When two dissimilar metals are directly contacting and immersing in conducting, corrosive solutions, it can lead to the accelerated corrosion of one metal and protection of the other more noble metal. The more noble metal will become the cathode and the more active metal will become the anode. The corrosion rate of the anode will be increased and the cathode decreased. The increased corrosion of the anode is called “galvanic corrosion”. For galvanic corrosion to occur there are three conditions which must be met:
1) Dissimilar metals
2) Metal-to-metal contact
3) Metals in the same conduction solution (electrolyte)
Metals are ordered in what is known as the galvanic series which is a table that described how reactive or noble metals are likely to be according to their potential. Galvanic table says that the anodic or less noble metals at the negative end of the series such as magnesium, zinc and aluminum (active) are more likely to be attacked than those at the cathodic or noble end of the series such as gold and graphite. The farther away the two metals are from each other in the series, the larger the voltage potential, and the more intense the reaction.
TABLE 1. Galvanic Series
Please watch video of an example of galvanic corrosion around our living area:
6061 Aluminum coupons of 4” by 4” in size were coated with the MICRALOX® process using either Sanford Quantum™ or Sanford Classic™. Type III conventional hard coating (aluminum passive) was also processed as a control sample. Sample matrix of coupling dissimilar metals is shown in Table 2.
Zinc or stainless steel 316 bolts were coupled to aluminum panel coated with MICRALOX as shown in FIG. 2. Centered small hole for hanging string was punched out before anodized coating while a large hole at right hand side was punched out after anodizing for metal to metal contacting. It is anticipated that small anode such as zinc jointed to a large cathode such as stainless steel or MICRALOX would result in a high current density on the zinc, and hence a high rate of corrosion.
Instant ocean sea salt was used as conduction medium. Marine Accelerated Life Test (MALT) experiments were carried out using a 10” by 20” aquarium. Total 6 test panels were submerged into electrolyte using a nylon string as shown in FIG. 3. Air bubbling filtration system was used to agitate the liquid medium. Temperature of the conduction medium was maintained at 52 °C using a hot plate heater. The experiment is continued to 24 hours and 7 days for 1 month. According to personal communication with Navy correspondent, 1 day of such a condition is equivalent to 27 days in actual marine life environment. Therefore, 31 days of testing is shown the result on 2.2 years in marine life condition.
MICRALOX fastened with zinc or stainless steel 316 bolts or conventional type III hard coat (aluminum passive) jointed with zinc or stainless steel bolts were thoroughly examined in term of cosmetic corrosion appearance, respectively. TABLE 3 summaries the results:
TABLE 3. Results on MALT
- Among the metals evaluated, i.e., zinc, stainless steel 316, MICRALOX and conventional type III hard coat (aluminum passive), MICRALOX acts as the cathode or noble metal regardless of the other metal. It is shown that MICRALOX coated surface fastened with zinc or stainless steel using either Sanford Quantum or Sanford Classic did not show any evidences of corrosion using Instant Ocean for one month during marine accelerated life testing.
- Sanford Quantum processed MICRALOX coupled with stainless steel fastener showed perfection in galvanic corrosion. In other word, no pitting or corrosion in both stainless steel fastener and MICRALOX coated surface were observed.
- Conventional type III hard coat anodic coating (aluminum passive) coupled with zinc or stainless steel showed corrosion failure providing pitting over the entire surface. The sea water environment promoted failure of the conventional type III aluminum oxide layer resulted in corroding both dissimilar metals.