At present, when bills about disposal of solid waste are beginning to be discussed in the country, evaluation of the impact of materials on the environment is of vital importance.

Recycling of post-consumption packaging is a viable solution practically all over the world. However, another important issue is raised: as collecting 100% of all post-consumption packages would be highly improbable, the option for recyclable packaging with less degradation time is more viable for impact reduction.

We know the intrinsic capacity of steel for degrading relatively fast in the environment when compared to other materials. However, many information without proper scientific source are found regarding the degradation time of the steel cans for beverage.

Seeking to solve these misunderstandings, the Companhia Siderúrgica Nacional - CSN and Companhia Metalic Nordeste contracted Fundação Núcleo de Tecnologia Industrial - NUTEC in order to determine the degradation level (corrosion) post-consumption in steel cans for drinks found at the seaside of the city of Fortaleza , in the state of Ceará.

Methodology

Metalic, CSN and NUTEC technicians collected steel cans at the seaside in Fortaleza , in the state of Ceará. Altogether, 156 cans were collected, and 126 were analyzed as being a representative sample of a hypothetical lot ranging from 1,201 to 3,200 units. All beverage cans were identified regarding their manufacturing date. Regarding time in the environment, the formula shown below was used:

Exposure Time (days) = Collecting Date – Validity Date – 365

This time can be considered the maximum exposure time, once time between filling process and consumption is added to total time in the environment.

The cans went through a cleaning process, in which they were first washed with detergent and solvent, for dirt and grease removal. After that, the coating was removed with paint remover.

With the clean cans, that is to say, only substrate, they went through oxidation (rust) removal. They were weighed, so as mass reduction in relation to new cans, could be verified. Tops and organic coatings, important parameter used when calculating steel corrosion rate and total estimate degradation time, were disregarded.

Conclusions

» Results of average corrosion rate in relation to the collected samples are not representative in the sense of estimating steel cans’ total degradation time;

» Based on obtained data, the average corrosion rate for distinct periods varied from 3.3 to 4.0 mpy;

» The number of collected samples diminishes potentially within maximum exposure time in places where samples were collected, tending towards zero for periods longer than 350 days;

» Both of the statements above raise two probable and non-exclusive hypothesis: older samples are either effectively collected and sent for recycling and/or they degrade in the environment and return to nature in the form of oxides, and, for any of the reasons above, are not found;

» Extrapolating the tendency curves related to the average minimum and maximum rates (the latter obtained with 95% of certainty with 2 standard deviations) of each period, it is estimated that degradation time for post-consumption steel cans in environment near the sea in the city of Fortaleza, in the state of Ceará, varies from 8 months to 3.6 years.

Source: DIQUI Techinical Report – 031/2002, issued by Fundação Núcleo de Tecnologia Industrial – NUTEC.