Corrosion rate is an important key performance indicator in any industrial water system and must be followed closely. Once corrosion is a natural process for any metal, plant personnel is entitled to constantly measure and provide corrective actions if necessary once the detected rate is above recommended limits.

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According to AWT - Association of Water Technologies, some guidelines are recommended as maximum limits for corrosion rates for the most common metallurgies (carbon steel and copper) used in industrial water systems. Figure 1 is describes the limits for open recirculating (cooling towers) systems. Figure 2 describes the same limits recommended for closed loop cooling systems.

Figure 1 - Corrosion rate allowances for open recirculating systems (source: AWT).

Figure 2 - Corrosion rate allowances for closed recirculating systems (source: AWT).

There are at least two different usual tools to measure corrosion rates: corrosion coupons and corrosion probes.

Details on corrosion coupons installation can be studied on the following link: https://www.linkedin.com/pulse/tips-when-using-corrosion-coupons-cooling-water-anderson-jos%C3%A9-beber/

Corrosion coupons characteristics:

• Inexpensive
• Different metallurgies can be tested
• Visual inspection can offer extra evaluations (e.g. biofilm, suspended solids, etc)
• Demands long time exposure (minimum 30 days)
• Measures accumulated/average corrosion rate during the exposure period

Corrosion probes such as Linear Polarization Resistance (LPR) are specific probes/analyzers capable to measure and monitor instantaneous corrosion rate for one or more different metallurgies (one probe for metal). The concept is similar to other probes such as pH, conductivity, ORP, etc, where online 24/7 measurement (and perhaps control) is achieved. Corrosion probes characteristics:

• Instantaneous corrosion rate
• Early upset detection (contamination, lack of corrosion inhibitor product, etc)
• Different metallurgies can be tested
• Possibility of automatic control
• Possibility of localized corrosion detection (if probe can measure imbalance)
• Higher associated cost

Sure the investment on corrosion rack and coupons is quite low compared to an automatic corrosion probe measurement system. However, one great advantage already described is the possibility of getting instantaneous measurement. Figure 3 is a great example. On this process, one can observe that the corrosion rate (carbon steel) at large size (> m3/h recirculation rate) cooling tower was very well controlled - under 0.5 mpy. Very suddenly, a spike is observed, which generated and alarm for the management team. The problem was quickly identified and once corrected, the corrosion rate started to go back to its common result.

Figure 3 - Online corrosion rate measurement (carbon steel) of a cooling water system, performed by a LPR corrosion probe.

Such feature is not possible with a corrosion coupon, given the fact that one has wait at least 30 days to remove the coupon and measure the average rate. Also, if the upset occurs for a short period of time during, perhaps the average corrosion rate does not change dramatically. Then it becomes very hard or even impossible to detect the upset.

As a standard recommendation, it is wise to have both measurement techniques in your system. The corrosion probe will allow rapid detection and counter measures. The corrosion coupons will allow post exposure period visual identification of the metal surface.

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Corrosion probe and corrosion coupon

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Corrosion probe and corrosion coupon

Corrosion probe and corrosion coupon

parthanml

(Mechanical)

(OP)

6 Aug 12 00:53

What is the difference between corrosion coupon and corrosion probe ?

Replies continue below

Recommended for you

RE: Corrosion probe and corrosion coupon

SJones

(Petroleum)

6 Aug 12 03:46 The coupon is a retrievable piece of metal and the probe is an electronic instrument.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

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RE: Corrosion probe and corrosion coupon

BigInch

(Petroleum)

6 Aug 12 04:00

Don't put either one in a piggable line.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

RE: Corrosion probe and corrosion coupon

zdas04

(Mechanical)

6 Aug 12 07:13

I'd go even farther and say that both were developed for lines that are liquid full, and you shouldn't put either one in a primarily gas line, because they are never where the corrosion happens.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

RE: Corrosion probe and corrosion coupon

BigInch

(Petroleum)

6 Aug 12 07:18

Even in liquid lines, corrosion seldomly happens right down the center of the pipeline

If it ain't broke, don't fix it. If it's not safe ... make it that way.

RE: Corrosion probe and corrosion coupon

zdas04

(Mechanical)

6 Aug 12 09:15

Same with corrosion chemicals in gas lines--they accumulate at the first sag because the gas can't move them farther and the chemicals never get to where the corrosion cells are, but we keep pumping that crap into gas lines to the tune of a few billion dollars/year. The attitude that "doing something probably ineffective is better than doing nothing" is pretty pervasive. I've cut open lines at sags that were pure (toxic or poisonous) chemicals and I have to disagree with the idea that injecting chemicals in gas lines is better than doing nothing. It is actually far worse than worthless.

Yeah, they go from not-good in liquid lines to horrible in gas lines. I have never gotten useful information from either probes or coupons. I guess people run them to make them feel like they are doing SOMETHING about corrosion.Same with corrosion chemicals in gas lines--they accumulate at the first sag because the gas can't move them farther and the chemicals never get to where the corrosion cells are, but we keep pumping that crap into gas lines to the tune of a few billion dollars/year. The attitude that "doing something probably ineffective is better than doing nothing" is pretty pervasive. I've cut open lines at sags that were pure (toxic or poisonous) chemicals and I have to disagree with the idea that injecting chemicals in gas lines is better than doing nothing. It is actually far worse than worthless.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

RE: Corrosion probe and corrosion coupon

SJones

(Petroleum)

6 Aug 12 09:41 Gents, Gents - spoken like true pipeline engineers. There are many variants of the two corrosion monitoring types, including flush mounted with the pipe wall to both 'get where the corrosion action is' and keep out of the way of pigs. Selection of specific types is based on what you are trying to monitor and what you are trying to achieve with the results. Gas line inhibition should also be reliant on maintenance pigging as part of the corrosion control strategy - that would shift the stuff down the line. My money is on your experience being based on pipelines where the instrument engineer or the mechanical engineer has been tasked with corrosion monitoring system design and execution: generally a sure bet for failure!

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

RE: Corrosion probe and corrosion coupon

zdas04

(Mechanical)

6 Aug 12 10:37

I've kept track of the corrosion failures I've been asked to investigate over the years and over 80% of the failures have been on "protected" lines (some combination of cathodic and/or chemical injection) that were not pigged. "Protected" lines make up about 30% of the lines I see in my practice, but they make up the lion's share of the failures. I have never seen a single failure on a line that was pigged regularly (and adding corrosion chemicals to the pig run just costs money without providing any real benefit since pigged lines don't rot).

As an aside; at a conference a decade ago I had occasion to ask the Chief Metallurgist of a major Oil & Gas chemical supplier to join me for a drink. After far too many drinks I asked him what was the transport mechanism for biocides in a low pressure gas line. He was just drunk enough to say "quarterly profit and loss statements". In vino veritas.

The first class I had on corrosion was . I've had several more since then (been a NACE member for 20 years). As a consultant I see a lot more issues where the corrosion "mitigation" was controlled by the Integrity Management department than when it is in the ham-handed control of pipeline engineers.I've kept track of the corrosion failures I've been asked to investigate over the years and over 80% of the failures have been on "protected" lines (some combination of cathodic and/or chemical injection) that were not pigged. "Protected" lines make up about 30% of the lines I see in my practice, but they make up the lion's share of the failures. I have never seen a single failure on a line that was pigged regularly (and adding corrosion chemicals to the pig run just costs money without providing any real benefit since pigged lines don't rot).As an aside; at a conference a decade ago I had occasion to ask the Chief Metallurgist of a major Oil & Gas chemical supplier to join me for a drink. After far too many drinks I asked him what was the transport mechanism for biocides in a low pressure gas line. He was just drunk enough to say "quarterly profit and loss statements". In vino veritas.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

RE: Corrosion probe and corrosion coupon

BigInch

(Petroleum)

6 Aug 12 10:50

Yes as you allude, the biggest problem is keeping up the gas flow to get good inhibitor transport in lower pressure lines out at the edge of the field where the water starts creeping in.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

RE: Corrosion probe and corrosion coupon

waterpipe

(Mechanical)

6 Aug 12 11:43

In few cross country water pipelines (not piggable), we've had "pipe monitoring arrangements". A short section parallel to the main pipe like a by pass at where you want to have your assessment. The main pipe can be inspected when needed (perhaps once every few years) to assess the corrosion and the lining of the pipeline by bypassing the flow through the by pass arrangement. I think at least you're inspecting the "real" pipe.

RE: Corrosion probe and corrosion coupon

brimmer

(Petroleum)

7 Aug 12 10:54

I have resorted to culverts with UT monitoring on some of the main transmission lines I manage, an expensive way to get corrosion rates, but much better than coupons or probes.

I don't know how the question became a discussion on chemical inhibition, but David I have certainly seen my share of corrosion failures on lines that were pigged regularly (twice per month) with no chemical inhibition. I can also say if I stopped applying chemical to most of the sour multiphase pipelines I manage I would see a spike in the number of failures, as this was the case before applying the inhibitor with everything else equal. Like anything else though, you have to know the details of what you are doing rather than just put chemicals in the line and hope for the best.

RE: Corrosion probe and corrosion coupon

zdas04

(Mechanical)

7 Aug 12 15:55
I would love to know the details on that. I've consulted on some pretty sour wet gas lines that were regularly pigged and when we've cut them open for tie-ins, the pipe looked fine every time. Some of the non-piggable laterals were a very different story, but in the absence of standing water there just isn't any place for the gas to facilitate the creation of a corrosive cell (either acid attack or MIC). It seems like twice monthly pigging would be adequate to disturb the liquid pools before they become a problem (I often have to pig more often than that for pressure-drop reasons, but not for corrosion).

I'm not saying you haven't seen what you've seen, it is just so counter to my experience that I would really like to know more.

Bimmer,I would love to know the details on that. I've consulted on some pretty sour wet gas lines that were regularly pigged and when we've cut them open for tie-ins, the pipe looked fine every time. Some of the non-piggable laterals were a very different story, but in the absence of standing water there just isn't any place for the gas to facilitate the creation of a corrosive cell (either acid attack or MIC). It seems like twice monthly pigging would be adequate to disturb the liquid pools before they become a problem (I often have to pig more often than that for pressure-drop reasons, but not for corrosion).I'm not saying you haven't seen what you've seen, it is just so counter to my experience that I would really like to know more.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

RE: Corrosion probe and corrosion coupon

SJones

(Petroleum)

8 Aug 12 04:26 There's a number of factors involved in H2S corrosion. Some are lucky and get a good protective sulphide film; other's aren't so lucky, particularly with high chloride water, and get corrosion failures.

The Canadians have the best experience with sour gas corrosion control

http://www.capp.ca/getdoc.aspx?DocId=&DT...

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

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