THE PROTECTIVE ACTION OF CALCIUM-MAGNESIUM COATING DURING CATHODIC PROTECTION OF STEEL AND IRON IN SEA WATER
Abstract
The protective action of white films formed on iron and steel surfaces in sea water under conditions of cathodic protection was studied. Three series of tests were performed with 16 Mn low-alloy steel. In the first series, specimens were exposed either to natural sea water or to 3% NaCl solution, and protected with a constant current. In the second series the specimens were first exposed to stagnant or stirred sea water and for 3 days protected with a high density, 40 microamp/sq cm, current to form uniform films and then current applied periodically in 24-hr cycles. Specimens of the third series were tested either in natural or artificial sea water with various current densities. The artificial sea water contained either Mg and Ca (pH = 7. 7), Mg alone (pH = 7.8), Ca alone (pH = 8.0), or no Mg and Ca (pH = 7.8). Experimental results showed that protection can be achieved in natural sea water at 25 microamp/sq cm and in 3% NaCl solution at 35 microamp/sq cm. Specimens pretreated with a higher current can be protected at 3.5 microamp/sq cm. The periodically applied current should have a density of 15-20 microamp/sq cm. Polarization of Ca-Mg films is 5-8.8 times that of Ca or Mg films, while polarization of Mg films is higher than that of Ca films. Polarization characteristics are affected by current density applied in forming the film. Film formation is more difficult on rusted metal surfaces and more complete in stirred sea water. Films formed at a pH exceeding 10 provide protection for metals; films formed at pH = 9 fail to protect.
Document Details
- Document Type
- Technical Report
- Publication Date
- Sep 20, 1967
- Accession Number
- AD0670297
Entities
People
- Ching-cheng Koo
- Chung-tao Tai
- Ming-tang Chi
Organizations
- National Air and Space Intelligence Center