Chlorides, Graphitic Corrosion, Holes at the Waterline

HVAC God

I am looking for a best approach, for prevention of holes at the waterline, on a 2.6 MBH steam boiler. The boiler, previous to my involvement, has failed three times, in the last six years. Each time it was due to holes at the waterline of the cast iron sections.

I am fairly certain the problem is an excessive amount of chlorides in the make up water, combined with high heat in the steam portion of the sections, has created a condition known as graphitic corrosion.

The system utilizes a 117-gal receiver/boiler feed pump and four remote condensate pumps. Make up water typically is limited to replacing blown-down discharge and evaporation from the venting of boiler feed and condensate pumps. The original piping and radiation system was installed in 1955, so leaks springing up are all to common. Leaks of this type, can attribute to more than normal make-up water needs. I have installed a water meter to monitor water usage.

As of the time of this post my customer is acquiring the funds to replace all 20 damaged sections. What is the best approach to prevent this condition from happening again? RO filtering? Chemical water treatment?

BobC

Where is the boiler located? I'd start by having the water tested and then contact Rhomar or someone like them to see what they suggest.

Bob

HVAC God

I have used some of Rhomar’s Chemicals before in hydronic systems, which had oxygen problems. Their advice and products seemed to have worked well.
I have been through their web site and didn’t see this problem addressed. But I’ll take your advice and give them a call.
I have attached a photo of one of the damaged sections.

Pumpguy

In my experience, graphitic corrosion to cast iron is caused by an acid condition leaching out the iron and leaving the free carbon behind. As a start, I would be checking the pH of the boiler water and adjusting it to be mildly alkaline, say between 7.5 to 8.5 or so. Obviously there's much more to boiler water chemistry, but the pH is one thing to be monitoring.

HVAC God

The makeup water source is the public water company. The water averages 7.3 pH units. I have tested it several times

Larry_52

Because the failure is localized at the water line this seems like oxygen pitting. But need more info for a real diagnosis. High makeup and many idle periods are going to attribute to the oxygen pitting.

If your return tank is allowed to cool and open to atmosphere it is going to suck up O2 from atmosphere and then escape at high heat area (boiler) liberating at water interface and killing the waterline. Also idle periods with boiler that is allowed to breathe will have high O2 and again attack mostly the water line. sodium sulphide is a basic O2 scavenger, hydrazine is even better but you aren't going to get this unless you want to be on the terrorist watch list, tannin is another used in low pressure applications.

You do need to check your water's pH at a minimum. As mentioned there needs to be a decent electrolyte (acid) to drive a de-alloy of the of the iron. All that makeup is going to attribute to carbonic acid. though if it was that bad your condensate system would be under attack too.

Larry_52

The boiler water pH is what needs to be checked, the feed by itself is not the issue. If pH is low, soda ash (sodium carbonate) will do for treatment. it might cause a little scale but that would also protect the metal from O2. Most will just use a straight sodium hydroxide solution to get pH in order. Dissolved oxygen and acid attack are not the same but two symptoms of high makeup. the chloride/acid idea would only be of issue if you where higher pressure/temp with the break down of magnesium chlroide into Muriatic acid and causing hydrogen embrittlement. High chlorides at low pressure just give you an unstable waterline while steaming. You may indirectly associate boiler conductivity to chloride concentration but that is not an exact measurement by any means.

HVAC God

@Larry, Great information!

I will be checking the "boiler system water's pH", from now on and look into treating the system with a straight sodium hydroxide solution.

I am also going to make a concentrated effort to have my customer be diligent in their maintenance, especially when it comes to how much makeup water is being used.

Larry_52

Tip on the NaOH, do not add direct. A tiny amount goes a long way. Dilute with water and add it to the water and not the other way around. Mixing caustic with water is exothermic and best handled with minute amounts added to water then solution added to the boiler. Aim for pH between 9 and 10.