What are the pitting acceptance criteria in each class?

Three defects, and the document that carries none of them

Pitting, grooving and edge corrosion get lumped together as "local corrosion" and then read off one number. They are three different things, gauged differently and judged differently, and the rule books keep them in three separate clauses for a reason.

• Pitting is judged on intensity first — how much of the area is pitted — and then on depth or remaining thickness. The gauge has to reach the bottom of a pit, so the transducer changes.
• Grooving is judged on breadth first. A narrow groove has its own remaining-thickness figure; a wide one is thrown back to the general corrosion criteria and read as an average across the member.
• Edge corrosion is judged on how far the wasting runs along or in from an edge — down the flange of a stiffener, or in from the rim of a manhole — and then on the thickness left at the edge and on the average across the member. Two tests, both to be met.

Now the thing that surprises people. The Common Structural Rules carry no acceptance criteria for any of the three. The renewal chapter is two sections — principles and survey requirements, then acceptance criteria — and the acceptance criteria are local corrosion against t_ren and global corrosion against the hull girder percentages, and nothing else. A full-text search of the 812 pages returns no occurrence of the word "pitting" and none of "edge corrosion". "Grooving" appears exactly once, and it is not a wastage clause: it is a newbuilding requirement to prevent wire rope grooving at cargo hold openings by fitting half-round bar on the hatch side girders, the hatch end beams and the upper portion of the hatch coamings. CSR Pt 1 Ch 13 Sec 2 [2] · Pt 2 Ch 1 Sec 2 [3.3.5]

CSR says where the criteria do live. Chapter 13 states that a CSR ship is surveyed to whichever rules or documents its own society uses to implement the Z10 series — UR Z10.2 where the ship is a single side skin bulk carrier, UR Z10.4 where it is a double hull oil tanker, UR Z10.5 where it is a double side skin bulk carrier — and [2.1.3] names those same three URs for alternative solutions in the reserve band. So CSR supplies the ingredients — t_ren, t_c and t_res — and the Z10 URs print the arithmetic that uses them. Every figure on this page is attributed to the document that prints it, never to CSR. CSR Ch 13 Sec 1 [1.1.4] · Sec 2 [2.1.3]

Pitting: two rules, and which one applies where

Two pitting rules are in circulation and they are not interchangeable. Which one applies depends on where the pitting is, not on how bad it looks.

The side-structure rule. It works on intensity and a percentage of the as-built thickness. If pitting intensity in an area where coating is required by CSR is higher than 15%, thickness measurements are to be taken to check the extent of the pitting — and the 15 % is based on pitting or grooving on one side of a plate only. Whatever is left at the bottom of a pit, a groove or any other local area then has to clear 75% of as-built in the webs and flanges of the frame and its end brackets, and 70% of as-built in the attached side shell, hopper and topside tank plating, taken in a band reaching 30 mm out on either side of the frame — and in neither case may the figure sit above the renewal thickness. That last condition is the one that gets dropped: the percentage is a floor, and t_ren is the ceiling on it. UR Z10.2 §8.2.1

The other-structures rule. It works on a formula against the renewal thickness and there is no percentage of as-built in it at all. For plates with pitting intensity less than 20%, the measured thickness has to meet the lesser of t_m ≥ 0.7 (t_as-built − t_{vol add}) and t_m ≥ t_ren − 1 mm. A second requirement rides on top of it. Slice the plating anywhere, average the thickness across that section, and the average still has to clear the general corrosion renewal criteria in CSR Chapter 13. Two tests again, and the second one is the one that catches a plate that passes at every individual point. UR Z10.2 §8.2.2

Both of those are CSR clauses. The acceptance criteria section of UR Z10.2 opens by splitting the two cases: for vessels built under the Common Structural Rules the criteria are those of CSR as specified in §8.2, §8.3 and §8.4; for vessels not built under CSR the criteria are those of the individual society's rules and specific IACS URs depending on the ship's age and the structural elements concerned, naming UR S18 for corrugated transverse watertight bulkheads, UR S19 for the bulkhead between cargo holds 1 and 2, UR S21 for cargo hatch covers and forward and side coamings in position 1, and UR S31 for side shell frames. If the ship is not a CSR ship, §8.2 is not your clause. UR Z10.2 §8.1.1–§8.1.2

And the reading itself, where the URs are unusually specific. Where the 15% is exceeded, the most pitted part of the plate has to be cleaned back to bare metal over 300 mm or more; the thickness is then taken at the five deepest pits inside that cleaned patch; and the least of the five is the figure recorded. Not the average of the five, not the average of the plate — the least. The tool comes from a narrower clause. Annex V of the same UR is the gauging annex for side shell frames and brackets on single side skin bulk carriers that have to comply with UR S31, and it is there that a 10 mm transducer is said not to suffice for pits or grooves, with a 3 to 5 mm miniature transducer, or a pit gauge and a calculation, called for instead. UR Z10.2 §8.2.1 · Annex V §3

Here is the whole set, document by document. The blank places are as important as the figures.

Two things fall out of that table that are worth saying on their own. First, the 15% and the 20% are not the same kind of number. The 15 % is a trigger — cross it and you gauge. The 20 % is a ceiling on the applicability of a criterion — above it, the individual-measurement rule is no longer offered and the plate goes back to general corrosion. A sheet that records "intensity 18 %" without saying which structure it was found in has not answered either question. Second, the tanker URs and the bulk carrier URs are not symmetrical. Z10.4 carries the grooving and edge sections in full but has no side-structure pitting clause and no cleaning procedure at all; those live only in Z10.2 and Z10.5.

Pitting intensity: the grid, and the diagrams

This is the part of the subject that gets flattened most often, and the distinction is the whole value of the section. Two societies turn intensity into a number. Everyone else turns it into a picture.

BV and RINA print the same nine-step grid of intensity against maximum average pitting depth, expressed as a percentage of the as-built thickness, and it applies to ships not built under CSR. On those ships intensity is not a trigger to go and gauge — it is an input that sets the acceptable pit depth. Read the intensity off the diagram, look up the row, and that gives you the depth the society will accept.

Everyone else prints the intensity diagrams only — the familiar plates showing 5 %, 10 %, 15 % of a surface scattered with pits — and uses them for one purpose: to decide whether the 15% or the 20% threshold has been passed. There is no depth figure attached to the picture. And the diagram sets are not the same from document to document: the ranges differ, and a step drawn in one document is not necessarily drawn in another. Naming which diagrams draw which step is not pedantry — if you match a plate against a 30 % diagram from one document and then apply another document's criterion, you are outside both.

Three qualifiers belong with that grid and none of them is optional.

• The depth grid is not the whole test. For ships contracted for construction before 1 July 2022, BV also requires the thickness outside the pits in the area considered to be assessed against the local, global and buckling criteria of [4.3] and [4.4]. RINA prints the same second test without the contract-date condition. BV [4.6.3] · RINA [4.5.3]
• Filler is not a repair. Both documents print the same note: plastic or epoxy compound is a way to stop or slow the corrosion, and is not an acceptable repair for pitting beyond the maximum allowable wastage limits. Welding repairs may be accepted under procedures agreed with the society. DNV says the same thing about plastic compound filler in its own words, and then puts a number on when filler is offered at all. Its repair clause allows filler only for widely scattered pitting, which it defines as intensity under 5% read off its Figure 1, and only where the thickness left in the pitting is at least 6 mm. The pits have to be blasted and dried first, filled completely, and coated over. Once the depth or the intensity falls outside its acceptance criteria, filler on its own is not a permanent repair — the choice there is welding to a stated procedure or an insert. Older edition — July 2022. BV [4.6.3] Note 1 · DNV Sec.9 [1.1] · [1.4] · older edition
• ABS prints no depth grid at all. FIGURE 5 is five diagrams — 5, 10, 15, 20 and 25 % scattered — and the only numbers ABS attaches to intensity are the 15 % gauging trigger and the 20 % ceiling of the individual-measurement criterion, both of which are CSR clauses. Do not go looking in Part 7 for a BV-style depth table; there is not one. ABS FIGURE 5

The BV and RINA grids carry identical values at every one of the nine steps. That is worth knowing, and it is also as far as the equivalence goes: both are non-CSR criteria and both work on pit depth against as-built thickness, not on a remaining-thickness formula, so neither can be substituted for the CSR pair in §8.2 of the URs.

Grooving corrosion

A groove is judged on its breadth first, and the breadth decides which rule you are in — not the depth, and not how alarming it looks. Under the CSR wording, where the groove breadth is a maximum of 15% of the web height and not more than 30 mm, the measured thickness in the grooved area has to meet the lesser of t_m ≥ 0.75 (t_as-built − t_{vol add}) and t_m ≥ t_ren − 0.5 mm, but not less than 6 mm. The 6 mm floor overrides both formulas and a thin member can fail on it alone. Anything wider than that goes back to the general corrosion criteria of CSR Chapter 13 and is read as an average across the plating or stiffener — the local groove figure is withdrawn, not relaxed. UR Z10.2 §8.4.1–§8.4.2 · Fig 3

The same clause pair, word for word, is in UR Z10.5 §8.4 and UR Z10.4 §8.4 — and note that the tanker UR carries the grooving section in full even though it has no side-structure pitting clause. It is also reprinted by ABS at 7-A1-4/35.1.3(c), by BV at [4.7.5], by KR at 105. 2 (3), by INSB at §1.13.3.1, by ClassNK in Appendix 4-5 §5 (3), by RINA in App 3 (three times: once for single skin bulk carriers, once for double skin, once for double hull oil tankers), by DNV in Sec.10 [4.1] and by LR in §1.5.7.6. The differences between those reprints are small but real, and three of them change the answer. the CSR grooving clause, ten reprints

• DNV runs to 100 mm on non-CSR ships, and prints a separate figure for L-profiles. Section 9, which is the non-CSR section, caps the groove breadth at 15 % of the web height and at 100 mm, whichever bites first — more than three times the millimetre cap the CSR wording allows — and gives t_min = 0.7 t_{as_built}, rising to 0.75 t_{as_built} for L-profiles, in both cases not less than 6.0 mm. It is the only document here that sets a different figure for an L-profile. The assumptions of [2.2] apply with it: grooves and edges are smooth and without sharp edges or notches, and the welding is intact with an acceptable remaining throat thickness. Corroded welded seams in shell plating carry their own figure, t_min = 0.7 t_{as_built}. And DNV prints a length test no one else does. Add up every transverse groove on one structural member inside the cargo area — deck, bottom, side plating or a longitudinal bulkhead — and the total is held to 20% of the ship's breadth or height. Where the ship has large deck openings, and the guideline names container ships as the case, the same total taken along the passageway is held to 10% of breadth. The same guideline therefore carries two different caps — 100 mm in Sec.9 and 30 mm in Sec.10 — and the CSR status of the ship decides which one is read. Older edition — July 2022. DNV Sec.9 [2.2]–[2.4] · Sec.10 [4.1] · older edition
• CCS prints a second criterion for a corroded weld seam on shell plating. For non-CSR ships, the web and face plate criterion is t_min = 0.75 T_as-built, not less than 6.00 mm, for a groove width not exceeding 15 % of the web plate height and not exceeding 30 mm. Separately, a corroded weld seam on shell plating takes t_min = 0.7 T_as-built where the groove breadth adjacent to the framing knuckle weld is not more than 30 mm — a separate criterion for the weld seam itself, not a relaxation of the first. CCS is also the only document that makes the shape of the groove and the state of the weld conditions of the criterion rather than assumptions stated elsewhere: the groove and its edge are to be smooth with no sharp concave or convex, and the weld seam complete with an acceptable surplus welding throat thickness. For CSR ships CCS prints no figure and refers out to CCS Rules Pt One Ch 5 Sec 1, 5.1.17(4), which is not held — but it does impose a separate reporting duty: separate diagrams of grooving corrosion are to be drawn showing the least remaining thickness, the influenced areas, the percentage and the length of the grooving. CCS Pt II §7.3 · Pt III §4
• RINA's non-CSR appendix has no grooving section at all. App 2 names grooving only as an example of what the isolated-area criterion is for — "a very local aspect such as grooving of a plate or web, or local severe corrosion" — so the groove is read from column 1 of the diminution table for the ship type and contract date, and no single grooving percentage is printed. Where that criterion is not met, the wasted part of the item is dealt with as necessary and the item is then assessed on column 2, its average thickness. The same clause explicitly excludes pitting from the isolated-area column and sends it to [4.5] instead, so the two defects are read from different places in the same appendix. BV does the same for its non-CSR ships, treating grooving of a plate or web through the isolated-area criterion of the diminution tables rather than through a groove-breadth rule. Older edition — RINA 1 July 2024 set. RINA [4.3.4]–[4.3.5] · BV [4.3.4] a)
• PRS gives one criterion for pitting and grooving together, with no breadth rule and no floor. Publication 63/P §2.5 is headed Pitting and Grooving and prints 75 % of as-built for the frame and bracket webs and flanges and 70 % for the attached side shell, hopper tank and topside tank plating over 30 mm each side. No groove breadth, no web-height fraction, no cap against the renewal thickness and no 6 mm floor is printed. Part I names grooving only in the repair clause, among damage in association with wastage over the allowable limits, and in the survey planning meeting list of findings to report promptly. The file held is the July 2008 text. PRS Pub 63/P §2.5 · Pt I §5.1.1.13
• ClassNK places the 6 mm floor ahead of the two formulas rather than after them; the arithmetic is the same. For ships outside CSR the guidance prints no groove criterion at all, and the standard limit for other local corrosion is 40 % of the original thickness. Older edition — 2014. NK App 4-5 §5 (3) · §2 (1)(iv) · older edition
• RINA App 3 and LR print inequalities and headings that are not quite the IACS text. RINA App 3 prints a strict > where the URs print ≥. LR's grooving criterion is printed under the heading CSR Tankers only, whereas the pitting part of the same clause carries a CSR Bulk Carriers heading as well; LR also writes t_own where the URs write t_{vol add}, and defines local corrosion in the same section as pitting, grooving, edge corrosion, necking effect or other corrosions of very local aspect. Older editions — RINA 1 July 2024 set, LR Ver 7.3, November 2016. RINA App 3 [3.2.4] · LR §1.5.7.6 · older editions
• Where nothing is published. UR Z10.1 and UR Z10.3 name grooving once each, inside the repair clause listing damage in association with wastage over the allowable limits; neither has an acceptance criteria section. IACS Rec 76 describes where grooving forms — at the transition between the thicker deck plating outside the line of cargo hatches and the thinner cross deck plating, where the step in thickness gathers water — and explains the mechanism as a galvanic current between the heat affected zone and the base metal, and prints no breadth limit and no thickness figure; its only numbers are repair advice. IACS Rec 96 carries a figure headed Grooving Corrosion labelling the groove breadth on the attached plating and on a flat bar stiffener, the same drawing the URs use, with no number set against it. And CSR itself, as above, carries none. Z10.1 · Z10.3 · Rec 76 · Rec 96 · CSR

So the first measurement at a groove is not a thickness. It is the breadth, and the web height it is a fraction of. Without those two numbers on the sheet the office cannot tell which rule the groove falls in, and both of them are easier to take on board than to reconstruct afterwards.

Edge corrosion

Edge corrosion is two rules in one clause, and they gate on different things.

The first covers the free edge of a stiffener — the flange, or the web in the case of a flat bar — and gates on how far down the edge the corrosion runs. So long as the corroded height along that edge stays under 25% of the flange breadth, or of the web height where it is a flat bar, the measured thickness has to meet the lesser of t_m ≥ 0.7 (t_as-built − t_{vol add}) and t_m ≥ t_ren − 1 mm. A second test rides with that one and does not replace it. Average the readings right across the stiffener — the full breadth of a flange, the full height of a web — and that average has to sit at or above the figure CSR Chapter 13 sets. Both have to be met. UR Z10.2 §8.3.1–§8.3.2 · Fig 2

The second covers plate edges at manholes and lightening holes and gates on how far in from the opening the thin band reaches. Such edges may be below the minimum thickness provided that the reduced band reaches in from the opening edge no more than 20% of the smallest dimension of the opening and no more than 100 mm — both are ceilings and the smaller one governs. Cropping a rough or uneven edge back is allowed, on two conditions: the opening's largest dimension may grow by no more than 10%, and the new edge has to be left at t_ren − 1 mm or thicker. UR Z10.2 §8.3.3 (a)–(b)

That pair is reprinted, in the same shape, by UR Z10.4 §8.3 and UR Z10.5 §8.3, by ABS at 7-A1-4/35.1.3(b), by BV at [4.7.4], by KR at 105. 2 (2), by INSB at §1.13.2, by ClassNK in Appendix 4-5 §5 (2), by RINA in App 3 (three times, single skin, double skin and double hull oil tanker) and by DNV in Sec.10 [3.1] and [3.3]. the CSR edge clause, nine reprints Where they differ, they differ in ways worth knowing:

• CCS and DNV write the limit as a fraction of as-built where the CSR wording writes a formula. Both forms, as printed:
  CCS, non-CSR ships, fractured edge or flat bar longitudinal, corroded height less than 25% of the web plate height — edge thickness not less than 2/3 T_as-built, and the average thickness of the longitudinal not less than the uniform corrosion limit. Two-thirds is about 66.7%, close to but not the same as the 0.7 (t_as-built − t_{vol add}) of the CSR wording — and it is set against the as-built with no voluntary addition deducted. At openings, CCS requires the edge thickness to be not less than 1/3 T_as-built and uniform, with the corroded edge extending no more than 20 % of the minimum size of the opening and no more than 100 mm; and it caps cropping at 10% of the area of the opening, where the IACS wording caps it at 10 % of the maximum dimension.DNV Sec.9, non-CSR ships, flat bar deck longitudinals, corroded height less than 25% of the stiffener web height — edge thickness not less than 1/3 t_{as_built} and well rounded, with the thickness of the remaining part of the longitudinal above the minimum allowable of Sec.2 to Sec.7. A third of as-built at the edge is far below the CSR figure; the second condition, the average across the member, is what holds it. The DNV clause is written for flat bar deck longitudinals, where the CSR wording covers any stiffener flange as well. CCS Pt II §7.2 · DNV Sec.9 [3.1.2]–[3.1.3] · older edition
• The same DNV guideline carries two different edge rules. The 1/3 fraction is Sec.9 and applies to ships not built under CSR; Sec.10 [3.1] prints the CSR formula for CSR oil tankers and bulk carriers. The CSR status of the ship decides which one is read. One printing quirk to be aware of: DNV Sec.10 [3.2], on the average across the stiffener, reads as printed that the average measured thickness shall be less than what is defined in IACS CSR — the reverse of UR Z10.2 §8.3.2 and of every other text here. Read it as the IACS wording: not to be less than. Older edition — July 2022. DNV Sec.10 [3.1]–[3.2] · older edition
• Two documents stop short at the cropped-back edge. DNV Sec.9 [3.1.3] b) and LR §1.5.7.6 (b) both allow cropping back to a 10 % increase in the opening but print no remaining-thickness figure for the new edge, where UR Z10.2 §8.3.3 (b) requires t_ren − 1 mm. DNV adds that special care is called for in areas with high shear stresses, including areas with adjacent cut-outs, and that the standard repair for excessive edge corrosion is to crop and renew with inserts of original scantling, with reinforcement rings with lap joint, additional flanges or closing the opening with collar plates as possibilities. Older editions — DNV July 2022, LR Ver 7.3 November 2016. DNV Sec.9 [3.1.3] · LR §1.5.7.6 · older editions
• ABS's tanker version cites two different CSR editions for the same figure. The bullet at 7-A1-4/35.3.3(b) iii) cites the minimum given in 1.4.2 of Section 12 of the CSRs for Oil Tankers where the sentence around it cites CSR Pt 5A Pt 1 Ch 13 Sec 2 [2] — the older and the current CSR for the same number. UR Z10.4 §8.3.1 handles the same point by naming both explicitly. ABS 7-A1-4/35.3.3(b) iii) · UR Z10.4 §8.3.1
• Where nothing is published. The term edge corrosion does not appear anywhere in UR Z10.1 (92 pages), UR Z10.3 (63 pages), the CSR (812 pages), PRS Publication 63/P (16 pages), PRS Part I (127 pages), RINA Part A Ch 2 App 2 (26 pages) or IACS Rec 76. Rec 96 carries the IACS definition — corrosion running locally along the free edges of plates, of stiffeners and of primary support members, and around openings — and a figure, and prints no percentage against either; its nearest approach to a criterion is naming structure in way of lightening holes or cut-outs as a place where accelerated corrosion is experienced from local drainage and flow. BV and KR print no edge criterion for ships not built under CSR, and ClassNK prints none for ships outside CSR either; BV does at least define edge corrosion and point at a figure. Z10.1 · Z10.3 · CSR · PRS · RINA App 2 · Rec 76 · Rec 96 · BV · KR · NK

The measurement that matters at an edge is therefore not one number either. It is the corroded height against the flange breadth or web height, or the reach in from the opening against the opening's smallest dimension — and then the thickness. Take the geometry first.

Requirement or guidance

Two of the documents most often quoted on pitting are not requirements, and the difference is not academic when a figure is being argued at a survey.

IACS Recommendation 76 (bulk carriers) and IACS Recommendation 96 (double hull oil tankers) are recommendations. A recommendation is not a unified requirement and carries no acceptance criteria. Both are on this page because both are read on board, and because their silence is itself the answer — if someone tells you the pit criterion is in Rec 76, the honest response is that Rec 76 gives repair advice and no acceptance figure. Its 15% is the ceiling below which weld repair of pits is offered: where intensity is below 15 %, pitting deeper than one quarter of the original thickness can be welded flush with the original surface, and renewal by plate insert is called for where deep pits are clustered together or the remaining thickness is less than 6 mm. That is a repair rule. Rec 96 prints no pitting acceptance percentage and no remaining-thickness figure anywhere in its 148 pages. It does print percentages — 75% and 100% for the substantial corrosion band, 20% and 10% for the coating condition scale — and neither set has anything to do with a pit. Rec 76 Area 3 §4.1.2 · Rec 96 Group 1 §5.1.2 · guidance

The Z10 URs are the mandatory side of it — once the member society implements them. A UR binds IACS members through their own rules; the text that governs your survey is the society's implementation of the UR, not the UR pdf. That is exactly what CSR Chapter 13 does when it hands the survey requirements over to whatever rules, or documents, each society has used to bring UR Z10.2, Z10.4 and Z10.5 into its own text, and it is why the class-by-class rows in the tables above are worth having: the society's version is the one that is applied, and several of them are not word-for-word the UR. CSR Ch 13 Sec 1 [1.1.4]

Two more status notes that belong on the face of the sentence. The CCS document is titled Guidelines and refers its criteria out to the CCS Rules for CSR ships, while printing its own non-CSR criteria in full — so the CCS figures quoted above are guideline text, and the CSR ones are not published there at all. The LR document is titled Guidance, and its §1.5 is headed Acceptance Criteria. Both facts are recorded rather than smoothed over. CCS Pt III §§2–4 · LR §1.5

The full grid

This page gives the reading — which rule applies where, what has to be measured before a thickness means anything, and which societies publish nothing. The table gives every cell: four datasets — pitting, pitting intensity, grooving and edge corrosion — across twenty documents, each cell carrying its document, its clause and its PDF page, and each guidance cell marked as guidance.

T04 — Pitting, grooving and edge corrosion, class by class →

Use the table when you need the cell; use this page when you need to know which of the three defects you are looking at and which rule it falls under.

What this means when you fill the sheet

Every criterion on this page needs something the office cannot reconstruct from a column of thicknesses. If the reading is recorded as a number and a location and nothing else, most of these clauses simply cannot be applied to it, and the ship gets a second attendance.

A pit reading that the office can actually use carries six things:

• The intensity, estimated against a named diagram — and say which diagram. "15 % against UR Z10.2 Figure 1" is a usable statement; "moderate pitting" is not. The ranges differ between documents, and above 25 % only the non-CSR figures draw a step at all.
• Which structure it is in, in the rule's own terms: side structure or other structure. That one fact decides whether the criterion is a percentage of as-built or a formula against t_ren. On a bulk carrier, whether the plating is the side shell, hopper tank or topside tank plating attached to a side frame within 30 mm of it is the difference between the 70 % rule and the general one.
• That the area was cleaned to bare metal, and how big it was. The URs say 300 mm or more at the most pitted part; CCS and DNV Sec.10 say 300 mm diameter. Record the figure you actually cleaned.
• All five deepest readings, not only the one you report. The recorded value is the least of the five, but the other four are what show the office that the procedure was followed and give it something to work with if the least one is queried.
• The transducer. A 10 mm probe will not reach the bottom of a pit and the UR says so; note the 3–5 mm miniature transducer or the pit gauge on the sheet, because a pit depth taken with the wrong tool is not a pit depth.
• The as-built, and whether a voluntary addition is documented. Every formula on this page is built from t_as-built, t_{vol add} and t_ren. LR calls the middle one t_own; it is the same thing.

And this is why "worst pit" alone is not enough. The deepest pit answers one clause out of four. It does not answer the intensity question, so the office cannot tell whether the 15 % trigger or the 20 % ceiling has been passed, and therefore cannot tell whether the individual-measurement criterion is even available. It does not answer the average across the cross section, which every version of the pitting clause requires as a second test and which a single deep reading actively misleads on. It does not answer the average across the breadth or height of the stiffener, which the edge clause requires. And on a non-CSR ship at BV or RINA the criterion is not a worst pit at all — it is the maximum average pit depth against a nine-step grid, with the isolated-pit figure of 35% reserved for genuinely isolated pits. A single number cannot serve all of that, and the UR's own symbol list makes the point: t_m is defined as the measured thickness on one item, meaning the average thickness on that item using the various measurements taken on it during periodical in-service surveys. UR Z10.2 §8.2.2, symbols

For a groove, take the breadth and the web height before the thickness, and note whether the groove runs along a weld seam — CCS and DNV both carry a separate criterion for a corroded weld seam on shell plating, and CCS also requires the groove and its edge to be smooth and the weld to be complete before its criterion applies at all. If the groove is wider than the rule covers, say so on the sheet: that is not a fail, it is a change of rule, and the member then has to be gauged for an average across the plating or stiffener, which is a different pattern of readings.

For an edge, take the corroded height and the flange breadth or web height it is a fraction of; at an opening, take the reach in from the edge and the smallest dimension of the opening. Then the thickness at the edge, and enough readings across the member to give the average the second test needs.

One last habit. When a defect is found, write down which of the three it is, in the rule's vocabulary — pitting, grooving, edge corrosion — and not "wastage". They are judged by three different clauses with three different thresholds and three different second tests, and the word you use on the sheet is what decides which clause the office opens.

Sources

1. UR Z10.2IACS UR Z10.2 Rev.37 (2023) — Hull Surveys of Bulk Carriersofficial text ↗
2. UR Z10.4IACS UR Z10.4 Rev.18 (2023) — Hull Surveys of Double Hull Oil Tankersofficial text ↗
3. UR Z10.5IACS UR Z10.5 Rev.20 (2023) — Hull Surveys of Double Skin Bulk Carriersofficial text ↗
4. UR Z10.1IACS UR Z10.1 Rev.25 (2023) — Hull Surveys of Oil Tankersofficial text ↗
5. UR Z10.3IACS UR Z10.3 Rev.21 (2023) — Hull Surveys of Chemical Tankersofficial text ↗
6. CSR Pt 1 Ch 13IACS Common Structural Rules for Bulk Carriers and Oil Tankers — 1 January 2024 version, in force 1 July 2024official text ↗
7. IACS Rec 76IACS Recommendation 76 Rev.2 Corr.1 — Bulk Carriers: Guidelines for Surveys, Assessment and Repair of Hull Structure (a recommendation, not a requirement)official text ↗
8. IACS Rec 96IACS Recommendation 96 Rev.2 (2023) — Double Hull Oil Tankers: Guidelines for Surveys, Assessment and Repair of Hull Structures (a recommendation, not a requirement)official text ↗
9. ABS Pt 7ABS Rules for Survey After Construction, Part 7 — July 2026official text ↗
10. BV NR467 Pt ABV NR467 Rules for the Classification of Steel Ships, Part A — R26, July 2026official text ↗
11. CCS TM GuidelinesCCS Guidelines for Thickness Measurement of Hull — Ver 6.6, March 2025, in force 1 April 2025official text ↗
12. KR Pt 1KR Rules and Guidance for the Classification of Steel Ships, Part 1 — 2026 editionofficial text ↗
13. PRS Pub 63/PPRS Publication 63/P — Replacement Criteria for Side Shell Frames and Brackets in Single Side Skin Bulk Carriers and Oil-Bulk-Ore Carriers (file held prints July 2008)official text ↗
14. PRS Pt IPRS Rules for the Classification and Construction of Sea-going Ships, Part I — January 2026official text ↗
15. INSB Pt IINSB Rules and Regulations, Part I — Issue 04 Rev.03, December 2024official text ↗
16. ClassNK GuidanceClassNK Guidance for Undergoing Class Maintenance Surveys — 2014 edition (older edition held)official text ↗
17. RINA Pt A Ch 2 App 2RINA Rules for the Classification of Ships, Part A Ch 2 App 2 — 1 July 2024 interactive set (older edition held)official text ↗
18. RINA Pt A Ch 2 App 3RINA Rules for the Classification of Ships, Part A Ch 2 App 3 — 1 July 2024 interactive set (older edition held)official text ↗
19. DNV-CG-0182DNV-CG-0182 Allowable thickness diminution for hull structure — Edition July 2022 (older edition held)official text ↗
20. LR TM Guidance Pt 1LR Thickness Measurement and Close-up Survey Guidance, Part 1 — Ver 7.3, November 2016 (older edition held)official text ↗

Class-society and IMO documents are indexed and linked, never re-hosted. Where a class rule is stricter than the IACS baseline, the class rule governs.

Related questions

• How do I read a thickness limit?K5.1
• What counts as substantial corrosion in each class?K5.2
• What are the permissible diminution limits for oil tankers, class by class?K5.3
• What are the permissible diminution limits for bulk carriers, class by class?K5.4
• What are the permissible diminution limits for general dry cargo ships?K5.5
• At what thickness must steel be renewed on a CSR ship?K5.7
• What thickness limits apply to ships under 90 m?K5.8
• What changes for thickness measurement on CSR ships?K3.9