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topic 11163

Effect of salt and/or acid concentration on the rate of rusting


I am a teacher. I need some numerical data (rather then descriptions) of the effect of salt concentration and / or acid concentration on the rate of rusting for use with my groups. We have tried, but can't generate reproducible data in our lab within the time scale we have to work on this topic. It would really help the students to understand rusting and what can be done to prevent it!

Thanks,

Dr. Dave Kent
- Leeds, Yorkshire, U.K.


 

Dave,

I don't think you'd ever get 'numerical' data. How in the HECK could you even imagine getting any when corrosion is not only based on the concentrations of acids, etc, but on the TEMPERATURE, too! Wouldn't you doubt that there'd be no corrosion at cryogenic temperatures?

Then, with sulphurics, iron will rust out BUT, and here's the conundrum, it will also work beautifully! This depends on the acid concentration which, if kept very high, then iron or steel is OK ... as long as a high concentration (90% +) is maintained.

As far as testing with salt, ow, I don't know if the eutectic affects the corrosion. Maybe the higher concentrations of salt are not so corrosive!

Rusting applies only to ferrous products. Rust is the natural oxidized state. Fe203 or Fe3O4. To prevent rust you have many options. Painting. Plating. Oiling ... or not using steel at all but use PLASTICS !

freeman newton portrait
Freeman Newton
White Rock, British Columbia, Canada

(It is our sad duty to
advise that Freeman passed away
April 21, 2012. R.I.P. old friend).



 

Have you tried consulting chemistry reference books in the library? I'm sure somewhere out there, some scientist has studied this and has some published results. I would start with the "CRC Handbook of Chemistry & Physics" [link is to info about book at Amazon] and "Chemical Engineers Handbook" [link is to info about book at Amazon].

tim neveau
Tim Neveau
Rochester Hills, Michigan


 
1 - Solution (1m old m-3)	
2 - pH	
3 - Temperature		
4 - Weight before (g)	
5 - Weight after (g)	    
6 - % of weight lost
1             2      3        4         5         6  
______________________________________________________
0.1 H2SO4        0.91	20°C	2.304	2.244	2.604
0.01 H2SO4       1.92	20°C	2.299	2.289	0.435
0.001 H2SO4      2.88	20°C	2.301	2.279	0.956
0.0001 H2SO4     4.51	20°C	2.309	2.285	1.039
1x10-5 H2SO4     5.01	20°C	2.297	2.279	0.784
1x10-6 H2SO4     6.59	20°C	2.290	2.289	0.044 
pH7 Buffer       7.00	20°C	2.298	2.298	0.000 
Tap water        6.45	20°C	2.296	2.272	1.045
0.1 NaCl          N/A	20°C	2.276	2.246	1.318
0.01 NaCl         N/A	20°C	2.295	2.258	1.612
0.001 NaCl        N/A	20°C	2.246	2.230	0.712
0.0001 NaCl       N/A	20°C	2.297	2.280	0.740
1x10-5 NaCl       N/A	20°C	2.251	2.229	0.977
1x10-6 NaCl       N/A	20°C	2.301	2.284	0.739
______________________________________________________
0.1 H2SO4        0.91	40°C	2.297	2.077	9.578
0.01 H2SO4       1.92	40°C	2.286	2.242	1.925
0.001 H2SO4      2.88	40°C	2.261	2.213	2.123
0.0001 H2SO4     4.51	40°C	2.210	2.186	1.086
1x10-5 H2SO4     5.01	40°C	2.293	2.285	0.349
1x10-6 H2SO4     6.59	40°C	2.303	2.281	0.955 
pH7 Buffer       7.00	40°C	2.289	2.289	0.000 
Tap water        6.45	40°C	2.287	2.265	0.962
0.1 NaCl          N/A	40°C	2.302	2.279	0.999
0.01 NaCl         N/A	40°C	2.282	2.219	2.761
0.001 NaCl        N/A	40°C	2.297	2.263	1.480
0.0001 NaCl       N/A	40°C	2.300	2.276	1.043
1x10-5 NaCl       N/A	40°C	2.297	2.284	0.566
1x10-6 NaCl       N/A	40°C	2.295	2.277	0.784
______________________________________________________
0.1 H2SO4        0.91	60°C	2.297	2.022    11.972
0.01 H2SO4       1.92	60°C	2.284	2.225	2.583
0.001 H2SO4      2.88	60°C	2.295	2.263	1.394
0.0001 H2SO4     4.51	60°C	2.299	2.281	0.783
1x10-5 H2SO4     5.01	60°C	2.296	2.256	0.436
1x10-6 H2SO4     6.59	60°C	2.297	2.276	0.914 
pH7 Buffer       7.00	60°C	2.243	2.243	0.000 
Tap water        6.45	60°C	2.258	2.250	0.354
0.1 NaCl          N/A	60°C	2.268	2.254	0.617
0.01 NaCl         N/A	60°C	2.299	2.256	1.870
0.001 NaCl        N/A	60°C	2.303	2.253	2.171
0.0001 NaCl       N/A	60°C	2.304	2.286	0.781
1x10-5 NaCl       N/A	60°C	2.290	2.278	0.524
1x10-6 NaCl       N/A	60°C	2.292	2.275	0.742
Marc Griffiths
- UK


 

Thank you Marc with his chart. I recently posted a question inquiring whether loss of mass occurs with rusting, and your chart clearly shows that it does, now I can proceed with my experiment. We are currently testing with different salt concentration's effect on the rusting of iron, what is the explanation that with an increase in salt concentration, increase of iron rusting occurs? And does it matter if the metal is submerged in water, will it still work if it was hung above the solution?

Joey
- Los Angeles, California


March 31, 2009

Hello,
I was wondering Joey, how did your experiment turn out? Did lower concentrations of NaCl produce higher amounts of rust, or was it the other way around. I am very interested to see what happened.

Thank you,

John Cook
- Sydney, Australia


September 24, 2009

I've done an experiment for 'does salt affect the rate of rusting' and put 4 steel nails in 4 test tubes. one in air, one in filtered water, one in water that has 1 teaspoon of salt in it and one in 1 tablespoon of salty water. I know salt makes thing rust more but when I finished the experiment the nail in filtered water rusted the most! I told my science teacher this and she just gave me a look that said this is what's supposed to happen. can you tell me what was supposed to have happened? I've done the experiment 4 times and they all had the same results.

Alanna B
student - Australia


September 23, 2009

Hi, Alanna. Your results are your results, and when you've done it four times it's not a fluke. I think your teacher's "look" may have been meant to convey that.

I don't know where Mark G. got the chart he has posted, but it shows that, depending on the temperature, tap water caused more corrosion than certain levels of salt in the water.

Salt water is more conductive than filtered water, and thus it probably accelerates corrosion when there are multiple metals in contact, because it allows ions to flow through the solution better. But when it's just steel in solution, I'm not sure that there is an easily explained scientific reason to expect salty water to rust it faster than tap water -- and, as mentioned, your results are your results and there is no reason to discount them.

Regards,

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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