6 replies to this topic

[CCH Concepts]

hey all

im working on a spreadsheet for work.

I calculating how much of a metal will be dissolved in the presence of an acid, that's the easy part.

what im struggling to find is the formulae to calculate how long a reaction will take, given know surface area, molar of acid, type of acid and metal, temperature etc.

any suggestions?

[digger]

Sorry, no universal equation.

You may want to read the wikipedia page.

Rates of reaction are often determined experimentally.

The rate of reaction will not be constant unless you keep the conditions constant such as in a CSTR (continually stirred tank reactor). Clearly the conditions will change as a batch reaction continues. There are many things that will influence the rate which you would have to build a mathematical model for, such as rate of change of particle size, concentration, level of agitation (this will be important on many levels) etc.

Looking further at your post, I assume that you are looking at an etching process. If so simply set up a series of experiments and measure it. You may also want to do a literature survey in this field, as this will have been covered already.

D

[CCH Concepts]

Sorry, no universal equation.

You may want to read the wikipedia page.

Rates of reaction are often determined experimentally.

The rate of reaction will not be constant unless you keep the conditions constant such as in a CSTR (continually stirred tank reactor). Clearly the conditions will change as a batch reaction continues. There are many things that will influence the rate which you would have to build a mathematical model for, such as rate of change of particle size, concentration, level of agitation (this will be important on many levels) etc.

Looking further at your post, I assume that you are looking at an etching process. If so simply set up a series of experiments and measure it. You may also want to do a literature survey in this field, as this will have been covered already.

D

hello digger

actually what i'm looking at is the acid content of smoke contamination of a given metal surface. for instance a copper busbar and nitrate.

i have so far calculated the maximum average depth of the copper that would be corroded, given a measurement of the contamination level in ug/in2 and the density of the copper.

so what i would like to calculate is using the humidity, volume of air in room, temperature, start/ finish surface area and make worst case assumption of any other variables what would be the total reaction time.

[digger]

OK,

This is a very complicated subject area.

I occaisionally get involved with corrosion problems. I am not sure why or what your intended purpose is for your question. What I can assure you is that you will find it near on impossible to calculate what the result will be of such a situation. Corrosion is far from uniform, this is because of the way in corrosion often occurs.

For instance we have recently had an issue with the corrosion of a titanium heat exchanger. The product selection would indicate that the excahanger should last for ever. However we had one fail after 12 months due to corrosion. We had all sorts of metalurgical tests done, the upshot being that we had a small piece of rust that got trapped at the surface this set up an electrical potential difference. Under normal conditions this should not have been a problem, but the cooling water pH dropped for a day or two in conjunction with a higher than normal chloride content. This set up a corrosion cell, the small cavity was then covered in fat imobilising the solution locally. This set up a self sustaining corrosion reaction where Titanium chloride was being formed, this being unstable it would break down in the presence of the water to titanium dioxide and hydrochloric acid which reduced the pH further which formed more titanium chloride etc etc etc. In this situation the rate of corrosion speeded up through the cycle and the reactants were never used up (auto catalytic reaction).

In your situation something such as a finger print will affect how a busbar would be attacked. The only way to determine the rate of reaction would be under lab conditions.

It would also be wise to work in mols before converting to whatever units the final report requires.

I am certain you will not be able to work out the reaction rate without experimental work. Then you can use the standard reaction rate equations thereafter.

D

Edited by digger, 24 February 2012 - 04:37 PM.

[CCH Concepts]

OK,

This is a very complicated subject area.

I occaisionally get involved with corrosion problems. I am not sure why or what your intended purpose is for your question. What I can assure you is that you will find it near on impossible to calculate what the result will be of such a situation. Corrosion is far from uniform, this is because of the way in corrosion often occurs.

For instance we have recently had an issue with the corrosion of a titanium heat exchanger. The product selection would indicate that the excahanger should last for ever. However we had one fail after 12 months due to corrosion. We had all sorts of metalurgical tests done, the upshot being that we had a small piece of rust that got trapped at the surface this set up an electrical potential difference. Under normal conditions this should not have been a problem, but the cooling water pH dropped for a day or two in conjunction with a higher than normal chloride content. This set up a corrosion cell, the small cavity was then covered in fat imobilising the solution locally. This set up a self sustaining corrosion reaction where Titanium chloride was being formed, this being unstable it would break down in the presence of the water to titanium dioxide and hydrochloric acid which reduced the pH further which formed more titanium chloride etc etc etc. In this situation the rate of corrosion speeded up through the cycle and the reactants were never used up (auto catalytic reaction).

In your situation something such as a finger print will affect how a busbar would be attacked. The only way to determine the rate of reaction would be under lab conditions.

It would also be wise to work in mols before converting to whatever units the final report requires.

I am certain you will not be able to work out the reaction rate without experimental work. Then you can use the standard reaction rate equations thereafter.

D

what you are saying confirms alot of what i have been reading.

so even if i try to calculate a range slowest to fastest, would this still be very difficult? basically what im trying to show is if a contaminated busbar is left with X concentration of X containment what's the best/worse case scenario for time taken for the full reaction to have taken place.

if there was a formulae that i could assume what ever variable i didn't know to be from best to worse, could this be done?

[Arthur Brown]

Assuming that your bus bar has a perfectly clean surface you may be able to offer some theories, in reality there will be many surface contaminants -fingerprints, grease, natural oxide formations etc which will form their own environmants for local corrosion issues, so theory may differ dramatically from practise! Remember also that bus bars carry current so generate heat, so as the current varies the temperature will vary, with temperature variation there could be a condensation - evaporation cycle to mess with your surface condition assumptions.

[CCH Concepts]

Assuming that your bus bar has a perfectly clean surface you may be able to offer some theories, in reality there will be many surface contaminants -fingerprints, grease, natural oxide formations etc which will form their own environmants for local corrosion issues, so theory may differ dramatically from practise! Remember also that bus bars carry current so generate heat, so as the current varies the temperature will vary, with temperature variation there could be a condensation - evaporation cycle to mess with your surface condition assumptions.

we can assume the busbars aren't being used and i would assume surface contamination such as finger prints would create localised issues, for this instance im more interested in a rough average time per square inch for the reaction to take place.