Feature

Inspection Trends / May 201520

By Robert Ward

Natural objects, such as granite,and man-made structures, such asbridges, cars, ships, refineries, andaircraft, are all subject to the sameenvironmental stresses. Light,temperature changes, water, and gasesin the air all play a role in thebreakdown of materials. The one majordifference is just how quickly thatbreakdown occurs.

A general term for the degradationof man-made structures is corrosion.Scientists try to understand themechanisms by which corrosionoccurs, design barriers to corrosion,find ways to monitor the progress ofcorrosion, and build processes for assetmaintenance and systems to reduce theoverall costs of corrosion to society.

The Cost of Corrosion

The economic impact of corrosionhas been researched at length. A well-known study published in 1999 byNACE (the National Association ofCorrosion Engineers) titled The UnitedStates Cost of Corrosion Studyindicates that the direct cost ofcorrosion is more than 3% of the GrossDomestic Product (GDP). Similarstudies report direct costs ranging from2 to 4.5% of the GDP. The real issue iswhere direct costs end and indirectcosts begin. If a steam pipe under astreet corrodes to an extent that greatamounts of water are lost, the directcosts would be the replacement cost ofthe pipe, including material and workerhours and possibly the cost to repairthe sinkhole made in the road above.But the cost of lost productivity byclosing the street and the lack of steambeing provided by that pipe are notcounted as direct costs. The NACE

Corrosion Monitoring, Detection, andMeasurement

Equipment manufacturers are helping organizations control costs

Fig. 1 — Diagram of a digital radiography system.

Fig. 2 — A typical application space for corrosion under insulation.

Inspection Trends / Spring 2015 21

publication suggests that for sometypes of structures (a bridge, forinstance), actual costs are greater thanfive times the direct costs.

Given these enormous costs, it isnot surprising that there are largeindustries centered on

1. Corrosion prevention (such asadditives in water systems, coatingmaterials like paint for automobiles, etc.)

2. Corrosion repair andmaintenance

3. Corrosion monitoring,detection, and measurement.

This article focuses on corrosionmonitoring, detection, andmeasurement as it pertains to pipes andvessels fabricated from metals, withand without protective coatings.

Corrosion Monitoring

Think of the industrialinfrastructure of a plant, mill, refineryor manufacturing facility as a humanbody; the metal pipes and vessels arethe circulatory system and organs.Pipes transport water, processedchemicals, and raw materials such ascrude oil and waste products. Vessels,like organs, accept the materialsbrought to them, and managetemperature and pressure to circulatematerials (product and wastes) backinto the system for distribution.

Engineers choose pipe and vesselmaterials that can withstand as much aspossible the elements that causecorrosion in the intended temperature,pressure, and product environment.While these pipes and vessels are still

subject to corrosion, the degrees andtypes can vary. Types of corrosiontypically fall into three maincategories:

1. Predictable. If a given materialis passing through a given pipe orvessel, at a given set of temperaturesand pressures, general corrosion isexpected and predictable. Materials inthis category are most often under acomprehensive maintenance andinspection program. This case isgenerally well known and statisticallysignificant inspection surveys willoften suffice rather than full assetinspections.

In some cases, engineers shouldexpect specific attack mechanisms suchas microbial-induced corrosion or otherpit-developing processes to take place.These conditions call for inspectiontechniques that can cover large areasquickly with precision measurements,such as phased array ultrasound. Spotchecking will not reliably find pits, anddigital radiography may not be asuitable option due to time, safety, anddefect size considerations.

2. Unpredictable, but expected.Certain conditions may give rise tolocalized corrosion environments, suchas the following:

• Corrosion under insulation(CUI). In the process of transformingraw materials to products, temperatureis often a major control requirementmeaning that pipes or vessels areinsulated. In many cases, theseprocesses occur outside in theenvironment, so insulation can bedamaged or degraded, allowing aningress of water or product into thespace between metal wall andinsulation. It is not always easy torecognize the areas of concern, and it iscostly to remove insulation, inspect for

damage, and then reapply insulation.During the time insulation is not on thepipe, the system must be shut down.

Flow accelerated corrosion (FAC)has known locations of likelihood (apipe elbow after a valve), but there is alow probability of knowing whichelbows have had FAC, so again a quickreliable tool for elbows is needed.

3. Unpredictable. An example ofunpredictable corrosion would be inthe production of crude oil. If a wellpicks up significant sand or otherabrasive material, it can cause veryfast-acting corrosion. The only reliablemeans of determining if this ishappening is to fully monitor the pipeor vessel with installed sensorsattached to the metal surface and underthe insulation, if any.

Corrosion Detection

Permanently installed sensors arethe early warning system ofunpredictable corrosion. In this case, arigorous process for measuring theproperties of the material entering theasset cannot be managed well enoughto detect fast-acting corrosive agents(sand, sulfur, acid, or base conditionswidely different than expected).

Corrosion under insulation andFAC are conditions under which theasset owner can expect the possibilityof corrosion, but cannot easilydetermine the most likely area where itmay have occurred. Digitalradiography (DR) is one of the bettertools to use in these cases, as it is fastand does not require the insulation beremoved and replaced. Digitalradiography also makes it easier tomanage large volumes of inspectiondata using DICONDE-compliant

Fig. 3 — Using digital radiographyfor flow accelerated corrosion canyield fast, accurate results.

Fig. 4 — With traditional thickness gauges such as the one on the left, finding a pitis as difficult as finding a needle in a haystack. Today’s phased array ultrasonicinstruments make finding the needle possible.

Inspection Trends / May 201522

digital reporting tools, such as GE’sRhythm Enterprise Archive. Figure 1shows a diagram of a system and Figs.2 and 3 show typical applications.

Digital radiography has been fieldproven to significantly reduceinspection times by more than 95%. Itshortens radiation exposure time,eliminates film chemical processing,and minimizes the safety-affected workarea. Digital radiography also reducesoverall image noise levels, therebyyielding improved image quality. Thesecombined factors improve the detectivequantum efficiency (DQE) metric, awidely accepted metric for full-fielddigital detectors.

Corrosion Monitoring andPit Detection Using PhasedArray Ultrasound

Regularly scheduled inspectionscan validate corrosion rates and allowengineers and operators to better planfor maintenance situations. Whileultrasound thickness (UT) readings canbe of occasional use with regularlyscheduled inspections, they do notprovide enough precision with thecollection of manual thickness readingsto adequately determine wall thicknesslosses from corrosion. Pitting cannot bereliably detected by conventional UTmethods simply because the size of thedefect is small compared to the areainspected.

Phased array ultrasound (PAUT)techniques can be developed toapproach the needed precision and getgreat coverage quickly — Fig. 4.

Historically, the issues with the useof PAUT for corrosion evaluation andpitting detection have been

1. Availability of trainedtechnicians

2. Uniformity of testing (i.e.,consistency between testing)

3. Equipment costs.Availability of trained technicians

is the most pressing issue. It has beenmuch more difficult to find and traintechnicians on phased array systemsthan other inspection tools. However,this also is changing as more traininginstitutes and colleges areincorporating phased array ultrasoundtechnology courses, and the greatbenefits of phased array have resultedin more companies investing intraining. Equipment manufacturers,such as GE Inspection Technologies,

are allowing the experts in a companyto easily customize the user interface ofa phased array device so that a lessexperienced technician can much moreeasily learn to set up the devicecorrectly and gather high value data.

These custom interfaces can beordered into a seamless workflow asthe less experienced technician workshis or her way through setting up thedevice, calibrating it, developing gaincompensation curves, and getting readyto collect data. These serialized custominterfaces allow the input of photos,documents, and other aids to help thetechnician verify the device is set upcorrectly. If a technician does notunderstand a step or a data signal, he orshe can push a button on the device andthe screen is immediately shared on thePC, phone, or tablet of the boss orcustomer — as long as both are in awireless area. These new developmentsmitigate the hurdles in takingadvantage of the precision andproductivity gains allowed by usingPAUT.

Conclusion

The direct and indirect costs ofcorrosion can be staggering. Withimproved inspection technologies, suchas digital radiography and phased arrayultrasound, and maintenance schedules,equipment manufacturers andproviders are helping organizationscontrol costs and get a better handle onthe health of their assets.

ROBERT WARDis a senior project manager for

Ultrasonic Testing (UT) andElectromagnetics (EM) Innovation at

GE Measurement & Control, adivision of GE Oil & Gas, Lewistown,

Pa., www.gemeasurement.com.

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