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MATERIALS SELECTION & DESIGN TABLE 1 Corrosion types in coated metallic cans used in the food industry Coatings
Climatic Factors (Ranges)
RH >80% and temperatures <15 °C
RH >50% and temperatures <25 °C
RH >30% and temperatures <35 °C
Atmospheric Pollution Concentration Levels
Cl– (A) SO2
NO2 Cl–
SO2 NO2
Cl–
SO2 NO2
(B) (B)
>100 mg/L >0.1 ppm
>0.050 ppm >50 mg/L
>0.05 ppm >0.030 ppm
>10 mg/L >0.03 pp
>0.015 ppm (A)BS ISO 16701:2003, chlorides standard 10 mg/L < Cl–
Dense black spots are formed in external zones and with less range in internal areas
Spots in internal and external zones of steel cans, causing defective seafood containers
< 1,000 mg/L (B)Air quality standard SEMARNAT, MEXICO-EPA, USA, http://www.epa.gov/ttncatc1/cica/airq_e.html
Effect of Winds on Climate The Santa Ana winds that occur in
October through April of each year in Guaymas8
cause drastic variations in
humidity and temperature, which in combination with sulfur, chlorides, and nitrogen oxide, lead to atmospheric cor- rosion of steel cans.9
These affect the
physiochemical properties of the cans.10 With the frequent variations in climate, I \PQV ÅTU WN UWQ[\]ZM Q[ NWZUML \PI\ combines with air pollutants to begin the corrosion process in some zones of the metallic containers.2
Atmospheric Pollutants Among pollutants, sulfur dioxide (SO2 )
is of special importance because of its direct impact on the corrosion rate of
metals exposed to the atmosphere. SO2 is more soluble than oxygen, which causes the corrosion phenomena, and occurs at very low concentrations in the atmo- sphere. The corrosion process is also af- fected by moisture, which generates the values of RH, causing the formation of \PQV ÅTU[ WV \PM []ZNIKM WN UM\ITTQK KIV[ and subsequent deterioration of the con- tainers.11
Atmospheric studies are based
on analytical chemistry techniques and high-level software that determine the composition of the atmosphere and the effect of the corrosion levels indoors and outdoors in the food industry. Climatic
NACE International, Vol. 51, No. 4
changes contribute to the corrosion rate.11 The calculation indicates that most of these substances are present in the atmo- sphere at levels higher than expected in thermodynamic equilibrium with nitro- gen, oxygen, carbon dioxide (CO2
), and
water vapor. The composition of this atmospheric pollution is controlled by a series of kinetic mechanisms and encom- passes a complex combination of chemi- cal, physical, and biological agents.12 Regarding toxicity, air pollutants pro-
duce harmful effects in materials, damag- ing painted surfaces and metallic struc- tures of cans used to preserve seafood.10 The principal air pollutants in marine environments located in northwest Mexico are sodium chloride (NaCl) ions from the sea, and carbon monoxide (CO), nitrogen dioxide (NO2
), and SO2 \ZINÅK ^MPQKTM[ 13 from Inorganic pollutants are
generated by vehicles, industry emissions, IVL TIVLÅTT[ )\UW[XPMZQK XWTT]\QWV KIV be measured using the appropriate tech- niques and equipment to determine the indoor corrosion level.14
The NaCl and SO2 from the atmosphere.15
between the concentration levels of SO2 and NaCl and the corrosion rate is high.
Another aggressive air pollutant is hydro- OMV []TÅLM 02
S), which is responsible for
tarnishing steel and aluminum used in metallic seafood cans. Nitrogen com-
XW]VL[ \PI\ Æ]K\]I\M _Q\P P]UQLa IVL temperature also increase the corrosion rate of food containers.
Laboratory Testing
Measuring the Corrosion Rate The seafood plants in this area are
located at distances of 0.5 to 3 km from the seashore. Steel plates used to fabricate steel cans, with dimensions of 30 by 20 mm and 5-mm thick, were cleaned by immersion in an isopropyl alcohol ultra- sound bath for 15 min per ASTM G1,16 ASTM G4,17
and ASTM G31.18 Imme-
diately after cleaning the steel, probes were placed in sealed plastic bags, ready to be installed in the indoor and outdoor test sites. After each exposure period, the steel specimens were removed, cleaned, and weighed to obtain the weight loss and to calculate the corrosion rate. The H2
S
was measured by the MSA-Altair Pro System†
. The SO2 and Cl–
are the main corrosive contaminants ASTM 140,20 The correlation
were measured
by the sulfate plate technique per ASTM G91,19
and the wet candle method per respectively.
SEM and AES Measurements Pieces of steel cans were evaluated
using the scanning electron microscopy (SEM) and Auger electron spectroscopy
†Trade name. April 2012 MATERIALS PERFORMANCE 53
No formation of cracks in coatings as in the steel cans
Small stains and bad appearance without damage
Metallic Generates cracks under the coatings Plastic Isolated black spots are formed