The comparison of Magnetic flux leakage and ultrasonic when detect pit corrosion of Ferromagnetic plate and tube stock

1 、summary 

Magnetic flux leakage testing (MFL) and ultrasonic flaw detection (ut) have been widely applied to ferromagnetic plate and tube stock pit corrosion detection。Users and detecting personnel have different understanding and expectation on sensitivity and accuracy。The article discusses fundamental principle of two methods and their effects on possibility and accuracy of flaw detecting 。

2. Pit corrosion 

There are many mechanism and types of corrosion,  in here，we specialized in discussing Corrosion that between the bottom of tank and waterproof layer or internal medium corrosion of water tank。In the 1960s,the ultrasonic flaw detection used for tube system is considerable success。It gives people a illusion that can detect pit corrosion accurately。For helping understand this difference，now illustrate erosion some classic pit corrosion shapes。

Erosion 、classic “lake type ”and ”cone type” corrosion pit。This forming steps of recording corrosion is meaningful。Normally the “lake type ”and ”cylindricity type” corrosion are mostly found at the bottom of tank,the reason why the formed is due to the moisture comes into between bottom board and waterproof layer，or the saved products have moisture content，the cylindricity type pit is not commonly seen ,usually it is the water in medium and SRB produce together。

3. Method and principle

The principle of MFL and UT have been detailed in some other place，due to the purpose of this article,now it introduces simply。

The fundamental principle of MFL,the magnet that put onto holder will produces strong induced magnetic field on the panels or tube wall。If the panel material or tube wall exists corrosion flaw,and form leakage magnetic field at relative surface，set a row of probe between magnetic pole  to detect the leakage magnetic field。The probe usually adopts hall element or coils,and every kind of probe has its own advantage and limitation。

The simple UT device of Pulse echo principle,it uses double crystal probe ,in this structure，a crystal is transmitter another crystal is receiver。Transmitter is dependent of receiver，so as to A scanning discovered flaw transmit signal can be displayed freely。When detecting panel material or rod materials thinned area,transmitted pulse result should not make the first intine reflected wave ambiguous 。So we will understand the simple digital thickness tester without A scan is not suitable for discovery or measurement of any corrosion pit 

4、the possibility of MFL can detect flaw

MFL method uses a row of probe ,the detecting range is overlapping of neighboring probe  。Possibility of Any leakage magnetic signal being detected is dependent on leakage magnetic field relative to amplitude of noise signal。In other word, signal noise ratio is the key factor to decide flaw detection。The parameter that affects signal noise ratio is connected with design and operation of detecting device 。Some of them is connected to geometrical shape of corrosion pit。

Device parameter     base board parameter  

Magnet  design       base board material

Probe kinds and arrangement  scanning plane condition 

Detecting speed control   the coverage layer of scanning plane

Vibration damping       cleaning degree    

signal handling        depth of corrosion pit 

flaw hint             corrosion pit volume

Shape of corrosion pit

5.1 . Device

5.1.1   magnet design

Magnet must has enough magnetic field strength and then make magnetic flux density of detected material close to saturation。When magnetic pole and testing plane don’t have much changes,the designed supporter must make magnet system move along with fluctuant scanning plane。No doubt,one advantage of using magnet is magnetic strength can compensate by adjustment when it under different thickness material or uplift change。 Another practical advantage is enable to close magnetic field on the testing surface，helping to move scan head equipment，its applicative biggest wall thickness is 12.5mm;if reduce sensitivity usage,its applicative biggest wall thickness is 20mm,if a system that  suitable 、safe and be convenient to place and move supporter on the testing plane can be designed ,its applicative wall thickness can be bigger。

5.1.2   probe types and arrangement

Commonly used probes have coil and hall effect element ,under any circumstance, the distance of neighboring two probe should be smaller，so as to make sure that there is no interval in the range of detecting ，if in order to eliminate noise signal and use differential coil probe ，then the practical condition should be considered：the leakage magnetic field that cross the probe can be expanded to 3--4 times of corrosion pit diameter，and just exist at the nearby of corrosion pit diameter of scanning direction 。 

In certain leakage magnetic field ,coil probe generates electric potential signals  has certain function relation with the speed ratio of magnetic field lines tangent direction。Coil and scanning instrument proceed speed appear digital change function relation。Therefore,it should consider coil types and speed sensitivity of probe  when designing device。Coil is more sensitive than hall effect on uplift change。A unique advantage of Coil probe is scanning instrument generated strong eddy current under conditions of speeding up and reducing speed has lower effects than hall element probe。

In principle,hall effect element probe has lower sensitivity on speed change,if using filtering to handle signal ,used for eliminating fake signal of low frequency and high frequency ,then it needs some restricted conditions of speed change  that set by waveband of up and down amplitude limiter。When use these equipment to find leakage magnetic field vertical direction ,they are not sensitive to uplift change，but very sensitive to eddy current signal,however,a advantage of this equipment is this equipment is has a very big adjustable space between detector and testing plane，so as to reduce detector abrasion ,detector is also can eliminate some surface spots,such as weld spatter。

5.1.3    speed control

Every kind of probe has control on speed at a certain level is necessary，but using coil probe ,control degree is lower。

5.1.4    vibration damping 

A source of Background noise and fake signal is attributed to surface roughness of scanning plane。It is very common at the bottom of tank and ground tube that without covering layer 。The corrosion that happened on the surface leads to magnet and probe on the scanning supporter vibratory and then generates noise，it can be eliminated by three methods:using wheel with suitable width,using joint shock absorber to handle signal according to vibration frequency higher than the frequency of flaw signal 。

5.1.5  signal handling   

due to signal that leakage magnetic field get is low,so the signal needs to be magnified。 They also need to be treated differentially with unwanted noise。By filtering waveband to eliminate low frequency and high frequency noise，all residual noise can be calculated by flaw detecting threshold value,or under detecting dynamic display condition ,assessing the whole noise level by operator。

5.1.6  flaw hint

At present ,there are three methods that flaw can cause operators’ attention：

1、 auto stop ,meet corrosion pit, and signal displayed probe discover this corrosion pit ,scanning instrument will automatically stop,before operator cancel this display, the scanning instrument will not rescan, operator make marks on the corrosion pit,so as to do measurement on the corrosion pit depth subsequently。

2、 Dynamic display ,operator observes dynamic displayed signal,the signal noise level predicts whether corrosion pit is exit or not。Operator can be reminded by the sound of limit value or graph warning。Operator marks on the corrosion pit  so as to do measurement on flaw depth 。

Computer data acquisition  ,for later period analysis and report ,some of systems use computer ,this can include software that allow to use colour to show material reduction to draw simple picture of tank base board。Operator can save data at the each ending of scanning,this is for showing base board with flaw so as to check the results of repeatability subsequently 

5.2 base board 

5.2.1    material

Apparently ferromagnetic is necessary to MFT,but the penetration of  ferromagnetic material will affect detecting result。The sample and sample management of equipment equipped should be produced by the same level steel as be detected device。The material of base board of tank normally is not a question。Because the tank adopts strong in low carbon steel when it begin to be built，where needs more attention is when choose sample tube we should make sure the right steel level has been chosen。For a special magnetic field condition ,the thickness of material will affect magnetic field and up to saturation。

Then affect signal amplitude of special corrosion pit in this magnetic field。

5.2.2  scanning surface condition

The scanning surface should be clean with no sundries,,surface roughness can lead to noise,when scanning ,it needs to set relative high threshold value ,when scanning on the thinner plastic covering layer can also reduce sensitivity ,other irregular parts ,such as being polished weld spatter or repair welding parts will have big fake index signal,these signal needs to be saved ,because leakage magnetic detection method can distinguish whether it is scanning surface corrosion pit display or detailed parts display，but the flaw of relative material thickness 50% depth or deeper flaw,leakage magnetic detection method has higher sensitivity on these specific surface corrosion pit。

5.2.3  the covering layer of scanning surface

A main advantage of MFT is enable to keep reasonable sensitivity on considerably thick surface covering layer，on the base board with 6.32mm,under the condition of fiberglass covering layer reaches to 6mm,MFL can detect and detect on 20% wall thickness reduction parts。

5.2.4  degree of cleaning

Relative to UT,floor surface condition has less effects on MFL, but thicker rip scaleplate can produces fake signal ,corrosive objects will gather to magnetic pole can cross cracked fake signal that generated by probe。Clear surface sundries and wash by water is enough。

5.2.5  corrosion pit depth

When there are some distances between above condition surface,corrosion pit depth is a main factor that affect leakage magnetic signal amplitude，the volume and shape of corrosion pit can also affect the amplitude of signal，this will be discussed later of this article,but under given condition,the amplitude of leakage magnetic field signal can be used for assessing percentage of wall thickness loss and reduce the amount of repetition。 

5.2.6  the volume of corrosion pit   

In other place has discussed that the volume of corrosion pit is main factor that affect signal amplitude，this is the reason that it can not qualify the result of MFL detected flaw 。Due to these discussing point is boring ,so we decide to based on technically mode and some experienced trial to deeply research the volume and depth of corrosion pit has what effects on amplitude。Producing  series of corrosion modes with different depth and volume。Under the conditions of 6.35mm  40%  50%  60% wall thickness,the volume of corrosion pit and change of magnetic induction strength relation curve。It expresses corrosion pit volume has effect on signal amplitude ,so we advice that for typical “cone type ””lake type”corrosion pit of tank,single use MFL can reasonably accurately detect serious composite corrosion,however, cylindricality type corrosion pit ,such as SRB corrosion, maybe will not get accurate result,the volume of cylindrically type corrosion pit corresponding curve will be gathered together

5.2.7  the shapes  of corrosion pit

When producing sample board,people commonly choose machine to process simple shape analog flaw,such as flat bottom hole,or simple tapered slot,the shape of corrosion pit has apparent effects on leakage magnetic field,from profile to see,due to corrosion pit usually as some mode and develop by trapezoid.for this purpose,we use manual simulation trapezoid flaw shape,above experience showed that experienced results have been used to calibrate MFL system.

5.2.8   human factors

The same as other nondestructive flaw detection MFL method,have to take people’s assessing ability into consideration , for tank without good external environment all the more so.inside tank is black、dirty and foreign taste，along with change of tank position and season,its internal temperature sometimes very hot  but sometimes very cold ,so according to operators’ requirements,fundamental thought of producing machine is as light as possible，but operators should also as much as possible maintain equipment well and accurately complete calibrating job。 

5.3  the summary of MFL can detect pit corrosion

Under certain condition,MFL method has very high chance to detect flaw， trained and responsible operators use equipment that well maintained 、clean，non pit surface to detect，wall thickness up to 10mm,reduction 20% can be accurately detected。When detect on the dirty surface ,wall thickness is 13mm,reduction 40% can be detected，in the above conditions,MFL can scan at the speed of 0.5m/s,scanning width is from 150mm to 450mm,compared with UT,surface condition has little effect on MFL, most of leakage magnetic detecting system seldom require operator follow the operation step by step 。

6、 the possibility of UT can detect pit corrosion

UT depends on many factors to detect pit corrosion  cause this method is slower than MFL,until now ,the screen with reseau point by point detecting method is widely used to detect tube elbow，apparently,the possibility can be neglected by using this technique to detect single pocking point。Now prefer to choose two-dimensional scan technique, it can manually directly touching scan,or flush probe automatic scan，classic pit corrosion provided suitable ultrasonic wave has little reflecting surface，operators must enable to understand the meaning of signal parameter，avoid erroneous judgement。That is why the simple digital thickness instrument is not suitable for corrosion detection，prefer to choose instrument that equipped with A-scanning function this instrument is superior to B scanning and C scanning instrument，like MFL,ultrasonic flaw detection,the factors that affect its pit corrosion detecting possibility includes relevant instrument and technique、relevant base board and possible existing corrosion pit。Equipment parameter    base board parameter

Flaw detection instrument   base board thickness

Kinds of probe    scanning surface condition

Coupling methods and couplant types    base board covering layer

Scanning technique   flaw characteristics

Calibration 

Train and experience

6.1 instrument 

6.1.1 flaw detector

As the lowest requirement,it should have A scanning display,but if use data saving technique for C scanner  and B scanner,this will greatly improve possibility of detecting flaw ，especially verify continuous coupling  when the equipment is detecting。   

6.1.2  probe types

Under many circumstances,the thickness of being detected material should not exceed 10mm,scanning surface is not very smooth。This means the first pulse of  single crystal probe will take up very important part of normal wall thickness signal ，therefore under this circumstance,this kind of probe is not suitable for this condition 。But double crystal probe can triumph over this problem，but have to remember that when doing probe design,need to consider that receiver can accept biggest applicable distance of emitted energy.Clearly showed that out of this distance ,it will achieve amplitude shrunken reflecting signal ,even though when flaw reflecting surface is flat，and be parallel to scanning surface ,it will be like this too。Operators should especially realize that corrosion pit is very ideal reflector，when inwall  reflecting wave is missing，it should prepare adjust gain，when meeting rough surface,it will quickly polish organic glass interface on the probe and change angle of incidence。So arrange a wear resisting coil is necessary。Crystal dimension should be in range of 10mm to 15mm。

6.1.3  coupling methods and types

Currently ,ultrasonic wave and material coupling  have two methods,for manual scanning,using directly contact coupling method;for automatic and semi-automatic ,prefer to choose flush coupling ,at any case,the fundamental requirements is enable to wet the detecting surface,when do manual coupling,it needs to use applicable colloid;when do flush coupling,it also needs to add to wetting agent（soap）。

6.1.4  scanning technique

Apparently, gradually read on the  screen is just suitable for extensive  corrosion detection,for single mottling is meaningless。Therefore,using two-dimensional code technique is necessary,in its probe effective range,it has to effectively overlap,So as to make sure scanning area has been completely covered，manual scanning uses matched fast speed displacement probe is much better than slow 、hard to close to flaw part ,this is because eyes has conditioned reflex to signal on the screen，therefore,once corrosion pit has been detected ,it can have more careful research on depth of corrosion pit。

6.1.5  calibration

 When using ultrasonic wave to do manual scanning,for discovered flaw under detecting condition,it is better to choose a part that has already known its wall thickness on detecting baseboard to calibrate flaw detector,then set three times reflected wave displayed position at time base 3、6、9.adjust gain to make the third time  ,Reflected wave can up to 80%full screen，from now on,using the described earlier fast speed sports scanning,on the achieved three times reflected wave,coupling attenuation will show the same vertical drop。The whole movement display of Existing flaw signal is decreasing gradually and then incline to zero，being practiced,the eyes can be able to identify these graph。

6.1.6  train and experience

Corrosion pit detection is easier than thickness detection、or much harder than washout or callenia detection。When using time base calibration and just display once reflected wave slow speed scanning technique,some of operators towards corrosion pit of low reflectivity such as cone corrosion pit ,,has tendency of missing inspection,When operators just meet a corrosion pit,and often exists missing signal phenomenon,this is attributed to bad scanning surface 。Corrosion detection ,requiring to do special train and experience。 

6.2  baseboard

6.2.1  thickness

Using ultrasonic wave method,thinner wall thickness is main difficulty,achieved signal from a good reflector that lower than 6mm baseboard is the same as described attenuation，operators must realize that it requires bigger gain,compare to WFL for thicker parts(more than 12mm) ,ultrasonic wave method measurement distance does not  restrain，but its flaw detects possibility has been restrained by the shape of corrosion pit and reflectivity。

6.2.2  scanning surface condition

Compare to WFL,UT is more sensitive than scanning surface condition,this is suitable for contacting scanning and flush interval scanning。Coupling layer reflection ,Generates noise that ,make time base ambiguous 。Due to sound speed of coupling layer is quarter of being detected material sound speed，flaw can give clear reflectivity to display surplus wall thickness。Showing a 1mm depth lake type corrosion pit,its bottom reflected wave position is equal to reflection on the 4mm depth steel material。 If not careful,operators maybe report detect a 6mm depth corrosion pit in 10mm thickness slap,automatic scanning system and semi-automatic system regardless of using interface trigger or echo monitor will be misinterpreted for the same corrosion pit。  

6.2.3    Baseboard covering layer

In comparison, Oil paint and epoxy floor covering  are equipped good conditions,if using echo technique to eliminate paintcoat thickness error,then reminder thickness measurement accuracy will be improved，when detecting ,thicker fiberglass covering layer exists more questions，although in theory,if supporting covering  metal surface condition is good,under not dismantle covering ,detection is possible ,but seldom suitable for practical detection 。

6.2.4  corrosion pit parameter

Easiest to be detected flaw is lake type corrosion pit ,because its deepest part is relevant to parallel to scanning surface,enable to get reasonable reflectivity。On the other hand,cone type corrosion pit is always reflected wave deviate to acceptor，corrosion pit central area is too small to get stronger signal。 These corrosion pit is very easy to miss inspection by ultrasonic operators。Normally callenia is very good reflector,flaw can be detected,but its depth will be underestimated。Cylinder corrosion pit ,such as SRB corrosion,exists small 、used for ultrasonic transmission reflector, it is also difficult to detect。At the advantageous part Of corrosion pit reflectivity,ultrasonic method can be better than leakage flux to find smaller thickness changes，but due to corrosion surplus is always wall thickness 50%, so this advantage maybe not important at any conditions。

6.3  the possibility of UT can detect pit corrosion  (POD summary)

On the good condition scanning surface,lake type corrosion pit is more possible to detect,for poor scanning surface and zone corrosion pit,detecting possibility is not satisfied,using C scanning displayed automatics technique with data saving and at least use colour to mark different thickness wave band，to some extent,it can enable to improve possibility of detecting corrosion pit。 

7、some practical conclusions

 After leakage flux detection, cut off partly baseboard of tank,this part baseboard is derived from baseboard in the detecting report , below it has corrosive area and corrosion doesn’t exceed wall thickness 20% area，partly baseboards use SilverWWing company Floormap system ,this system can draw baseboard image,use different color to mark corrosion condition,every color represents certain waveband wall thickness percentage loss 。Corrosive parts get affected by mechanism Processing flaw deep dimension ,put its result and MFL into comparison。The discovered corrosion pit including lake type zone type,on the approximate position of corrosion pit corresponding to other side do a mark,require two groups UT staffs to detect to mark the position of corrosion pit and detect its depth 。 From average level,MFL system overestimates 10% to corrosion pit flaw depth,but ultrasonic methods underestimates 10%。But there is a ultrasonic misses to inspect two marked smooth corrosion pit。

8 、Conclusion

The two methods are able to reasonably detect,and the smallest corrosion pit detected effective thickness detecting range is limited ,in the MFL detecting conditions ,MFL is better than single flaw detection,and faster than UT, therefore it is more economic ,flaw depth measurement accuracy ,by comparing,this two methods have same percentage of error,due to existing baseboard material maybe not middle strength steel ,so the baseboard may exist magnetism that different from sample， So before making sure WFL corrosion pit depth assessment result,using UT to do effective reexamination to MFL results 。