degree of corrosion

Date 03-22-2012 00:29

When does exactly the light corrosion stops being light and starts being moderate? How many mils should have been corroded to make the difference between them? If less than so many, the corrosion is light; if more, the corrosion is moderate.

I suggest that you make a search into the API site: www.api.org
API is the American Petroleum Institute.
Good luck

Giovanni S. Crisi
Sao Paulo - Brazil

By bert lee (**)

Date 03-22-2012 00:40

thanks, but i could not find the answer in any api codes.
do you know any engineering referrence other than api or asme codes?

By Joseph P. Kane (****)

Date 03-22-2012 03:30

I don't think it will be appropriate for the chemical or refining industry, but the Steel Structures Painting Council (SSPC.org) publishes a color photo booklet of various corrosion conditions used in preparation for blasting and painting.

By CWI555

Date 03-22-2012 09:28

Do you know how much that one runs?

By RANDER

Date 03-22-2012 06:14

Bert,
Your answer will come from the client. Light , Medium, and Severe corrosion are very subjective terms that should be further clarified or defined by the client whether in written form or by example from photographs or actual specimens. I have had clients in the past give us photographs and descriptions for varying levels of degradation of components. When in doubt as to what you could call a degree of corrosion, you should consider giving actual numbers, I.e., actual thickness as compared to nominal.

By eekpod

Date 03-22-2012 10:20

SSPC (The society for Protective Coatings)
SSPC-VIS2
Standard Method of Evaluating Degree of Rusting on Painted Steel Surfaces.

By CWI555

Date 03-22-2012 11:12

Will have to look those up. Thanks.

By bert lee (**)

Date 03-22-2012 15:24

the client is the one who demanded that the inspector should be able to know the degree on corrosion,
the client only want to know your advise on when they have to repair, recoat the lines, when the inspector say "severe", the advise given is to repair or replace the corroded area, whereas light to moderate corrosions need recoating.

what term you normarly used when classifying the degree of corrosions? and what recommendations you normally provide?

thanks
bert

By dbigkahunna (****)

Date 03-25-2012 16:11

Reading your comment again.
You are in a refinery
API applies
You are working on a piping system that has been in service
API 570 applies
You are wanting to determine if the pipe requires replacement or can be recoated.
Agan, API 570 applies.
Your client requires all corrosion determined to be "Severe" to be replaced.
Under API 570 if the piping requires replacement, use the undefined term "Severely Corroded".
If it does not require replacement, use the undefined term "Light Corrosion"
Throw in the undefined term "Moderate Corrosion" on areas every once and a while to break up the pencil whippin.

By bert lee (**)

Date 03-26-2012 15:26

nope, not at all...you have to apply your measurement policy before deciding, don’t forget you are paid to do measurement as 653 inspector.
hope you enjoy the measurement all day long…concentrate on those corroded areas having through holes, bend your knees and make a serious measurement to determine whether these are the so called "severe corrosion".

By dbigkahunna (****)

Date 03-22-2012 15:20

Those are relative terms and there is no reference in API for those terms. A 0.250 pit may be light corrosion with a 1.0 base but may be severe on a 0.375 base. The term should have something to measure itself by. A report that states "moderate corrosion in the bottom quarter of the plate" means nothing except the writer does not know how to write.
A report that states " Isolated areas of corrosion ranging from 0.188 to 0.250 were found in the bottom quarter of the plate were found during the inspection. The base material thickness is 0.500." This statement allows the reader to determine if this could be considered light, moderate or severe. And that determination would be based on other things like structure type (vessel, tank, pipe, structural, etc), length of service (corroded areas were found after a 20 year inspection interval or has occured in the last 6 monts), service (elevated temp, pressure, immersion, atmospheric, etc) and other things that need to be considered when evaluating and determining service life or continued service suitability.

By bert lee (**)

Date 03-22-2012 15:35

if you have thousands of pitting scattered on shell courses & bottom plates, will you measure them one by one?

By qcrobert (***)

Date 03-22-2012 18:34 Edited 03-22-2012 21:48

Is this an existing tank that requires inspection? If so, how long has it been in service? Type of service? Material of tank? Specific information will lead to specific answers.

You state that the Owner says the Inspector should know the difference between the levels of corrosion. I disagree. The Owner-Operator must establish the inspection interval and acceptance criteria. This RBI assessment is established by the AI and an Engineer knowledgeable & experienced in tank design & corrosion.

As an Inspector, I examine then acurately record findings in detail. I do not make assessments. That is the responsibility of the Engineer.

When repairing cyclones, tanks, etc. the Client must use specific terms (i.e., percentage of original material thickness). I have had Clients specifiy such percentage in terms of "wasted" areas of 2", 4" or 6" diameters with further acceptance criteria as, "in no case shall pitting of any size be permitted if below a certain percentage of original matl thk.."

Sorry for not being able to give you a definitive answer to your original question.

QCRobert

By bert lee (**)

Date 03-22-2012 23:45

my issue is not specific on tank inspection, i just want to know any reference or std other than sspc that will assist me in definition of light, moderate & severe corrossion.

thanks
bert

By ziggy

Date 03-23-2012 14:43

i would suggest you contact Heather Stiner at SSPC for assistance in your quest. Her number is 412-281-2331 X2224

ziggy

By js55

Date 03-23-2012 17:23

There isn't one. It isn't done that way. You want to determine the degree of corrosion you measure the remaining thickness and extrapolate the corrosion rate.

By eekpod

Date 03-23-2012 19:31

Heather at SSPC has access to an network worldwide that can help you with this question, she is very helpful.

By dbigkahunna (****)

Date 03-25-2012 04:12 Edited 03-25-2012 04:18

If that is the SOW then yes. I would and HAVE measured hundreds of pits on a few occasions. In fact I have a inspection group on a project next week that will take 2 techs up to 3 days to measure and map. I looked at the steel after blasting and damed if I can tell you if it is light, moderate or severe. I can tell you there is corrosion, but until it is measured I will not know what the extent of the pitting and depth is
Again, light, moderate and severe are not qualitative terms. You missed my point. Sorry for attempts to give you context.
The answer you are wanting is NO there is no reference or standard from SSPC, NACE or API for the terms, light, medium and severe relating to corrosion.

By bert lee (**)

Date 03-25-2012 05:12 Edited 03-25-2012 05:47

sure you can do measurement, it seems to me that only when you do some measurement then only you can tell whether it is light, moderate or severe corrosion.

however, if you refer to fig 37 & 38 of api 575 second edition 2005 (page 30 & 31), do you think you need to do some measurement before you conclude that the condition of projection plate is severely corroded?

another example, you entered a tank not inspected for 20years, you noticed corrosion flakes on the floor, you look up to shine the condition of roof rafters and found out where the flakes come from......have you read this in the above api which states " usually the roof rafters can be measured with calipers, unless severe corrosion of the rafters is evident, these measurements should suffice"

you have a problem if you don't know how severe corrosion look like prior to measurement.

By dbigkahunna (****)

Date 03-25-2012 15:44 Edited 03-25-2012 15:47

"if you refer to fig 37 & 38 of api 575 second edition 2005 (page 30 & 31), do you think you need to do some measurement before you conclude that the condition of projection plate is severely corroded?"
YES! As the owner of the equipment I am PAYING for that measurement. Not the opinion of the inspector!
The other from API 575;" usually the roof rafters can be measured with calipers, unless severe corrosion of the rafters is evident, these measurements should suffice"
In the example with corroding rafters, the indication of flaking coming off the rafters may or may not indicate repairs need to be done. I have yet to see a inspector that can stand on the floor of a 40 foot tall tank and can determine if the rafters are corroded unless the rafters have failed. In that case, the obvious had been defined.
The rafters have failed.
If the roof is sagging is it corrosion of the rafters or has there been a weld failure of the rafter supports. Unless there is failure, nothing is evident. The inspector has to look and measure to make that determination.
I do not want an inspectors opinion of what the inspectors sees and believes. I want measurements and recommendations made only on that. If the corrosion cannot be measured, then I want to know that. I understand sometimes parts of the equipment cannot be accessed. If I am the equipment owner it is my responsibility to make the equipment accesable for the inspection. All three of the referenced documents give continued service based on corrosion rates. The only way a corrosion rate and continued service can be determined is by measuring the corrosion and calculating the remaining life.
Suppose the inspection interval is three years. Is that corrosion light, moderate or severe? In my opinion the corrosion may be severe but the measurements of the rafters and the corrosion rate indicates the rafters have sufficient remaining thickness. As someone that reviews inspection reports and bases repairs and replacement based on these inspection reports I do not want a inspectors opinion.
In this case the term severe corrosion is useless.
Can the structure safely support the load for three years?
What may be acceptable for three years may not be if the inspection interval is twenty years.
It is right and proper for a inspector to offer recommendations based on the inspection results and the inspectors experience but that recommendation must be based on the inspection of a specific piece of equipment and the measurements of the corrosion on that specific piece of equipment based on the service and length of service. And the inspection report should clearly indicate if the recommendation is based on measurements and experience or just on the experience of the inspector. I have no problem with the inspection report having recommendations based on the experience of the inspector as long as it is clearly stated this recommendtion is not based on measurements(fact) but experience (opinion).
Take a nonfired vessel operating at 25 PSI. The thickness of the vessel is 1 inch. The bottom third of the vessel has extemsive corrosion in the bottom third. Based on the terms Light, Moderate or Severe, what is the corrosion rate, next inspection interval and remaining life of the vessel? Does the vessel require repairs? How extensive?
This applies to piping and tanks. Both product and atmosphere covered by API 510, 570 and 653. Working on a bridge or in a power plant, then these do not apply.
If your client wants you to define corrosion and base repairs on a visual indication and the inspectors opinion of Light, Moderate and Severe then go ahead. But there is no reference defining those terms.
That is the question you asked.
If you are sitting for the API inspector certification for any of the three documents, those terms will not be on the test.
Promise you.

By ozniek

Date 03-24-2012 11:15

Hi Bert

When an inspector does an inspection of process plant equipment, s/he gets a "feeling" for the state of the equipment. This is often expressed in subjective (or emotive) terms to convey that feeling. (e.g. Shiny appearance; moderate corrosion; isolated pitting etc.) These "emotive terms" are obviously useless by themselves, and a report only stating such subjective terms are useless. They do however add an additional dimension for the reader of the report when backed by facts. (e.g. Isolated pitting (approx 10 pits per 100cm2) of 0.5mm to 1.0mm deep in the vapour space; OR Moderate corrosion around the outlet nozzle, taking remaining wall thickness to 8.5mm from an original thickness of 10mm. Corrosion allowance is 3mm etc...) A report without the emotive terms will not have the same effect as one with the emotive terms. The emotive terms will convey a sense of "all OK here" or "lets keep an eye on this one" much better than facts alone. Within this context, I believe that experienced inspectors add value in that they have a better idea of when things need some attention than many engineers that are simply studying the facts off the report.

Please understand that I am NOT saying that a report made up solely of these emotive terms are OK. They are not. All the facts on which to make the decisions need to be included, but these emotive terms can guide less experienced personnel in the right direction when it comes to deciding foreward actions.

Regards
Niekie

By bert lee (**)

Date 03-25-2012 05:22

will you advise the terms mentioned are useless when you are using a borescope and have no access to carry out measurement?

By ozniek

Date 03-25-2012 11:22

Hi Bert

The problem when you are unable to do measurements, is not with the terms, but rather with the inability to do measurements. In fact, where the ability to perform quantitative measurements are reduced, the value of the subjective terms become greater, as they increase the sense of "OK / NOT OK" that is being conveyed by the inspector. In this instance, a report of severe corrosion with extensive pitting will convey the need for more information to be gathered by the inspection team.

Just a few comments on borescope inspections in general:

1) It takes a lot of experience with borescope work to actually interpret what you are seeing to any degree. This makes it much more important for the inspector to convey an interpretation on a "qualitative" basis to the engineers that do not have this experience. (A small pit can sometimes look like the grand canyon to the inexperienced.)
2) Any potential issues identified with the borescope inspections need to be quantified through other means. (External UT thickness measurement; See Scan etc.)
3) There are fancy borescope systems available that enable reasonable quantitative measurements to be made, but these need to be operated by skilled technicians.

In summary, where there is an inability of the inspection to collect quantitative data, more emphasis is placed on the inspector's ability to interpret what s/he sees, and consequently the more value there is in these "emotive" statements, as they convey some of the inspector's experience to the readers of the report. Where there are any alarm bells rigning, other inspection techniques are needed to gather the quantitative data for engineering decision making.

Regards
Niekie

By bert lee (**)

Date 03-25-2012 15:13

thanks for your input ozniek

let make it simple, you want to change the tire of your bicycle but you could not remove it because the bolting connection is severely corroded. Do you require measurement before you convince yourself that the bolt / nuts suffered severe corrosion?

By ozniek

Date 03-26-2012 11:46

Hi Bert

If you need to remove the wheel, then clearly you need to replace the bolt, (The operational ability of the component is therefore lost.) but if you are only concerned if it will break while you are riding down the road, then knowing the extent of corrosion (and therefore the remaining load carrying capacity of the bolt) will let you make the decision if replacement is necessary or not.

Most of the time in pressure equipment, you need to decide if it is going to break while you are using it. If not, then all good to "ride" a while longer!

Regards
Niekie

By G.S.Crisi (****)

Date 03-26-2012 20:33

The table that follows below has been taken from the book "Materiais para Equipamentos de Processo" (Materials for Process Equipment), by Prof. Pedro Carlos Silva Telles. Prof. Silva Telles is a highly respected engineer here in Brazil and enjoys high prestige among engineering professionals. He's now retired, but he's been for many years the manager of Petrobras' Engineering Division as well as a Professor at Rio de Janeiro Federal University. Petrobras is the Brazilian State oil company.

Prof. Silva Telles says that the figures included in the table are common practice at Petrobras' Engineering Division and, although they don't make part of any Petrobras official rule (specification, engineering standard, recommended procedure and the like) they are usually employed by Petrobras process and design engineers. The table is used to choose the corrosion allowance of vessels and piping.

   Light corrosion Moderate corrosion    Severe Corrosion
   Pressure vessels and similar equipment 1.5 mm 3.0 mm    6.0 mm
   (e.g. chemical reactors)
   Piping 1.2 mm 2.0 mm    4.0 mm

Prof. Silva telles warns the book reader that in some circumstances there may be some factors that preclude the use of those figures.
Hope this is of help to you.

Giovanni S. Crisi

By ozniek

Date 03-29-2012 13:06

Hi Prof Crisi

In design circles, the corrosion terms are much more exact, as these are actually related to the expected wall loss per year, multiplied by the design life of the equipment. Corrosion loss figures related to the "terms" such as those shown in your post, then make sense from a design perspective. To see if this is helpfull from an inspection perspactive, let us "work backwards" while making some assumptions. (Purely intellectual here, not based on any codes or standards!)

The first assumption relates to the design life of the equipment. While this can vary widely where extreme conditions need to be handled, but in most petro-chem applications, the design life would be around 25years. The second assumption is that we are looking at a general corrosion mechanism, not localized. So:

1.5mm / 25y = 0.06mm loss per year
3.0mm / 25y = 0.12mm loss per year
6.0mm / 25y = 0.24mm loss per year

If we further assume (third assumption) that any corrosion products formed is not removed, but stays in place as an oxide scale, then there is a rough rule of thumb that for every mm of wall loss, the oxide build-up is 7mm thick. (Oxide scale on steel.) We can make a further assumption (fourth assumption) that the inspection interval of the equipment is 2 yearly. (Based on the legislated inspection intervals in some jurisdictions, unless RBI studies show it is not necessary.) So the oxide scale thickness for the differing corrosion terms after this 2 year interval will be:

Light = 0.06mm/y = 0.84mm scale over 2 years. (Scale this thickness will tend to be relatively tightly adherent and hard, making a near continuous film, possibly with "steps")
Moderate = 0.12mm/y = 1.68mm scale over 2 years. (Scale this thickness will tend to be visible as a number of different layers or steps.)
Severe = 0.24mm/y = 3.36mm scale over 2 years. (Scale this thick will tend to be relatively flaky and look rather bad.)

We have obviously made a whole lot of assumptions, but many of these will hold in typically stagnant areas in equipment subjected to regular inspections. I believe that quite probably the terms the inspector might use for the quantities of corrosion product as given above, may very well be in line with the design intent, so this may be quite instructive for inspectors. Obviously a vessel with a "light corrosion" rate of 0.06mm/y, if only inspected every 6 years (Totally plausible) will have a corrosion product of 2.5mm thick, which would almost certainly be described as "severe" by most inspectors. This shows us that the terms are very relative when used by the inspector in isolation. It needs to be backed up by other data such as thickness testing. Obviously a surface can be subjected to erosive effects from the fluid, which would remove the corrosion products, and the result is that the inspector will see no corrosion at all. Again we need more data. Generally though, corrosion is rarely totally even. As corrosion products build up, they actually have a temporary protective effect, but due to their inherent stresses, they spall off, leaving certain areas exposed to experience the previous corrosion rates. This sort of mechanism then leads to the corroded surface having a "step like" appearance, or laking or pitting can also result. This makes the corrosion more obvious, and will lead the inspector to ask more questions.

Thanks Prof Crisi, I think this is not a bad starting point from which to think of the corrosion terms. (Nothing exact, but interesting.) - Now come on, I know there are a whole lot of you that do not agree at all, and think this exercise is full of holes, so let us hear!!!

Regards
Niekie

By G.S.Crisi (****)

Date 03-29-2012 22:43

Niekie,
A brilliant lesson on Corrosion!

Giovanni

By swsweld (****)

Date 04-30-2012 14:20

I know this thread is more than 1 month old and lots of good replies have already been made but I saw this info recently and it was helpful and relevant to this topic so...

Also, I'm not an API AI but I am studying 510 and 570 with hopes to test later this year or next.

Quantifying Corrosion And Pitting On Your Inspection Report

When you have pitting and corrosion to describe in your report always give the size of the area and the depth of the pitting.

For describing the size:
General – Wide spread over the entire surface. The corrosion or pitting covers more than 10% of the surface area and the wall thickness loss is less than 50 mils.

Localized – Confined to one specific area. Corrosion or pitting affects less than 10% of the surface area and wall thickness variation is greater than 50 mils.

Scattered – Several separate areas over the surface but not enough to be considered general.

Isolated – Only one small area of wastage or a single pit.

For quantifying the corrosion or pitting:
Light – Depth is measurable but no immediate action is considered necessary. The average depth should be estimated, maximum depth and location recorded in a manner which will permit subsequent reassessment at the next inspection. An example would be a piece of pressure equipment which was built with a corrosion allowance of 0.125” and the corrosion loss us up to 40 mils.

Moderate – Depth is measurable but no immediate action is required depending on the inspection intervals and rate of loss. This wastage would not be at or near the retirement limit. An example would be a piece of equipment which was built with a corrosion allowance of 0.125” and the corrosion loss is 40 to 80 mils.

Heavy – Action is required to determine the rate of corrosion versus the next internal inspection interval to insure the equipment is adequate for this interval. Consideration should also be given to the repair impact at this time period versus a larger repair at the next interval. An example would be a piece of equipment which was built with a corrosion allowance of 0.1256” and corrosion loss is 80 to 100 mils.

Severe – The vessel retirement thickness has been reached or exceeded. Repairs or complete renewal will be required before the equipment is returned to service.

Remember:
Pitting – Should be evaluated using API 510 code section 7.4.3

By Joey

Date 05-03-2012 10:16

Can you apply this to insulation jacket? This is the item you will see first when inspecting a vessel or pipe with insulation.

~Joey~

By swsweld (****)

Date 05-04-2012 16:30

Joey,
I know that you are well versed in API work so I'm not sure if this is what you are asking but yes API 510 6.3 External Inspection states:

Inspection for corrosion under insulation (CUI) shall be considered for externally-insulated vessels subject to moisture ingress and that operate between 25°F (–4°C) and 250°F (120°C), or are in intermittent service. This inspection may require removal of some insulation. It is not normally necessary to remove insulation if the entire vessel shell is always operated at a temperature sufficiently low [below 25°F (–4°C)] or sufficiently high [above 250°F (120°C)] to prevent the presence or condensation of moisture under the insulation. Alternatively, shell thickness measurements done internally at typical problem areas (for example, stiffening rings, around nozzles, and other locations which tend to trap moisture or allow moisture ingress) may be performed during internal inspections.

By G.S.Crisi (****)

Date 05-04-2012 20:39

Corrosion under insulation due to low temperature service can be prevented by a well done moisture barrier.
Giovanni S. Crisi

By Joey

Date 05-14-2012 09:30

swsweld

I'm not referring to the base metal under insulation, my point is on whether the term stated can be applied to describe the condition of insulation jacket, which is not measurable.

~Joey~