Machine OEM needs help with corrosion

[mschlough]

My company builds machines for processing granite countertops. We are experts in machine design and not corrosion. I'm trying to establish a water specification so our customers don't accidentally cause excessive corrosion. I'm familiar with the Ryznar index but don't claim to be very keen with it. The machines operate in a very wet environment (like a car wash) with only water - no additives. We use very good paint. In some isolated cases the machines experience excessive corrosion (maybe 5% of the time). We build the machines out of painted steel, aluminum, and stainless. What would be an acceptable pH range to minimize corrosion of these materials? I've been told 7.8-8.2. Is that correct?

 

[CoryPad]

Can you use additives in the water to reduce corrosion? For Al, pH between 6 & 8 is a good rule. For iron, pH of 8 to 10 is a good rule. Galvanic corrosion is a potential problem for you - how do you connect the stainless steel to the other two materials?

Regards,

Cory

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

 

[unclesyd]

Doesn't the PH go up to around 10-11 when cutting Granite?

 

[strider6]

i think that the best way to protect you machine is to use a good coating and make inspetion and maintenace of the coating. it would be necessary to know exactly where the corrosion takes place. And also what you mean with 5% of the time?

S


Corrosion Prevention & Corrosion Control

 

[mschlough]

Thanks for your feedback everyone. I'll answer the questions you've asked. CoryPad--we rarely experience any galvanic corrosion. The stainless parts are bolted to the painted surfaces. I don't know much about additives but would think they'd be acceptable. Unclsyd, I don't know what the typical pH values are cutting granite, but I do know in the few cases where we experience the extreme corrosion the pH is between 6-7. Therefore I would guess that if the pH is often around 10 we wouldn't see the corrosion. Strider6, we use very high quality paint, but a small percentage of customers (5%) still have excessive corrosion--even on stainless. My objective is to provide an acceptable range and let the customer work with a local provider to solve the problem. Based on what you've said it seems as if 7.8-8.2 would work. What is an acceptable range for stainless?

 

[CoryPad]

Stainless is fine with the pH levels we are discussing. You have to keep chloride ion concentrations low to avoid stainless corrosion.

Regards,

Cory

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

 

[mschlough]

CoryPad,

You've been very helpful.

Mike