Metal Samples MS2500L, Руководство оператора

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43
Galvanic Series
The table below is a simple version of the galvanic series of alloys in seawater. Because electrode
(oxidation-reduction) potentials only apply to pure elements and true compounds, another system was
developed to compare the relative reactivity of alloys in an environment. This series has the added
advantage of allowing you to predict the galvanic behavior of certain alloy pairs in an environment.
If a pair of alloys listed in the series are coupled, the alloy higher in the list will be corroded more rapidly
than if it were uncoupled, and the alloy lower in the series will be protected, or corrode more slowly
than if it were uncoupled. The table shows why alloys of aluminum and magnesium are galvanically
coupled to steel to protect the steel. Coupling steel to copper, brass, or stainless steel accelerates the
corrosion of steel.
Active / Anodic
Magnesium
Zinc
Aluminum
Iron / Steel
Lead
Brass
Admiralty
Copper
70-30 Copper Nickel
Stainless Steel (passive)
Noble / Cathodic
7. Common Corrosive Agents
Most, but not all, of the common corrosive agents encountered in industrial waters are gases. The
reactions of importance are:
Other corrosive agents occur in industrial waters, although infrequently. Among these are the ferric ion
(Fe
+++
), sulfide ion (S
-2
), bromine (Br
2
), and cupric ion (Cu
++
).
2H
+
+ 2e- 2H H
2
O
2
+ 4H
+
+ 4e- 2H
2
O
O
2
+ 2H
2
O + 4e- 4OH-
HCO
3
- + e- H
0
+ CO
3
-2
Cl
2
+ 2e- 2Cl-