Aqueous Corrosion Mechanism

Corrosion process on metal with access of air in aqueous environment could be describe by deterioration of iron which can be written as:

2Fe + O₂ + 2H₂O à 2Fe(OH)₂                                                                 [1]                                                         

The product, ferrous hydroxide, is commonly further oxidized to magnetite (Fe₃O₄) or a hydrated ferric oxide (FeOOH), that is, rust. It is convenient to consider separately the metallic and nonmetallic reactions in Eqn [1]:

2Fe à 2Fe²⁺ + 4e                                                                        [2]                                      

O₂ + H₂O + 4e à 4OH^-                                                              [3]

To balance Eqns [2] and [3] in terms of electrical charge, it has been necessary to add four electrons to the right-hand side of Eqn [2] and to the left-hand side of Eqn [3]. However, simple addition and rationalization of Eqns [2] and [3] yield Eqn [1]. We conclude that corrosion is a chemical reaction [1] occurring by an electrochemical mechanism (Eqns [2] and [3]), that is, by a process involving electrical and chemical species. Figure 1 is a schematic representation of aqueous corrosion occurring at a metal surface. Equation [2], which involves consumption of the metal and release of electrons, is termed an anodic reaction. Equation [3], which represents consumption of electrons and dissolved species in the environment, is termed a cathodic reaction. Whenever spontaneous corrosion reactions occur, all the electrons released in the anodic reaction are consumed in the cathodic reaction; no excess or deficiency is found. Moreover, the metal normally takes up a more or less uniform electrode potential, often called the corrosion or mixed potential (E_corr).

Figure 1 : Schematic illustration of the corrosion of steel in an aerated environment. Note that the electrons released in the anodic reaction are consumed quantitatively in the cathodic reaction, and that the anodic and cathodic products may react to produce Fe(OH)₂.

Anode Efficiency Test for Magnesium Sacrificial Anode

Anode efficiency was measured using the ASTM G97 test method. Sample in Figure 3.7 prepared for this test. This sample act as an anode placed in the test pot as in Figure 3.8 for 14 days under impressed current at current density 0.039 mAcm^-2 supplied by YEW Type 2553, Japan DC power supply. Current output recorded in Ampere per hour using Ampere hour meter model, Hokuto Denko H115, Japan. A steel plate 10 cm x 9.3 cm prepared for the cathode. The steel plate made from low carbon steel which similar to the buried pipeline material. This plate coated with paint leaving a 9.3 cm x 6.5 cm bare surface.  This is to simulate the protection of steel pipe with minor coating damage.

FIGURE 3.7: Sample dimension for Anode Efficiency Test

During the test, hydrogen gas being collected in the volumetric cylinder. The quantity of gas produced during the test proportional to the anode efficiency value. After 14 days, the open circuit potential measured after 1 hour current has been disconnected. Sample then cleaned using the solution containing 200 g CrO₃, 10 g AgNO₃, 20 g Ba(NO₃)₂ and distilled water to 1000 ml by ultrasonic bath. This solution prepared as stated in ASTM G1 cleaning procedure. Sample then rinsed with distilled water and blow dry before being weight to measure the weight loss.

Anode capacity determined by measuring charge transfer in Ampere hour divided by massed of specimen. The calculation following the ASTM G97 as in Equation 3.3.

Anode capacity  =  Ah x 01 x 1000   Ahkg^-1                                                       (3.3)

                                                M₁  - M₂

Where              Ah = Readings at the Ampere hour meter

                        M₁ = Sample weight before test, g

                        M₂ = Sample weight after test, g.

While the anode efficiency calculated using next equation:

Anode Efficiency =     Actual anode capacity      x100                                      (3.4)

                                       Theoretical anode capacity                                                        

Where  theoretical anode capacity equals to 2206.4 Ahkg^-1(Hock, 2001).

                                    

FIGURE 3.8: Schematic diagram of Anode Efficiency Testing.