Young-Ran Yoo

Influence of corrosion properties on electrochemical migration susceptibility of SnPb solders for PCBs

Electrochemical migration is caused by the adsorption of water and the bias voltages between the electrodes or pads which form an electric circuit or in the solders used to connect the parts. This work focused on the elucidation of the mechanisms of electrochemical migration of pure Sn, Sn37Pb, and Sn55Pb solders in electronics. The electrochemical migration behavior was discussed on the basis of the polarization behavior of SnPb solders. After the water drop test, the time to failure decreased with increasing Pb content in Cl− solution and increased with increasing Pb content in SO42− solution. In the case of the SnPb solder alloys, the pitting potential, the passive current density, the cathodic current density and the efficiency of cathodic deposition of the alloying elements were closely related to the resistance of the electrochemical migration.

Influence of cathodic protection on the lifetime extension of painted steel structures

For corrosion to occur on a coated metal surface, an electrochemical double layer must be established. Hence, the adhesion between the substrate and the coating must be weakened to enable a separate thin layer of water to be formed at the interface from water that has permeated the coating. To prevent the failure of a painted coating, we applied a cathodic protection method. This method has been controversial for a few years because cathodic protection can induce cathodic delamination of the entire surface, especially near the anode, and also because it is not effective on a not-wetted surface from the anode. We therefore evaluated the efficiency of cathodic protection for 700 days in an atmospheric environment and performed surface observation, AC impedance measurements and corrosion tests. In the case of a noncathodic protected painted steel specimen, blisters formed after 100 days and grew in number for the remainder of the test. However, cathodic protection of the painted steel increased the coating resistance and extended the lifetime of the coating. According to our calculation with the BEASY program, the thickness of the water film under a rain condition barely influenced the protection potential. The high voltage of the cathodic protection and the subsequent cathodic delamination caused the paint to peel off near the anode. Hence, the protection voltage should be controlled in accordance with changes to the environmental condition.

Corrosion Behavior of 316L Stainless Steel in Supercritical Water Environment

There is a need to destroy both military and civilian hazardous waste and urgency, mandated by public concern over traditional waste handling methodologies, to safe and efficient alternative technologies. One very effective process for the destruction of such waste is supercritical water oxidation (SCWO). Nevertheless, corrosion of the materials of fabrication is a serious concern. This work intends to obtain the fundamental data for developing the corrosion resistant steel for the construction of SCWO system. The effects of various factors on the corrosion resistance of flat, welded, and U-bend 316L stainless steels in Trimsol solution were studied. Corroded product on surface was composed of multi-layer with oxides and salts, and dealloying was observed. Major corrosion phenomena of 316L stainless steel under SCWO condition were intergranular corrosion, pitting corrosion, SCC, and erosion corrosion. This work focused on the elucidation of corrosion mechanism of 316L stainless steel in SCWO environment.

Role of Alloying Elements on the Cytotoxic Behavior and Corrosion of Austenitic Stainless Steels

Super stainless steel has been used to solve corrosion problems of biomaterials because it shows the excellent corrosion resistance as like Ti and Ti alloys and has better mechanical properties than Ti and Ti alloys. We designed high Mo and Ni bearing super austenitic stainless steel. To obtain desirable microstructure, Cr, Ni, Mo, N contents were controlled. This work focused on the role of alloying elements on cytotoxic behavior and corrosion of stainless steel. In acidic chloride solution, when the alloys had high PRE values, the alloys showed high pitting resistance and low critical current density. However, in Hanks’ solution, the higher PRE’s alloys showed high critical passive current density. Namely, it was revealed that EDTA among Hanks’ solution played an important role to increase the critical passive current density of high Mo and Ni bearing super stainless steels, regardless of PRE’s value of the alloys. Therefore, even if the PRE values of the alloys were higher, high Ni and Mo bearing alloys would reveal more cytotoxic and high metal ion release rate than 316L stainless steel.

Dominant migration element in electrochemical migration of eutectic SnPb solder alloy

In harsher condition with elevated temperature/humidity and complex chemical species, electronic components can be electrochemically ionized and migrate by electric field. The migrated ions form conducting filament between anode and cathode across a nonmetallic medium. The filament can cause a failure of electronic system. This phenomenon is called as the electrochemical migration (ECM). A model test system for water-drop test was devised to assess the ECM characteristics of eutectic SnPb (63Sn-37Pb, wt%). The result for model test system was compared with that for real electronic component on printed circuit borads (PCBs). Although the filament formation mechanism was different, it was found that Pb atoms are more susceptible to polarization than Sn atoms for both cases. This tendency agrees well with the composition distribution of the filament on the model system and PCBs

Effect of SO 4 2- Ion on Corrosion and Electrochemical Migration Characteristics of Eutectic SnPb Solder Alloy

Electrochemical migration phenomenon is correlated with ionization of anode electrode, and ionization of anode metal has similar mechanism with corrosion phenomenon. In this work, in-situ water drop test and evaluation of corrosion characteristics for SnPb solder alloys in solutions were carried out to understand the fundamental electrochemical migration characteristics and to correlate each other. It was revealed that electrochemical migration behavior of SnPb solder alloys was closely related to the corrosion characteristics, and Sn Ivas primarily ionized in solutions. The quality of passive film formed at film surface seems to be critical not only for corrosion resistance but also for electrochemical migration resistance of solder alloys.ጊ吀Ѐ㘹〻Ԁ䭃䑎䴀

Effect of Co-Content on the Corrosion of High Performance Stainless Steels in Simulated Bio-Solutions

This work focused on the effect of Co content on the corrosion resistance of high pitting resistance equivalent (PRE), super ferritic, Ni-free stainless steels in simulated biosolutions. The effect of Co in Ni-free alloys was evaluated by cytotoxicity test. Anodic polarization test and AC impedance measurement were performed to evaluate the effect of Co on corrosion resistance of the alloys. The cytotoxicity test result for 4 experimental alloys shows non-cytotoxic but mild cytotoxic for 316L stainless steel due to relatively poor corrosion resistance. However, the effect of Co on the passivity was positive in bio-solution but it was negative in acidic chloride solution.

Electrochemical Migration Characteristics of Sn-3.0Ag-0.5Cu Solder Alloy in NaBr and NaF Solutions

Electrochemical migration characteristics of Pb-free solder alloys are quantitatively correlated with corrosion characteristics in harsh environment conditions. In-situ water drop test and corrosion resistance test for Sn-3.0Ag-0.5Cu solder alloys were carried out in NaBr and NaF solutions to obtain the electrochemical migration lifetime and pitting potential, respectively. Sn-3.0Ag-0.5Cu solder alloy shows similar ionization and electrochemical migration behavior with pure Sn because of Ag and Cu do not migrate due to the formation of resistant intermetallic compounds inside solder itself. Electrochemical migration lifetime in NaBr is longer than in NaF, which seems to be closely related to higher pitting potential in NaBr than NaF solution. Therefore, it was revealed that electrochemical migration lifetime of Sn-3.0Ag-0.5Cu solder alloys showed good correlation to the corrosion resistance, and also the initial ionization step at anode side is believed to be the rate-determining step during electrochemical migration of Pb-free solders in these environments.

Influence of Thermal Treatment on the Caustic SCC of Super Austenitic Stainless Steel

In general, thermal treatment at 500oC ~ 900oC ranges depending upon alloy composition of stainless steels can sensitize the steels and promote the intergranular cracking, and their intergranular corrosion resistance is decreased. These behaviors seem to be related to the change of microstructures. So, heat treatment at that temperature range should be avoided in fabrication, especially welding of stainless steels. In this work, it is focused on the effect of thermal treatment on caustic stress corrosion cracking of super austenitic stainless steel - S32050 The low temperature thermal treatment increased greatly the resistance to caustic SCC than those of annealed specimen. This enhancement might be closely related to the reduction of residual stress and slightly large grain, but its resistance was not affected by the anodic polarization behavior.