CORROSION BEHAVIOUR OF Al-12Si-1Mg AUTOMOTIVE ALLOY … · fect on the corrosion resistance of the aluminum alloys. Corrosion is affected by many factors that are related to the environment

Mohammad Salim Kaiser

423

CORROSION BEHAVIOUR OF Al-12Si-1Mg AUTOMOTIVE ALLOY IN ACIDIC, ALKALINE AND SALT MEDIA CONTAINING Zr TRACES


ABSTRACT

The corrosion behavior of Al-12Si-1Mg automotive alloy in absence and presence of zirconium traces is inves-tigated in acidic, alkaline and salt media. The heat treated cast alloy specimens are allowed to stay in 0.5M H2SO4, 0.5M NaOH and 0.6M NaCl solutions for 47 days. The work is carried out using conventional gravimetric and electrical resistivity measurements as well as optical microscopy, SEM and EDX investigations. It is concluded that the alloys corrosion rate is greater in an acidic solution than in an alkaline one and that the anodization provides to decrease the corrosion rate of the alloys in all media. All alloys studied corrode less in a salt medium. The presence of Zr refines the grains and stabilizes the aluminium matrix phase up to 47 days of an exposure with no measur-able effect on the alloys corrosion behavior. Formation of pores, a black corrosion product layer and a thin film of aluminum oxide and hydroxide on the surface is observed in the different media investigated. The microstructures obtained show that the process of alloys corrosion is localized at the area surrounding the precipitates of Al (Si, Mg) and Al-Mg which form hemispherical pits. The formation of crystallographic pitting is not found in a salt solution within the exposure times studied.

Keywords: Al-Si alloys, corrosion, gravimetric analysis, solution, SEM.

Received 21 February 2018Accepted 10 September 2019

Journal of Chemical Technology and Metallurgy, 54, 2, 2019, 423-430

Directorate of Advisory, Extension and Research ServicesBangladesh University of Engineering and Technology, Dhaka-1000, BangladeshE-mail: [email protected]

INTRODUCTION

Aluminum alloys containing silicon are very im-portant among the majority of casting alloys. They are widely used in the automotive and aerospace industries. This is mainly due to the outstanding effect of silicon in the aluminium alloys referring to the improvement of the casting characteristics and the corrosion resistance as well as with other mechanical properties [1,2]. It also improves the fluidity, the hot tear resistance and the feed-ing characteristics. Sometimes other alloying elements such as magnesium, copper, nickel, manganese, tin, etc. are added to the alloys aiming an intended application [3, 4]. It is necessity to study the corrosion resistance and the mechanical properties of the alloys because they are used in different environments.

Corrosion is a continuous process which could

be difficult to control and terminate. It is a destructive phenomenon that occurs in practically all environments [5]. It is caused by an electrochemical reaction between the metal or the alloy and the aqueous phase. The metal corrosion results from two simultaneous reactions: oxi-dation of metal ions and a process of reduction of the depolarizer in the medium (hydrogen ions to hydrogen gas in an acidic medium or oxygen to hydroxide ions in neutral and alkaline media). The corrosion processes not only influence the chemical properties of a metal but also generate changes in its physical properties and its mechanical behavior [6, 7]. Aluminium and its alloys are very good corrosion resistant materials in a neutral aqueous solution due to the formation of a passive film [8]. The addition of transition elements such as Zr and Ti is an effective way to improve the mechanical properties of aluminium alloys [9, 10]. It is found that trace ele-

Journal of Chemical Technology and Metallurgy, 54, 2, 2019

424

ments and aluminum can form dispersoids improving the alloys recrystallization resistance, controlling the grain boundary structure, etc. It is reported that the refined microstructure and the precipitated phases have an ef-fect on the corrosion resistance of the aluminum alloys.

Corrosion is affected by many factors that are related to the environment and the metal [11]. The nature of the corrosive medium has also an effective role on the corrosion behavior [12].

The aim of present study is to investigate the ad-sorption and the inhibitive properties of Al-12Si-1Mg automotive alloy in H2SO4, NaOH and NaCl media using the weight loss method. The research has been directed towards studying the effects of Zr traces on the corrosion behavior of the alloy.

EXPERIMENTAL

Melting was carried out in a resistance heating furnace under a suitable flux cover (degasser, borax, etc.). The commercially used aluminium pistons were initially melted in a clay-graphite crucible which was used as the master alloy. Two heat regimes were used to develop Al-12Si-1Mg base alloy and Al-12Si-1Mg alloy containing Zr. Zirconium was taken in the form of powder (of 99.98 % purity) in an aluminium foil and was added by plunging. The final temperature of the melt was always maintained at 750 ± 15oC. The casting was done in a cast iron metal mould preheated to 200oC. The mould size was 16 mmx 150 mm x 300 mm. The chemical composition test was carried out by Optical Emission Spectrometer (Table 1 shows the chemical composition of the developed alloys). The cast alloy was homogenized for 18 h in a muffle furnace at 400°C and air cooled to relieve the internal stresses and ho-mogenization. The homogenized samples were melted at 530°C for 2 h. Then salt ice water quenching followed to get a super saturated single phase [13]. The samples of 50 mm x 16 mm x 3 mm size were obtained and were artificially aged for 90 min at 225oC to conduct the tests for studying the corrosion behavior. The samples were mechanically wet-sanded with SiC papers of 220 grit and 1200 grit. Prior to use, the samples were de-greased with ethanol of 99 % purity and then rinsed with plenty

of water. Afterwards, they were dried, weighted (initial weight, Wint) and exposed for periods up to 47 days. Stagnant solutions of 0.5M H2SO4, 0.5M NaOH, and 0.6M NaCl were used as an aggressive medium. After the designated exposure in the solution medium, the samples were removed from the test solution, thoroughly washed with a solution of acetone and deionised water, dried well and weighted again (final weight, Wfin). The weight-loss measurements were carried out in triplicate and the weight loss was calculated by taking an average of the values obtained [14]. The weight loss, ΔW, was calculated in accordance with:

int fin(W W )WA−

∆ = (1)

while the corrosion rate, Kcorr , was evaluated on the ground of:

corr

(K W)K(T D)×∆

=×

(2)

where Wint (mg) was the initial weight prior to the im-mersion, Wfin (mg) was the final weight after exposure, A was the area in (cm2), Kcorr was the corrosion rate in mmpy, K was the unit conversion constant (K = 87.6 for the mmpy unit), T was the time of exposure (h), while D (g/cm3) was the density of metal.

Electrical conductivity of the alloy was measured under different conditions using Electric Conductivity Meter, type 979. The electric resistivity was calculated on the ground of this conductivity data. The washed and dried samples were observed carefully by OPTIKA Microscope and some selected photomicrographs were taken. The SEM investigation and EDX analysis of the surface of the alloy specimens were obtained after im-mersion in the different medium for 47 days. The SEM images were collected by using a JEOL scanning electron microscope with an energy dispersive X-ray analyzer (Model: Link AN - 10000) attached.

Table 1. A chemical composition of the experimental alloys (wt. %).

Si Mg Cu Ni Fe Zr Al

Alloy 1 12.280 0.919 1.120 1.306 0.521 0.001 Bal

Alloy 2 12.410 0.938 1.130 1.328 0.554 0.024 Bal


425

RESULTS AND DISCUSSION

Gravimetric analysisFig. 1(a) and 1(b) depict the change of the weight loss

ΔW (mg cm-2) calculated using Eq. (1) as a function of the immersion time for Al-12Si-1Mg (automotive Alloy 1) and Al-12Si-1Mg containing Zr (automotive Alloy 2) immersed in 0.5M H2SO4, 0.5M NaOH and 0.6M NaCl solution respectively. It is observed that the samples weight loss immersed in an acidic solution is higher than that of the samples immersed in an alkaline solution. The result in the salt solution seems neutral- neither weight gain nor weight loss is observed. The weight gain is at-tributed to the formation of passive films on the surface of the samples which lead to the temporary seizure of the corrosion attack, while the weight loss is due to the

gradual breakdown of the passive films brought about by the corrosion attack within several days. The weight loss increases in an acidic solution with the increase of the days of exposure because of the continuous aggres-sive attack. However, the weight loss is slightly lower than that observed at the earlier stage of immersion suggesting that the passive films formed on the surface of the composites are stable. In case of an alkaline solu-tion the weight loss at the initial stage could be due to the alumina particles segregation in the matrix which is very common in as-cast composites. Furthermore, it promotes nucleation of pits which facilitates the corro-sion proceeding. The negative weight loss is ascribed to the protective nature of the film formed on the metal surface [15].

The values of Kcorr for Alloy 1 and Alloy 2 calculated with the application of Eq. (2) are plotted versus time as shown in Fig. 2(a) and Fig. 2(b), respectively. The values of KCorr referring to 0.5M H2SO4 increasing to 8 mmpy after 3 days of exposure are the highest. They are followed by those obtained in 0.5M NaOH which reach a value of 6.5 mmpy after 3 days of immersion. Then they start to decrease with time and attain a roughly constant value. In case of salt solutions a negative cor-rosion rate is observed. The results observed in H2SO4 media are ascribed to the high aggressive attack of the sulphide ions leading to the disruption of the passive film formed on the alloys surface [16, 17]. The higher sulfates concentrations imply a higher concentration of Al3+ ions in the aqueous environment. They catalyze the reduction reactions on the cathode, which in turn affects the oxidation reactions and hence the corrosion rate. A part of the cast dissolves in a dilute sulphuric acid providing the presence of Al3+ ions and hydrogen gas, H2, in accordance with Eq.(3):

2Al +3H2SO4 → 2Al3+ + 3SO42– + 3H2 (3)

The weight gain for the alloys after the 3rd day of immersion is more pronounced in NaOH solution as expressed by the large negative corrosion rates observed. The corrosion rate is thereafter much decreased indicat-ing that the passive films formed on the surface of the specimens remain stable to a large extent till the com-

Fig. 1. The weight loss (mg cm-2) versus the immersion time (days) in different solutions for (a) Al-12Si-1Mg automotive Alloy 1 and (b) Al-12Si-1Mg automotive Alloy 2 containing traces of Zr.

a)

b)


426

pletion of the immersion test. The weight loss increases with time within three days. Then it starts to decrease showing a curvature until its maximum value is reached. This tendency is attributed to the depletion of the hy-droxyl ions in the solution, which are consumed in the corrosion process. A black corrosion product layer is formed on the surface of Al-12Si-1Mg alloy during the corrosion process and it becomes thicker, particularly at higher NaOH concentrations. There is a difference in the solubility of aluminium and its alloy elements. The black layer formed on pure aluminium in NaOH solution is mainly aluminium oxide containing some particles of metallic aluminium [18]. Although the protecting effect of this layer is limited by its fragility, it has a protective nature to a certain degree. The corrosion reactions of the

Al alloy in NaOH solutions can be described by:2Al + 6H2O → 2Al(OH)3 + 3H2 (4)Al(OH)3 + Na(OH) → Na+ + [Al(OH)4]

- (5)Al2O3 + 2NaOH + 3H2O → 2Na+ + 2[Al(OH)4]

– (6)In the early stages of the corrosion in 3.5 % NaCl

solution an oxide film grows on the aluminum surface. This is indicated by the passive region shown on the curve. The process is expressed as follows:Al + 3OH– → Al(OH)3 + 3e- (7)2Al + 6OH- → Al2O3 . 3H2O + 6e- (8)Al3+ + 4Cl- → AlCl4

- (9)In case of Alloy 2, Zr refines the grain structure to

some extent but no remarkable difference in the alloy corrosion behavior is observed in the different corrosive media examined.

Fig. 2. The corrosion rate (mmpy) versus the immersion time (days) in different solutions for (a) Al-12Si-1Mg au-tomotive Alloy 1 and (b) Al-12Si-1Mg automotive Alloy 2 containing traces of Zr.

Fig. 3. The resistivity versus the immersion time (days) in different solutions for (a) Al-12Si-1Mg automotive Alloy 1 and (b) Al-12Si-1Mg automotive Alloy 2 containing traces of Zr.

a) b)

a) b)


427

The resistivity of the alloys in an acid solution in-creases due to the diffusion of aggressive ions leading to porosity of the surface as shown in Fig. 3(a). The increase of the magnitude of porosity increases the sur-face resistivity. In NaOH solution it increases linearly because of the formation of a layer of a black corrosion product. It becomes thicker with the immersion time in-crease. This results in resistivity values increase. A trend of resistivity decrease and increase is observed in 3.5 % NaCl solution within 47 days. Aluminum forms a very thin film of aluminum oxide and hydroxide bonded to its surface decreasing the resistivity. The corrosion attack is restored after some days due to the gradual disruption of the passive films which causes resistivity increase [19]. Prior to the immersion in the corrosive medium the resistivity is higher for Alloy 2 due to its fine grains. But its resistivity changes besides in a similar way in all the corrosive environments studied. This is illustrated in Fig. 3(b). This means that the grain refinement does not affect the corrosion behavior of Alloy 2.

Optical micrographic observationFigs. 4(a) and 4(b) show the optical micrographs of

polished Al-Si-Mg automotive Alloy 1 and automotive Alloy 2 containing traces of Zr prior to the immersion to the corrosion medium. Both micrographs show a polished surface with a few scratches due to the polishing. The presence of an Al-rich dendritic matrix, α-Al phase and a eutectic mixture in the interdendritic region formed by silicon particles is evident. The latter are coarse and distributed in a plate-like morphology set in the Al-rich phase [20]. Precipitates of Al(Mn, Fe, Cu) appear in a dark tone in these images, while those of Al(Si, Mg) are seen in a lighter tone. In case of Alloy 2 containing traces of Zr the tones are relatively refined and uniformly distributed. Furthermore, the particles of Al3Zr formed as a primary phase during the rapid solidification of the melt act as nuclei for solidification of Al and Zr operating thus as grain refiner of Al [21, 22].

Figs. 4 (c) and 4(d) show the micrographs of Alloy 1 and Alloy 2 immersed in an acidic solution for 3 days. They evidence some defects generally found like pits in the conversion layers. The intensity of the pits is higher. They are uniformly distributed for Alloy 2 containing

traces of Zr. The microstructure of the corroded samples after immersion for 3days is illustrated in Figs. 4(e) and 4(f). The latter show the degradation of the alloys. The attack observed in a sodium hydroxide medium is more or less uniform. Figs. 4(g) and 4(h) show some corro-sion after 3 days of immersion in NaOH medium. These findings suggest primarily that NaCl can be an aggressive medium inducing corrosion of the aluminium alloys.

(e) (f)

(c) (d)

(a) (b)

Fig. 4. A microstructure of the polished alloy prior to corrosion (a) Al-12Si-1Mg automotive Alloy 1 and (b) Al-12Si-1Mg automotive Alloy 2 containing traces of Zr after an exposure for 3days; (c) Alloy 1 in H2SO4; (d) Alloy 2 in H2SO4, (e) Alloy 1 in NaOH; (f) Alloy 2 in NaOH; (g) Alloy 1 in NaCl solution; (h) Alloy 2 in NaCl solution.

(g) (h)

SEM and EDX observationThe SEM images of the alloys studied after 47 days

of exposure in 0.5M H2SO4 solutions are shown in Figs. 5(a) and 5(b). Those referring to 0.5M NaOH solution


428

are presented in Figs. 5(c) and 5(d). The SEM images obtained in 0.6M NaCl solution are shown in Figs. 5(e) and 5(f). The images referring to H2SO4 solution show clearly pores due to the uniform degradation of the al-loys. The aggressive attack of the sulphide ions disrupts the passive film formed on the surface of the alloys. A black corrosion product layer is formed on the alloys surface in NaOH solution. This film grows thicker with the corrosion proceeding, particularly at higher NaOH concentrations [23]. It is worth noting that the corrosion layer in the alkaline solution seems more compact and homogenous. It is not porous. The SEM images obtained in NaCl solution show the presence of passive films on

the alloys surface. There is no crystallographic pitting. Only some spots are observed due to the gradual disrup-tion of the passive film [24].

There is no remarkable difference in the SEM im-ages of the alloys studied. In case of the acid solution the SEM image depicted in Fig. 5(f) presents a visible grain structure for Alloy 1. The structure is relatively fine for Alloy 2 because of the additive effect. The EDX provides the determination of the surface composition (in weight %) of Alloy 2 (Fig. 5(b)). It is as follows: 40.71 % of O, 0.38 % of Mg, 15.74 % of Al, 31.24 % of Si, 2.34 % of S, 1.36 % of Ti, 0.68 % of Fe, 2.27 % of Ni, 5.16 % of Cu and 0.12 % of Zr. The SEM image of

Fig. 5. SEM images after exposure for 47 days of (a) Al-12Si-1Mg automotive Alloy 1 in H2SO4 solution; (b) Al-12Si-1Mg automotive Alloy 2 containing Zr as an additive in H2SO4 solution; (c) Alloy 1 in NaOH solution; (d) Alloy 2 in NaOH solution; (e) Alloy 1 in NaCl solution; (f) Alloy 2 in NaCl solution.

(b) (a)

(c) (d)

(e) (f)


429

Alloy 2 (Fig. 5(d)) shows that the black surface formed in NaOH solution refers mainly to aluminium oxide containing some particles of metallic aluminium [12]. The corresponding surface composition (in weight %) determined by ЕDX refers to 51.69 % of O, 13.58 % of Na, 0.24 % of Mg, 16.36 % of Al, 15.85 % of Si, 0.87 % of Fe, 0.82 % of Ni, 0.58 % of Cu and 0.02 % of Zr. A typical localized corrosion is seen on the SEM image of Alloy 2 after an exposure in NaCl solution (Fig. 5(f)). Corrosion products formed on the sample surface are discontinuously distributed. The EDX analysis suggests that the corrosion products contain 64.4 % of O, 0.28 % of Na, 1.47 % of Mg, 28.47 % of Al, 3.62 % of Si, 0.25 % of S, 0.06 % of Cl, 0.82 % of Fe, 0.23 % of Ni, 0.13 % of Cu and 0.28 % of Zr. The data shown indicate that the oxygen present is most probably resulting from the formation of aluminum oxide layer on the surface of the alloy, which in turn leads to decrease of the corrosion of the alloy.

CONCLUSIONS

The careful analysis of the results obtained shows that Al-12Si-1Mg automotive alloy has a higher corro-sion rate in an acid solution than in an alkaline one due to gradual disruption of the passive films and the segre-gation of the alumina particles in the matrix. The alloys exhibit superior corrosion resistance in a salt medium because of the formation of passive films on the surface. In case of the alloy containing Zr, there were no sig-nificant differences in the corrosion behavior observed in all corrosive media studied. The alloys resistivity increases in alkaline and salt media due to formation of a black corrosion product layer, while an intensive pit formation is observed in an acidic medium. Aluminum forms a very thin film of oxide in a salt medium. This results in resistivity decrease. No evidence of formation of crystallographic pitting is found in NaCl solution for exposure times up to 47 days.

AcknowledgementsThis work is supported by CASR of Bangladesh

University of Engineering and Technology (BUET) and is part of project “Effect of trace zirconium on the

chemical, electrical, mechanical and physical proper-ties of cast and heat treated aluminium-silicon piston alloy”. Thanks to the Department of Glass and Ceramics Engineering at BUET for providing laboratory facilities.

REFERENCES

1. R.S. Rana, R. Purohit, S. Das, Reviews on the influ-ences of alloying elements on the microstructure and mechanical properties of aluminum alloys and aluminum alloy composites, International Journal of Scientific and Research Publications, 2, 6, 2012, 1-7.

2. M.S. Kaiser, M.R. Qadir, S. Dutta, Electrochemi-cal corrosion performance of commercially used aluminium engine block and piston in 0.1M NaCl. Transaction of the Mech. Eng. Div., The Institution of Engineers, Bangladesh, 45, 1, 2015, 48-52.

3. M.M. Hamasha, A.T. Mayyas, A.M. Hassan, M.T. Hayajneh, The Effect of Time, Percent of Copper and Nickel on Naturally Aged Al-CuNi Cast Alloys. Journal of Minerals & Materials Characterization & Engineering, 11, 2, 2012, 117-131.

4. F. Stadler, H. Antrekowitsch, W. Fragner, H. Kauf-mann, P.J. Uggowitzer, Effect of main alloying ele-ments on strength of Al-Si foundry alloys at elevated temperatures, International Journal of Cast Metals Research, 25, 4, 2012, 215-224.

5. M.G. Fontana, N.D. Greene, Corrosion Engineering, McGraw-Hill Book Co., New York, 1967

6. P.G. Sheasby, R. Pinner, The Surface Treatment and Finishing of Aluminum and Its Alloys. 6th Edition, ASM International, 2001.

7. M. Okayasu, S. Takeuchi, T. Shiraishi, Corrosion and mechanical properties of cast aluminium alloys, International journal of Cast Metals Research, 26, 6, 2013, 319-329.

8. S. Rajendran, C. Thangavelu, A. Angamuthu, S. Jayakumar, Inhibition of corrosion of aluminium in alkaline medium by glutaric acid in conjunction with zinc sulphate and diethylene triamine penta (Methylene phosphonic acid), Archives of Applied Science Research, 5, 1, 2013, 202-212.

9. M.S. Kaiser, Grain refinement and precipitation hardening of cast Al-6Mg alloy through ternary


430

scandium and quaternary zirconium and titanium addition. International Journal of Research in Me-chanical Engineering and Technology, 1, 1, 2011, 57-62.

10. M. Emamy, A.R. Daman, R. Taghiabadi, M. Mah-mudi, Effects of Zr, Ti and B on structure and tensile properties of Al–10Mg alloy (A520). International Journal of Cast Metals Research, 17, 1, 2004, 17-22.

11. J.C. Scully, The Fundamentals of Corrosion, 3rd Edition, Pergamon Press, Oxford, UK, 1990.

12. A.S. Ismail, M.A. Abbas, The corrosion performance of Al-Si and Al-Cu casting alloys in H2SO4 and Na2CO3 media. Silicon, 9, 33, 2016, 1-7.

13. M.S. Kaiser, Solution treatment effect on tensile, impact and fracture behaviour of trace Zr added Al-12Si-1Mg-1Cu piston Alloy. Journal of the In-stitution of Engineers, India, Series D, 99, 1, 2018, 109-114.

14. H.H. Strehblow, Mechanisms of pitting corrosions in corrosion mechanism in theory and practice, Marcel Dekker, New York, 1995.

15. P.C.R. Nunes, L.V. Ramanathan, Corrosion behavior of alumina–aluminium and metal–matrix compos-ites, Corrosion, 51, 1995, 610-617.

16. O. Balogun, J. Borode, K. Alaneme, M. Bodunrin, Corrosion behaviour of alpha phase aluminium bronze alloy in selected environments, Leonardo Electronic Journal of Practices and Technologies, 24, 2014, 113-125.

17. W.R. Osorio, L.C. Peixoto, L.R Garcia, A. Garcia, Corrosion behavior of hypoeutectic Al-Cu alloys in H2SO4 and NaCl solutions, Acta Metallurgica

Sinica, 22, 4, 2009, 241-246.18. T. Notoya, T. Ishikawa, Corrosion of aluminum-

copper alloy and inhibition action of benzotriazole, tolyltriazole and hydroquinone in sodium hydroxide solutions, Bulletin of the Faculty of Engineering, Hokkaido University, 105, 1981, 1-7.

19. R.A. Rodríguez-Diaz, J. Uruchurtu-chavarín, A.M. Cotero-Villegas, S. Valdez, J.A. Juarez-Islas, Cor-rosion Behavior of AlMgSi Alloy in Aqueous Saline Solution, International journal of electrochemical science, 10, 2015, 1792-1808.

20. A. Pardo, M.C. Merino, S. Merino, M.D. Lopez, F. Viejo, M. Carboneras, Influence of SiCp content and matrix composition on corrosion resistance in cast aluminium matrix composites in salt fog, Corrosion Engineering Science and Technology, 39, 1, 2004, 82-88.

21. T.R. Ramachandran, P.K. Sharma, K. Balasubrama-nian, Grain refinement of light alloys, 68th World Foundry Congress, 2008, 189-193

22. A.E. Mahmoud, M.G. Mahfouz, H.G. Gad- Elrab, Influence of zirconium on the grain refinement of Al 6063 alloy, International Journal of Engineering Research and Applications, 4, 7, 2014, 188-194.

23. M. Mouanga, P. Berçot, J.Y. Rauch, Comparison of corrosion behaviour of zinc in NaCl and in NaOH solutions. Part I: Corrosion layer characterization, Corrosion Science, 52, 12, 2010, 3984-3992.

24. M.S. Kaiser, S. Dutta, Corrosion behaviour of alu-minium engine block in 3.5% NaCl solution, Journal of Materials Science and Chemical Engineering, 2, 2014, 52-58.

Documents

CORROSION BEHAVIOUR OF Al-12Si-1Mg AUTOMOTIVE ALLOY … · fect on the corrosion resistance of the aluminum alloys. Corrosion is affected by many factors that are related to the environment