Does Corrosion Inscreases Exponentially? 2

[Logan82]

Hi!

Would you say that corrosion increases exponentially in a steel element where corrosion has started, given the following parameters:
- Environmental conditions are constant (humidity, temperature, wind speed, exposure to wind etc.).
- The structure is not repaired, not repainted, not modified.

 

[dhengr]

Logan82:
Yes, exactly what exponential I’m not sure. The one square inch surface area quickly becomes 1.5 and 2sq.in. as the corrosion, pitting, small intergranular cracking, flaking of layers of rust which will hold moisture btwn. the layers, etc. progresses. There are a few steels and materials which tend to form a protective layer which inhibits further corrosion when they are exposed to the environmental conditions

 

[Greenalleycat]

Going to take the opposite stance to the guy above me and say no
Disclaimer: I am certainly not an expert on this

The basis for this being that our code provides two decay rates: decay over the first 10 years (average mm/yr loss) and decay over the first 30 years (average mm/yr loss)
The rates are lower for 30yrs than 10yrs, implying that corrosion from 0-10yrs occurs more quickly than from 10-30yrs
I assume the logic for this is that the initial corrosion passivates the exposed steel and forms a protective layer that stops active corrosion occurring as easily

There is a disclaimer in the code that the equation does NOT account for localised pitting corrosion, which is what dhengr has talked about, so perhaps if pitting is a legitimate risk (I don't know how you assess this?) it is exponential..?

 

[kingnero]

@ OP, are you asking about exponential increase in corroded surface, or exponential increase in loss of material?
For the surface, galvanized surfaces start to increase decaying when the sacrificial working of the zinc decreases (e.g. HDG layer is worn away), so yes, in some cases you will see faster and faster expansion of corroded surface.
For loss of material, I'd concur and say that this will be fairly constant over the long run (years).

 

[Logan82]

Thank you all for your answers.

@ OP, are you asking about exponential increase in corroded surface, or exponential increase in loss of material?

I am asking about exponential increase in loss of material over time. For example, does the depth of corrosion in a beam flange increase exponentially?

 

[CrabbyT]

Corrosion rates are generally regarded as constant and are defined in terms of "mils per year".

Scroll down to page 9 in the doc at this link for an example of how to calculate a corrosion rate.

https://www.usna.edu/NAOE/_files/documents/Courses/EN380/Course_Notes/Corrosion Basics.pdf

 

[MotorCity]

Just curious, how do you plan to use the answer to your question? A project you are working on?