roughness with the corrosion properties of the various post-treated samples. Analytical techniques used included needle profilometry, light optical microscopy (LOM), critical pitting temperature (CPT), cyclic potentiodynamic polarisation (CPDP) and immersion in 1M NaCl and 1M NaOH for two different durations for each solution. While some results were expected, others were surprising. Even though tumbled samples resulted in the lowest surface roughness, the results for immersion and CPDP curves tests indicated the lowest corrosion resistance compared to the other post-treatments. On the other hand, electropolishing resulted in the lowest surface roughness reduction among the post-treated samples. However, these samples had comparatively high corrosion resistance in the CPT and CPDP tests. The DryLyte® samples were tested using two DryLyte® medias where one of them resulted in the second lowest surface roughness and showed highest corrosion resistance in all three corrosion tests even though the process parameters are still to be developed. The results for samples post-treated by Hirtisation® and chemical milling were inconsistent and need further investigations. The reference as-built set showed high corrosion resistance in its untreated state, indicating post-treatments having minor impact on its performance from a corrosion perspective. / Additiv tillverkning (AM) har möjlighet att konkurrera ut konventionell produktion av rörsystem i rostfritt stål typ 316L avsett för bioprocessapplikationer genom att använda pulverbäddsinfusionslaserstråleteknik (PBF-LB). För tekniken kvarstår dock utmaningar för att uppfylla branschstandarder då tillverkningen medför material med hög ytråhet och sämre korrosionsegenskaper än traditionella metoder. Detta kan åstadkommas genom olika typer av efterbearbetning av de printade ytorna. Ett syfte med denna studie var att utvärdera hur olika typer av efterbehandling av PBF-LB 316L (trumling, elektropolering, kemisk etsning, Hirtisation® och DryLyte®) påverkar ytråheten. Som referensmaterial användes obearbetade printade ytor. Studiens andra huvudmål var att undersöka möjligheten för korrelation mellan ytråhet efter behandling och korrosionsegenskaperna. Analyser gjordes med hjälp av nålprofilometri, ljusoptisk mikroskopi (LOM), bestämning av kritisk gropfrätningstemperatur (CPT), framtagning av cykliska potentiodynamiska polarisationskurvor (CPDP) samt nedsänkning i 1M NaCl och 1M NaOH under två olika tidsperioder per lösning. Resultaten var delvis väntade och delvis överraskande. Trots att de trumlade proverna resulterade i lägst ytråhet, visade de elektrokemiska studierna (CPDP) i de olika lösningarna på att lägre korrosionsbeständighet jämfört med övriga efterbehandlingar. Å andra sidan resulterade elektropolering resulterade i den lägsta minskningen i ytråhet bland de efterbehandlade proverna. Dessa prover visade dock på en jämförelsevis hög korrosionsbeständighet i både CPT och CPDP testerna. DryLyte®-proverna studerades med två olika DryLyte®-medier, varav en av dem gav näst lägst ytråhet samt visade högst korrosionsbeständighet i samtliga korrosionstester trots att processparametrarna inte ännu är optimerade för denna nya teknik. Resultaten för prover som efterbehandlats med Hirtisation® och kemisk etsning var tvetydiga och behöver studeras vidare. De obehandlade referensproverna indikerade hög korrosionsbeständighet, vilket tyder på att efterbehandlingar hade minimal påverkan ur ett korrosionsperspektiv.