×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Steel Corrosion due to Atmospheric Moisture
2

Steel Corrosion due to Atmospheric Moisture

Steel Corrosion due to Atmospheric Moisture

(OP)
I'm an HVAC professional, trying to get some good information regarding room design to avoid steel corrosion. Over the years, I've heard people say that maintaining a room's relative humidity below 55% or 60% will minimize the corrosive effects of water vapor in the atmosphere. What I'd like to know is whether it's really relative humidity, or absolute humidity that's the key. I would suspect absolute humidity, but I've never seen any guidelines that refer to limits on either absolute humidity or dewpoint.

If the critical parameter is relative humidity, then raising the room temperature would help. But this obviously would not work if absolute humidity is the key.

Any info would be appreciated.

---KenRad

RE: Steel Corrosion due to Atmospheric Moisture

I can't point you to a reference, but absolute humidity is the real factor.  Corrosion occurs like any other chemical reaction and is based on concentrations, so the higher the concentration of water in contact with the steel, the greater the corrosion extent.

The reason relative humidity is mentioned is because indoor temperatures are maintained to a relatively small window.  

You will need less than 50 % RH at 22 °C to avoid corrosion of unprotected steel (again, no reference, just my anecdotal evidence).

Regards,

Cory

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Steel Corrosion due to Atmospheric Moisture

(OP)
CoryPad,

Thanks; you confirmed what I had suspected, that reducing relative humidity without removing absolute moisture would not reduce the potential for corrosion. I never thought about why relative humidity was always referred to, but your reason makes sense.

KenRad  

RE: Steel Corrosion due to Atmospheric Moisture

Reducing relative humidity DOES work if the air might at some point have a dew point higher than the temperature of the steel itself...condensation of liquid water is far worse than atmospheric vapour phase moisture!

RE: Steel Corrosion due to Atmospheric Moisture

Usually the value used to asses the corrosivity of atmosphere is the RH. I've never seen absolute humidity. The limit assumed for control of corrosion is RH =60 %.
For reference see this docs from NPL:
Atmospheric Corrosion
"Control of RH Most atmospheric corrosion can be prevented by maintaining RH below 60%."
http://www.npl.co.uk/upload/pdf/atmospheric_corrosion.pdf

and in the Book "Corrosion" - Shreir - Chap "The atmosphere".

S.

Corrosion Protection & Corrosion Control
 

RE: Steel Corrosion due to Atmospheric Moisture

(OP)
bimr,

45% seems excessively low to me, and in a southern US climate, would come with a large HVAC energy penalty. Since the number came from a company that sells dehumidification equipment, I'm skeptical.

---KenRad

RE: Steel Corrosion due to Atmospheric Moisture

One of the other issues will be how the steel is stored in the room - if the steel parts are not touching you can get better results but if the parts are touching then you need to worry about a galvanic cell as well.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources