4.1    Potentiodynamic polarization and weight loss techniques :

Corrosion rates and inhibition efficiency using potentiodynamic and weight loss technique  for AISI-1020 in 3% NaCl solution   at 25 ºc , in the absence and presence of different concentrations of aromatic hydrocarbon and quaternary amine are listed in the next table .

 

Table: Corrosion rate and inhibition efficiency using potentiodynamic and weight loss technique for AISI–1020 in 3% NaCl solution at 25 ºc in the absence and presence of inhibitors.

 

 

 

 

Potentiodynamic  Polarization Technique :

From the polarization behavior of carbon steel at different concentrations of Quaternary Amine inhibitor and Aromatic Hydrocarbon in 3%NaCl solutions at 25˚C which are shown in figure a. and fig b respectively. The current densities on the anodic polarization curve were significant reduced as inhibitor concentration is increased. Generally, the inhibition was of a mixed type.  The suppression of the anodic process being greater.

figure .a: polarization curves for AISI 1020  in 3% NaCl solution in the absence and presence of quaternary amine at 25 ºC.

 

 

 

             

  

   

        Figure b: polarization curves for AISI-1020 in 3% NaCl solution in the absence and presence of Aromatic Hydrocarbon at 25 ºc

                                           By comparing the last two figures  quaternary amine inhibition is more effective than aromatic hydrocarbon in 3%NaCl solutions.The measured (corrosion rate) CR by the potentiodynamic method of AISI  1020 carbon steel specimen in saltwater solution as a function of corrosion inhibitor concentration (quaternary amine) is shown in the following fig E.

Fig c

Effect of Aromatic Hydrocarbon inhibitor concentrations on the corrosion rate of AISI 1020 carbon steel at 25^oC using the potentiodynamic method is indicated in fig c.

Fig d

Effect of aromatic hydrocarbon  and quaternary  amine  inhibitor concentration on the  corrosion rate of AISI-1020 carbon steel in 3% NaCl solution at 25 °c  using potentiodynamic technique indicated in fig d.

Fig E

 

 

It is clear from figures c and d that the corrosion rate (CR) is decreased with increasing inhibitor concentration and in figure e the corrosion rate (CR) is reduced  more when using Quaternary Amine than Aromatic Hydrocarbon at all concentrations.

 

 Weight loss measurements :

 The mild steel unmounted cylindrical steel samples with 1cm²cross section area and 1cm height washed with acetone using an ultrasonic cleaner. Surface preparation is handled by using abrasive paper of 320, 600, 800, 1000 grade. Then the samples dried and weighed before immersion in solutions of 3%NaCl solution in the absence and presence of inhibitors for recording weight loss measurements. The results were indicated before in table 1.

The effect of organic inhibitors  “Quaternary Amine and Aromatic Hydrocarbon”  concentrations on the corrosion rate of carbon steel AISI 1020 at 25^oC using weight loss technique is indicated in fig f, g

 

 

 

Figure f indicates the corrosion rate of mild steel AISI 1020 as a function of the concentration of both quaternary amine and aromatic hydrocarbons corrosion inhibitors.

 

It is clear from the figure 4 , 5 and table 2 that the measurements of potentiodynamic technique are the same as weight loss technique , both the corrosion rates decreases by increasing inhibitor concentration because the exposed surface area decreases by forming a film by corrosion inhibitors and the covered surface increases by increasing the inhibitor efficiency .

 

 

4.2    Scanning Electron Microscope (SEM)

a

SEM images for the AISI 1020 in 3% NaCl solution saturated  at 25˚C for one week in the absence of inhibitors, presence of 50 ppm Quaternary Amine and 20 ppm Aromatic hydrocarbon are shown in Figures 4.13a, b and c, respectively. As observed from Fig. 4.13a, corrosion appeared on the metal surface in the form of scattered pits, while in Figures 4.13b and c, most of corrosion pits disappeared from the metal surfaces due to the presence of inhibitors.

 

 

 

b

c

 

 

 

 

 

 

 

Figure 4.13: SEM images for AISI 1020 in 3% NaCl solution at 25˚C for

One week  in the absence of inhibitors (a), presence of 50 ppm Quaternary amine(b)

20 ppm Aromatic hydrocarbon  (c)

4.3    Energy Dispersive X-ray Analysis (EDX)

Figures 4 a, b and c show the EDX spectrum for the AISI 1020 in 3% NaCl solution  at 25˚C for one week in the absence of inhibitors, presence of 50ppm Quaternary Amine  and presence of 20 ppm Aromatic hydrocarbon, respectively. Tables 4 a, b and c reflect data obtained from the related spectrums for each element on the metal surface. In the absence of the inhibitors, the EDX spectra show the characteristics peaks of some of the elements constituting the AISI 1020 sample.

In inhibitor containing solutions, the EDX spectra showed an additional line characteristic for the existence of nitrogen (N). In addition, the intensities of carbon (C) and oxygen (O) signals are enhanced. The appearance of the N signal and this enhancement in the C and O signals upon adding inhibitors to the solution is due to the N, C and O atoms of the adsorbed compounds as shown in fig 4 a , b & c. These data show that a material containing N atoms has covered the metal surface. This layer is undoubtedly due to the inhibitor, because of the existence of N signal and the high contribution of the C and O signals observed in presence of the inhibitors.

The N signal and this high contribution of the C and O signals are not present on the metal surface exposed to uninhibited solutions . The spectra show also that the Fe peaks are considerably suppressed relative to the specimens inserted in the uninhibited solution. The suppression of the Fe lines occurs because of the overlying inhibitor film. These results confirm those from electrochemical measurements which suggest that a surface film inhibits the metal dissolution, and hence retarded the hydrogen evolution reaction. Therefore, EDX examinations of the metal surface support the results obtained from electrochemical methods that quaternary amine and aromatic hydrocarbon are good inhibitors for AISI 1020 carbon steel in 3% NaCl solution . It is appeared from the results that the amount of N atoms adsorbed on metal surface in the presence of quaternary amine is higher than that of aromatic hydrocarbon which may be due to the formation of thicker film and so, higher inhibition efficiency .

b

a

c

Figure 4.14: EDX Spectrum for the AISI 1020 in 3% NaCl solution at 25˚C for

One week  in the absence (a), presence of 50 ppm quaternary amine (b) and 20 ppm  aromatic hydrocarbon (c)

Table 4.8: Data obtained from EDX for the AISI 1020 in 3% NaCl solution  in the absence (a), presence of 50 ppm quaternary amine  (b) and 20 ppm of aromatic hydrocarbon (c)

	Element	Weight %	Atomic %	Net Int.
Table 3. a	C K	1.74	5.69	36.90
	O K	11.81	29.05	1064.70
	NaK	3.28	5.62	132.00
	ClK	2.44	2.71	465.80
	MnK	0.60	0.43	57.30
	FeK	80.14	56.50	6036.10

O %

Element	Weight %	Atomic %	Net Int.
C K	23.50	38.44	55.50
N K	13.45	18.86	16.80
O K	20.20	24.81	61.90
NaK	5.38	4.60	23.30
ClK	0.56	0.31	8.50
MnK	0.78	0.28	5.40
FeK	36.12	12.70	195.40

 

                          Table 3. b

 

 

 

O %

N %

 

 

 

Element	Weight %	Atomic %	Net Int.
C K	0.00	0.00	0.00
N K	5.42	11.77	4.10
O K	17.88	33.98	36.90
NaK	14.25	18.85	19.20
ClK	4.43	3.80	23.20
MnK	3.36	1.86	8.40
FeK	54.65	29.75	110.30

 

                           Table 3. C

 

 

 

 

 

4.4    X-Ray Diffraction Analysis (XRD)Low Angle

Figure 4.15 a shows the XRD pattern for the AISI 1020 carbon steel specimen after immersed in 3% NaCl solution saturated  at 25˚C, while Figures 4.15b and c show the specimen after immersion in 50 ppm quaternary amine and 20 ppm aromatic hydrocarbon inhibitors, respectively. Figure 4.15 a shows the formation of oxonium aqua iron chloride (H₃O) ₂ FeCl₅ (H₂O) and goethite (Fe₂O₃.H₂O) on the metal surface and iron in the core. These compounds represent the corrosion products due to immersion in the test solution. Intensity of these compounds is much higher on the metal surface as indicated by the related peaks.

Figure 4.15 b and c show the formation of iron nitride complex compounds on the metal surface and iron in the core. Iron nitride compounds represent the protective film formed due to interaction of iron with the inhibitor molecules. Intensity of these compounds is much higher on the metal surface as indicated by the related peaks and percent of more than (50%) which mean that the metal surface is composed mainly of iron nitride compounds. Also, narrow peaks of these compounds indicated that the formed films have crystallized character. This led to increase in the level of protection which is also, confirmed by the absence of corrosion products in the pattern in the presence of iron nitride compounds.

Nitride coatings have been used in numerous applications to increase the hardness and improve the wear and corrosion resistance of structural materials, as well as in various high-tech areas, where their functional rather than mechanical properties are of prime importance .

 

Figure 4.15 a: XRD pattern for the AISI 1020  specimen after immersion in 3% NaCl solution  at 25˚C for  one week

Figure 4.15 b: XRD pattern for the AISI 1020 specimen after immersion in 3% NaCl solution and 50 ppm quaternary amine at 25˚C for one week

 

 

Figure 4.15 c: XRD pattern for the AISI 1020 specimen after immersion in 3% NaCl solution and 20 ppm aromatic hydrocarbon at 25˚C for one week