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Continued from The
Blog, p. 13.
The following items relate to cathodic & anodic protection.
Please be advised that the items are
not peer-reviewed, and opinions and suggestions are entirely those of the in- quirers and respondents. NACE Interna- tional does not guarantee the accuracy of the technical solutions discussed.
welcomes additional responses to these items. They may be edited for clarity.
Anodes for seawater- compensated fuel tanks
late" fuel. The original plan would have involved using bladders to separate the two liquids, but that idea fell victim to the old upfront cost problem. A ques- tion has arisen concerning the need for anodes in these tanks. There is a fear that Zn anodes would not perform well as they would build up a layer of oily sludge every time the tank was pumped out. Both the fuel and the seawater are clean, so I can only assume that the oily sludge was pro- duced by microbiological activity in the water/fuel interface. The seawater is "clean" because it is unpolluted and from the open ocean. Using Mg anodes is also strictly forbid-
Q
den for safety reasons. The tank is lined with a two-part epoxy that seems to be standing up quite well (I personally prefer to use anodes wherever possible, but there Q[ \PM WTL ILIOM ¹1N Q\ IQV¼\ JZWSM LWV¼\ Å` it."), hence the question about the need NWZ IVWLM[ QV \PM ÅZ[\ XTIKM My questions are, will Zn perform under these circumstances? If not, what is the usual practice/anode material for seawater-compensated fuel tanks?
NACE International, Vol. 51, No. 4 A
Where does one obtain "clean" seawater? When closely exam- ined, it contains sediment. The sludge is probably from this sedi-
Some time ago, some of our ships started using seawater-compensated tanks for "navy distil-
ment, which settles out. Under the cir- cumstances and the assumption that the tank is lined, you can't use Mg anodes. Because the sludge would have a rela- tively high resistance, can you use im- pressed current?
next batch of liquid is pumped into the tank, although I don't see why they should foul up. On the other hand, if it's coated with a good-quality coating and inspected periodically, leave it alone.
strike steel, will produce sparks whereas Zn will not.
A A
A
There has been some thought about thermite sparking between Al alloys (especially higher-Mg types) and rusty steel. This has
been debated within the coal mining in- dustry where methane gas (CH4
) formed
may be present. Perhaps the most accurate description is "frictional smearing of alu- minum alloy onto a steel surface under PQOP [XMKQÅK XZM[[]ZM[º 2 + *IQTMa Trans. of the Institution of Mining Engineering, 2IV]IZa 1959). From my cursory glance at this and other literature, the pressure needs to be very high for this reaction to occur. Al- though the spark can be produced in a laboratory, I believe that there is no proven evidence of it having caused a real- TQNM ÅZM WZ M`XTW[QWV ;\MMT WV [\MMT IVL steel-on-rock can also spark, of course.
Hot-spot cathodic protection
the following characteristics? Continued on page 31
Q April 2012 MATERIALS PERFORMANCE 29
Has anyone heard of a cathodic protection (CP) method for un- derground pipes with
Please note it is not the high po- tential difference between the steel and Mg that causes sparks. Both Mg and Al anodes, if they
Usually, Zn anodes have been used on ballast tanks with no problems. If the anodes foul up, they can be cleaned before the
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