научная статья по теме STUDY FOR PREVENTION OF STEEL CORROSION BY SACRIFICIAL ANODE CATHODIC PROTECTION Химическая технология. Химическая промышленность

Текст научной статьи на тему «STUDY FOR PREVENTION OF STEEL CORROSION BY SACRIFICIAL ANODE CATHODIC PROTECTION»

ТЕОРЕТИЧЕСКИЕ ОСНОВЫ ХИМИЧЕСКОЙ ТЕХНОЛОГИИ, 2013, том 47, № 3, с. 323-330

УДК 669:620.19

STUDY FOR PREVENTION OF STEEL CORROSION BY SACRIFICIAL ANODE

CATHODIC PROTECTION © 2013 г. A. S. Yaro", K. W. Hameed4, A. A. Khadomc

aChemical Engineering Department, College of Engineering, University of Baghdad, Baghdad, Iraq bBiochemical Engineering Department, Al-Khawarzim Engineering College, University of Baghdad, Baghdad, Iraq cChemical Engineering Department, College of Engineering, University of Diyala, Diyala, Iraq

aneesdr@gmail.com Received 26.09.2011

Corrosion of steel tube in sea water was controlled by cathodic protection. Sacrificial anode technique was used. In this technique, weight loss method was used to determine the rate of zinc consumption as a function of temperature, time, pH and solution velocity. Reaction kinetics studies showed that the rate of zinc consumption was first order. Activation parameters were obtained from Arrhenius equation and transition state equation. Two mathematical models were suggested to represent the consumption data. Statistical analysis proved that the second-order multi-terms model was better than the one-term model.

DOI: 10.7868/S0040357113030159

INTRODUCTION

Many techniques have been used to minimize the corrosion of steel, one ofwhich is cathodic protection. Cathodic protection is widely used to protect immersed metallic structures from corrosion processes [1]. It is possible to apply cathodic protection in most aqueous corrosive environments, although its use is largely restricted to natural near-neutral environments (soils, sands and waters, each with air access). Thus, although the general principles outlined here apply to virtually all metals in aqueous environments, it is appropriate that the emphasis, and the illustrations, relate to steel in aerated natural environments [2, 3]. Ca-thodic protection involves the application of a direct current from an anode through the electrolyte to the surface to be protected. This is often through of as "overcoming" the corrosion currents that exist on the structure. Cathodic protection eliminates the potential differences between the anodes and cathodes on the corroding surface. A potential difference is then created between the cathodic protection anode and the structure such that the cathodic protection anode is of a more negative potential than any point on the structure surface. Thus, the structure becomes the cathode of a new corrosion cell [4, 5]. There are two proved methods of applying cathodic protection: sacrificial anode (galvanic) and impressed current. Each method depends upon a number of economic and technical considerations. For every structure, there is a special cathodic protection system dependent on the environment of the structure [6]. In the present work, the rate of zinc consumption, which used as a sacrificial anode, as a function of temperature, flow rate, pH and time was studied using the weight loss technique.

EXPERIMENTAL WORK

Experimental work of sacrificial anode system was carried out to determine the consumption rate of zinc in artificial sea water (4% NaCl/distilled water) using weight loss for various conditions of temperature (0— 45°C), flow rate (5-900 L/h), pH (2-12) and time (1-4 h). Working electrode was tube specimen of low carbon steel with dimensions of 13.50 cm length, 2.68 cm inside diameter and 0.31 cm thick. The composition of steel specimen was as follows, wt %: C, 0.1648; Si, 0.2540; Mn, 0.5101; S, 0.0062; Cr, 0.0253; Ni, 0.0090; Cu, 0.1511, V, 0.0034 and the remainder is Fe. Anode electrode was zinc strip with dimensions of 12.50 cm length, 1.00 cm width and 0.60 cm thick. The composition of zinc specimen was as follows, wt %: Al, 0.12; Pb, 0.0034; Cu, 0.0017; Cd, 0.0033; Fe, 0.0032; Sn, 0.0023 and the remainder is Zn. The low carbon steel tubes (cathode) and zinc strips (anode) were cleaned by washing with liquid soap and tap water followed by distilled water, degreased by analar benzene and acetone. They were stored in a desiccator over a silica gel until use. Before each test, specimens were abraded in sequence on emery paper grades 220, 320, 400 and 600 under tap water on METASERN hand grinder tray (England), washed with running tap water followed by distilled water, dried with clean paper tissues, degreased and rinsed with benzene and acetone respectively and dried with paper tissues. They were then left to dry for one hour over silica gel ready for use. The apparatus shown in Fig. 1 was used to obtain the experimental data. After each run the zinc strip was rinsed in distilled water and brush to remove the corrosion products, dried with clean tissue then immersed in the benzene and acetone, dried again,

323

6*

Fig. 1. Schematic diagram of apparatus used in sacrificial anode test system: 1 — electrolyte (seawater), 2 — feed forward pipe, 3 — pump, 4 — valve, 5 — rotameter, 6 — tube steel, 7 — rubber stopper, 8 — zinc strip, 9 — copper wire, 10 — feedback pipe, 11 — heater or chiller, 12 — variac, 13 — electric source, 14 — insulated vessel, 15 — overflow.

and then re-weighted to determine the weight loss. The steel tube is also rinsed and dried by the same way above in order to re-use again. After each run the vessel is emptied from the solution and washed with distilled water, then filled with a new prepared solution for new run.

RESULTS AND DISCUSSION

To investigate the rate of zinc consumption during the cathodic protection of carbon steel pipe carrying 4% NaCl solution, 256 experiments were conducted using the factorial experimental design, each variable

W, mg/cm 4

2

1

2

3

4 t, h

Fig. 2. Zinc consumption with time for different temperatures at a flow rate of 600 L/h and pH 8: T = 45 (1), 30 (2), 15 (3), and 0 (4) °C.

was discrete into four levels, such that for temperature (0, 15, 30, 45°C), flow rate (5, 300, 600, 900 L/h), pH (2, 5, 8, 12) and time (1, 2, 3, 4 h). For the present system the electrochemical cell responsible for cathodic protection is Zn/NaCl/Fe. The anodic reaction is

Zn ^ Zn+2 + 2e.

(1)

The cathodic reaction is one of the following reactions:

2H+ + 2e ^ H2T, 2H2O + 2e ^ H2T +2OH-, O2 + 2H2O + 4e ^ 4OH

(2)

(3)

(4)

The cathodic reaction is depending on the nature of seawater but reaction of O2 reduction towards the wall of the carbon steel pipe is assumed predominate.

Time effect. Figure 2 shows the rate of zinc consumption (dissolution), which is instead of corrosion rate of steel, with time at different temperatures, different flow rates and different pH, respectively. The rate of zinc dissolution increases with increasing time and this is a normal case. But this increasing is not equally with time, where the dissolution rate in the first hour is more than second hour and so on. The reasons of that are attributed to continuous growth of the corrosion products layer with time, which affects the transport of oxygen to the metal surface and the activity of the surface and hence the corrosion rate. Also, the cathodic reactions will result in an increase in pH with time either by the removal of hydrogen ions (Eq. (2)) or by the generation of hydroxyl ions (Eqs. (2) and (4)), both reasons are reduced the corrosion rate of steel and hence the dissolution rate of zinc.

3

2

1

0

Temperature effect. Figure 3 shows the effect of temperature on the rate of zinc dissolution with time with different flow rates and with different pH values, respectively. The increase in the rate of zinc dissolution with increasing seawater temperature (particularly from 15 to 30°C) may be explained in terms of the following effects.

1. A temperature increase usually increases the reaction rate which is the corrosion rate and according to the Freundlich equation [2]:

r = к CO

(б)

The rate constant k varies with temperature according to the Arrhenius equation [2]

k = k0e

-Б/RT

(6)

Equation (6) indicates that k is increased with increasing temperature and then the corrosion rate which leads to increasing the rate of zinc dissolution.

2. Increasing seawater temperature leads to decreasing seawater viscosity with a consequent increase in oxygen diffusivity according to the Stokes—Einstein equation [7, 8]

T

= const.

(7)

As a result of increasing the diffusivity of dissolved oxygen, the rate of mass transfer of dissolved oxygen to the cathode surface increases according to the following equation [8]:

J — kdCO2 — DC o2 Od

(8)

W, mg/cm б

2

2 -

200

400

600

800 1000 Q, L/h

Fig. 3. Zinc consumption with flow rate for different temperatures at t = 4 h and pH 5: T= 45 (1), 30 (2), 15 (3), and 0 (4) °C.

3. The decreases in seawater viscosity with increasing temperature improve the seawater conductivity with a consequent increase in corrosion current and the rate of corrosion.

4. On the other hand, increase of temperature reduces the solubility of dissolved oxygen with a subsequent decrease in the rate of oxygen diffusion to the cathode surface and the rate of corrosion.

It seems that within the present range of temperature effects 1, 2 and 3 are predominating.

Flow rate effect. Figure 4 shows the effect of solution flow rate on the zinc dissolution with time, with different temperatures and with different pH values, respectively. It can be seen that the dissolution rate of zinc increases with increasing the flow rate. This may be attributed to the decrease in the thickness of hydro-dynamic boundary layer and diffusion layer across which dissolved oxygen diffuses to the tube wall of steel with consequent increase in the rate of oxygen diffusion which is given by Eq. (8). Then the surface film resistance almost vanishes, oxygen depolarization, the products of corrosion and protective film are continuously swept away and continuous corrosion occurs. The flow rate of seawater may also cause erosion which combined with electrochemical attack.

W, mg/cm 16

14

12

10

8

6

4

0

2

2 О З

A 4

200

400

600

800 1000 Q, L/h

Fig. 4. Zi

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