Stress Corrosion Cracking (SCC) is induced by the combined influence of tensile stress and a corrosive environment. SCC can occur for specific metal/environment coupling, and its effect is higher than the sum of environmental corrosiveness and stress state. Usually, the metal surface remains almost unattacked, but with fine cracks penetrating into the material. These cracks can have intergranular or transgranular morphology. SCC is classified as a catastrophic form of corrosion, since the detection of such fine cracks can be very difficult and the damage is not easily predicted. In recent literature, several SCC cases occurring in tinplate cans for the conservation of highly proteic food (meat, tuna or petfood) are reported. In these cases, the specific SCC damage can be very dangerous due to fine cracks that can pass through the can wall, causing the air ingress. The present work is focused on the analysis of stress corrosion damage suffered by two different types of tinplate cans (called A and B) for oil-conserved tuna. Tinplated steel cans came from two different producers, and also the tuna meat came from different companies. Metallic cans are usually produced using low carbon steel, tinplated and then coated with epoxyphenolic resin. From thin strips, the cans are often produced by welding. After welding, both internal and external welding regions are re-protected with epoxyphenolic film; the cans are then filled with tuna meat, filled with oil under low pressure, and finally closed and sterilized around 115-120°C. In both examined cases, fine and branched SCC cracks were observed close to the welding regions. A deep microscopic analysis, performed by Scanning Electron Microscope (SEM) equipped with a energy dispersive system (EDS), highlighted the presence of resin delamination in regions close to SCC cracks. Moreover, inside the microcracks, a mixture of corrosion products and phosphates was detected. A chemico/physical analysis was performed on liquid phases contained in the cans. A large amountof water was present in both can types, higher for sample B than A. Moreover, the chemical analysis of the aqueous phases showed a massive presence of phosphates and bicarbonates, together with chlorides. The typical pH value was around 5.8. Experimental results are discussed in order to explain the occurrence of SCC in the carbon steel. First of all, the occurrence of oil-water separation inside the can is necessary to induce corrosive phenomena. Moreover, the only region prone to SCC is near the welding, as a consequence of residual stresses induced by the thermal treatment. In these regions a water phase comes in contact with the steel surfaces whatever discontinuities are present in the protective epoxy-phenolic deposit. Among possible chemical environment carrying to SCC on low carbon steel, phosphates in deareated water are the most probable in this case. Some experimental evidence suggest that phosphates promote a polarization inversion between steel and tin. A possible way to control SCC damage in these applications is the use of more continuous, thicker and more protective laquers on welding regions, such as powder layers (around 80 m in thickness) obtained with thermoplastic modified polyesters.
Stress corrosion cracking of tinplated cans for conserved tuna in oil
De Sanctis M.;Lovicu G.;Valentini R.
2011-01-01
Abstract
Stress Corrosion Cracking (SCC) is induced by the combined influence of tensile stress and a corrosive environment. SCC can occur for specific metal/environment coupling, and its effect is higher than the sum of environmental corrosiveness and stress state. Usually, the metal surface remains almost unattacked, but with fine cracks penetrating into the material. These cracks can have intergranular or transgranular morphology. SCC is classified as a catastrophic form of corrosion, since the detection of such fine cracks can be very difficult and the damage is not easily predicted. In recent literature, several SCC cases occurring in tinplate cans for the conservation of highly proteic food (meat, tuna or petfood) are reported. In these cases, the specific SCC damage can be very dangerous due to fine cracks that can pass through the can wall, causing the air ingress. The present work is focused on the analysis of stress corrosion damage suffered by two different types of tinplate cans (called A and B) for oil-conserved tuna. Tinplated steel cans came from two different producers, and also the tuna meat came from different companies. Metallic cans are usually produced using low carbon steel, tinplated and then coated with epoxyphenolic resin. From thin strips, the cans are often produced by welding. After welding, both internal and external welding regions are re-protected with epoxyphenolic film; the cans are then filled with tuna meat, filled with oil under low pressure, and finally closed and sterilized around 115-120°C. In both examined cases, fine and branched SCC cracks were observed close to the welding regions. A deep microscopic analysis, performed by Scanning Electron Microscope (SEM) equipped with a energy dispersive system (EDS), highlighted the presence of resin delamination in regions close to SCC cracks. Moreover, inside the microcracks, a mixture of corrosion products and phosphates was detected. A chemico/physical analysis was performed on liquid phases contained in the cans. A large amountof water was present in both can types, higher for sample B than A. Moreover, the chemical analysis of the aqueous phases showed a massive presence of phosphates and bicarbonates, together with chlorides. The typical pH value was around 5.8. Experimental results are discussed in order to explain the occurrence of SCC in the carbon steel. First of all, the occurrence of oil-water separation inside the can is necessary to induce corrosive phenomena. Moreover, the only region prone to SCC is near the welding, as a consequence of residual stresses induced by the thermal treatment. In these regions a water phase comes in contact with the steel surfaces whatever discontinuities are present in the protective epoxy-phenolic deposit. Among possible chemical environment carrying to SCC on low carbon steel, phosphates in deareated water are the most probable in this case. Some experimental evidence suggest that phosphates promote a polarization inversion between steel and tin. A possible way to control SCC damage in these applications is the use of more continuous, thicker and more protective laquers on welding regions, such as powder layers (around 80 m in thickness) obtained with thermoplastic modified polyesters.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
