What counts as substantial corrosion, class by class
Substantial corrosion is a band, not a limit: the plate is still acceptable, but the finding changes what the surveyor has to do. Twenty-three rows, carrying twenty-four documents, are read side by side here — the wording each one uses, the number that opens the band, and what the finding obliges. Several societies do not word it the same way, and one of the most quoted sources does not use the term at all.
- table
- T02
- last verified
- 2026-08-04
- verified by
- —
- cycle
- each Z10 revision, each class edition, and each January
The definition, the trigger and the duty — one row per document
Read the wording column first. Where a society adds a second sentence for CSR-built ships, it is shown in the same cell, because on a CSR ship both sentences are in the same clause and only one of them applies to the plate in front of you. 23 rows · every value carries its clause and PDF page
| Society · document | How the society words itverbatim scope of the definition | Numeric triggerthe number that opens the band | What the finding obligesbeyond recording the reading |
|---|---|---|---|
| IMO 2011 ESP CodeRes. A.1049(27) as amended by Res. MSC.525(106) — bulk carriers and oil tankers | Wastage that has run past 75 % of the allowable margins while the member is still inside the acceptable limits. Three of the Code's four parts add a sentence for ships built to the IACS Common Structural Rules, and they write that band with t_net rather than t_ren: a gauged or measured thickness lying between t_net + 0.5 mm and t_net. The part covering oil tankers other than double-hull tankers stops at the percentage and adds nothing for CSR ships. The only text in this table that binds through SOLAS rather than through a class contract — the Code is made mandatory by SOLAS chapter XI-1, so it reaches the flag State as well as the society. The term is carried on 66 of the Code's 326 PDF pages. MSC.525(106) leaves the definition untouched: its amendments run to coating condition, space protection, the condition evaluation report and a new regime for double-side skin void spaces on bulk carriers over 20 years of age and 150 m and upwards. Annex A Pt A §1.2.9 · PDF p. 11, Annex A Pt B §1.2.9 · PDF p. 99, Annex B Pt A §1.2.9 · PDF p. 175, Annex B Pt B §1.2.8 · PDF p. 251 | 75 % of allowable margins — CSR ships: t_net + 0.5 mm t_net, as in IACS Rec 96 and DNV-CG-0182, where the UR Z10 series writes t_ren. CSR does not define the two symbols alike, so the choice of letter is not cosmetic. Annex A Pt A §1.2.9 · PDF p. 11 | The measurement is widened over the area, before the survey may be credited as finished. Suspect areas carried forward from earlier surveys are examined, and areas already recorded as substantially corroded are gauged again. Where the pattern for that widening is printed depends on the part: the two bulk carrier parts put it in their Annex 10, the two oil tanker parts in their Annex 4. Annex numbering is not shared across the four parts. In the oil tanker parts, Annex 10 is the recommended procedure for recording measurements — PDF pp. 219 and 293 — and not the substantial corrosion extent at all. Annex A Pt A §2.6.3 · PDF p. 16 with Annex 10 · PDF pp. 83–85; Annex B Pt A §2.5.2 · PDF p. 178 with Annex 4 · PDF pp. 194–198 |
| IACS UR Z10.1 Rev.25Oil tankers and ore/oil ships, ESP | A percentage band, and nothing else. What is judged is the corrosion pattern across an area, never one reading; the band opens once that pattern shows more than 75 % of the allowable margin gone, and it closes where the member stops being acceptable. One form only. Z10.1 carries no second sentence for CSR-built ships, unlike Z10.2, Z10.4 and Z10.5. §1.2.9 · PDF p. 7 | 75 % of allowable margins The allowable margin itself is set by the individual Society, not by Z10.1. §1.2.9 · PDF p. 7 | The measurement widens to Table IV, and to whatever else the Survey Programme calls for; none of it can be left until after the survey has been credited. Suspect areas carried forward from earlier surveys are examined, and anywhere already recorded as substantially corroded is gauged again. §2.4.2 · PDF p. 11, Table IV · PDF pp. 28–31 |
| IACS UR Z10.2 Rev.37Bulk carriers, ESP | Two tests, one clause. Off CSR, the pattern has to show more than 75 % of the allowable margin consumed with the member still acceptable. On a ship built to the Common Structural Rules the test is a thickness instead: the measured figure has to land inside the 0.5 mm band that sits directly on top of t_ren. t_ren is fixed one clause earlier, at §1.2.10 — the thinnest, in mm, a structural member may become before it has to be renewed. §1.2.11 · PDF p. 7 | 75 % of allowable margins — CSR ships: t_ren + 0.5 mm §1.2.11 · PDF p. 7 | The measurement widens to Table VIII, and it has to be finished before the survey is credited. On a CSR ship the areas found may be handled two other ways instead: coated to the coating manufacturer's requirements and looked at every year to confirm the coating is holding, or gauged every year. §2.4.2 · PDF pp. 14–15, Table VIII · PDF pp. 60–64 |
| IACS UR Z10.3 Rev.21Chemical tankers, ESP | The percentage band alone. Corrosion assessed over a pattern has to have taken more than 75 % of the allowable margin, with the member still acceptable. One form only. No CSR sentence — CSR does not cover chemical tankers. §1.2.9 · PDF p. 7 | 75 % of allowable margins §1.2.9 · PDF p. 7 | The measurement widens to Table IV, and it has to be finished before the survey is credited. Where the hull structure or the piping is stainless steel, Z10.3 lets the gauging be waived altogether — clad steel plating excepted. §2.4.2 · PDF p. 12, Table IV · PDF pp. 33–36 |
| IACS UR Z10.4 Rev.18Double hull oil tankers, ESP | Both forms sit in the same clause. The pre-CSR one turns on a pattern that has eaten more than 75 % of the allowable margin while the member is still acceptable. For a CSR-built tanker the clause switches to a measured thickness caught between t_ren and t_ren + 0.5 mm. §1.2.10 · PDF p. 6 | 75 % of allowable margins — CSR ships: t_ren + 0.5 mm §1.2.10 · PDF p. 6 | The measurement widens to Table IV, and it has to be finished before the survey is credited. On a CSR ship the areas found are examined and gauged again at each annual and intermediate survey — and here no coating route is offered in place of that. This is the sharpest split in the Z10 series: the bulk carrier texts allow coating in place of annual gauging, the double hull tanker text does not. §2.4.2 · PDF p. 13, Table IV · PDF pp. 41–44 |
| IACS UR Z10.5 Rev.20Double skin bulk carriers, ESP | The same pair as Z10.2. Percentage first — more than 75 % of the allowable margin consumed, the member still acceptable — then, for CSR-built ships, a measured thickness lying in the 0.5 mm band immediately above t_ren. §1.2.11 · PDF p. 7 | 75 % of allowable margins — CSR ships: t_ren + 0.5 mm §1.2.11 · PDF p. 7 | The measurement widens to Table IV, and it has to be finished before the survey is credited. Suspect areas carried forward are examined and anything already recorded in the band is gauged again. On a CSR ship the areas found may instead be coated to the coating manufacturer's requirements and looked at every year to confirm the coating is holding, or gauged every year. The Rev.20 text prints "Table III IV" with III struck through: Table IV is the operative reference. §2.4.2 · PDF p. 15 |
| IACS UR Z7 Rev.29 Corr.1Hull classification surveys — the general requirement | The parent wording of the whole Z series, and the percentage form only: a corrosion pattern showing that more than 75 % of the allowable margin has gone, with the member still acceptable. One form only. No CSR sentence, although Z7 carries an Annex II on net scantling design. §1.2.8 · PDF p. 5 | 75 % of allowable margins §1.2.8 · PDF p. 5 | Readings in the band oblige the surveyor to widen the gauging until the affected areas have been marked out, and Table 2 is put forward as a guide for the extra readings rather than as a rule. The widening has to be complete before the survey can be credited. Z7 says Table 2 "may be used as guidance" — the Z10 series says the Table IV / VIII provisions "are given". §2.2.11 · PDF p. 10, Table 2 · PDF p. 18 |
| IACS UR Z7.1 Rev.15 Corr.1General dry cargo ships | The Z7 sentence, carried over word for word and with no second form: wastage past 75 % of the allowable margins, the member still inside the acceptable limits. Numbered §1.2.9 here and §1.2.8 in the parent Z7; the sentence is the same. No CSR wording is added, and none would bite — general dry cargo ships are outside the Common Structural Rules. §1.2.9 · PDF p. 5 | 75 % of allowable margins §1.2.9 · PDF p. 5 | Once measurements show the band has been entered, the extent is increased until the affected areas have been mapped, with Table III offered as guidance for the extra readings. The same widening reaches suspect areas carried forward from earlier surveys and areas already recorded as substantially corroded, and it has to be finished before the annual survey can be credited as completed. Table III is headed "Guidance for Additional Thickness Measurements in Way of Substantial Corrosion" and prints the same two lines as Z7 Table 2: plating, suspect area plus adjacent plates, five points over a square metre; stiffeners, three readings in line across web and flange. §2.4.2 · PDF p. 10 and §3.2.4 · PDF p. 13, Table III · PDF p. 22 |
| IACS UR Z7.2 Rev.8Liquefied gas carriers | Again the Z7 sentence unchanged, and again one form only: wastage past 75 % of the allowable margins, the member still inside the acceptable limits. Liquefied gas carriers sit outside CSR, so no second form appears. BV, by contrast, pulls gas carriers contracted on or after 1 July 2022 into its own renewal-thickness band — see the BV row. §1.2.8 · PDF p. 5 | 75 % of allowable margins §1.2.8 · PDF p. 5 | The extent is increased until the areas of substantial corrosion have been delineated, with Table V offered as guidance. Suspect areas carried forward are gauged on the same footing, and the extra readings have to be taken before the annual survey is credited as completed. Table II carries a third note to the same effect, worded without a table: the extent is to be increased to the Surveyor's satisfaction — PDF p. 18. §2.4.2 · PDF p. 10 and §3.2.5 · PDF p. 11, Table V · PDF p. 22 |
| IACS Common Structural RulesBulk carriers and double hull oil tankers, contracted on or after 1 July 2015 | The term "substantial corrosion" does not appear in the Rules. [2.1.3] states the band and heads the consequence "Alternative solutions". The label is attached to this band by UR Z10.2, Z10.4 and Z10.5, not by CSR. Pt 1 Ch 13 Sec 2 [2.1.3] · PDF p. 723 | t_ren ≤ t_m < t_ren + t_res, with t_res = 0.5 mm and t_ren = t_as-built − t_c − t_vol_add Pt 1 Ch 13 Sec 2 [2.1.1] and [2.1.3] · PDF pp. 722–723, Symbols · PDF p. 721 | Renewal is not the only way out of this band. CSR allows alternative solutions but never spells them out — it defers instead to whatever the individual Society's own rules or documents provide, meaning the texts in which that Society has enacted IACS UR Z10.2, Z10.4 and Z10.5. CSR supplies the arithmetic and hands the survey consequence back to the Society. Pt 1 Ch 13 Sec 2 [2.1.3] · PDF p. 723 |
| IACS Recommendation 76Bulk carriers — surveys, assessment and repair | guidanceThe percentage form only, and worded a shade apart from the URs: material wastage read off the pattern has to run past 75 per cent of the allowable margins while the member stays acceptable. "material wastage", and "per cent" written out — the only IACS text in this set to word it that way. Rev.2 predates CSR, so no second form is given. §3.2.1 (h) · PDF p. 12 | guidance75 per cent of allowable margins §3.2.1 (h) · PDF p. 12 | guidanceWhere the coating in a salt water ballast space has broken down, or an area of structure is substantially corroded, the item is written down so that it gets particular attention through the next cycle of surveys — unless it is put right at the survey itself. The clause joins the two limbs with "and/or": the area may qualify because it is substantially corroded, because the surveyor judges it liable to waste away fast, or on both counts. It is not an either-or. §2.4.4 · PDF p. 10 |
| IACS Recommendation 96Double hull oil tankers — surveys, assessment and repair | guidanceBoth forms, as in UR Z10.4, but the CSR one is written on t_net. Percentage first: the pattern showing more than 75 % of the allowable margins gone, the member still acceptable. Then, for CSR-built ships, a gauged or measured thickness falling between t_net and t_net + 0.5 mm. t_net, where UR Z10.4 writes t_ren for the same band. The two symbols are defined differently in CSR. §3.2.1 (h) · PDF p. 16 | guidanceBoth ends given as percentages: from 75 % to 100 % of the diminution allowed for the member being judged Rec 96 is the only source in this set that states the band with both ends as percentages. §2.1.5 · PDF p. 11 | guidanceFind it in a ballast tank at one survey and that tank comes up for examination every year afterwards. It is one of four triggers §2.2.3 lists for annual examination of a ballast tank, the other three all turning on the coating. §2.2.3 (c) · PDF p. 12 |
| ABSRules for Survey After Construction, Part 7 — July 2026 | ABS carries both forms in one definition. The pattern has to show wastage past 75 % of the allowable margins with the member still inside the limits ABS accepts; on a CSR-built ship that percentage gives way to a measured thickness lying between t_ren and t_ren + 0.5 mm. The clause carries a note naming where the allowable margins live: 7-A1-4/35.3 TABLE 1A through TABLE 4. 7-1-1/3.64 · PDF p. 21 | 75 % of allowable margins — CSR ships: t_ren + 0.5 mm — Great Lakes vessels: 80 % Worked example in the appendix: a 12 mm transverse bulkhead plate with a 25 % allowance is in the substantial corrosion range from 9.75 mm down to 9.0 mm. Great Lakes vessels are carved out at 7-A1-4/17.3 and given 80 % of the allowable margins instead of 75 %, which narrows the band; the carve-out is withdrawn from vessels trading east of Montreal in the St. Lawrence River and from vessels carrying salt cargoes, both of which follow the salt water rules. 7-1-1/3.64 · PDF p. 21, with 7-A1-4/17.3 · PDF p. 586 | Extra readings to the extent 7-3-2/7 sets, taken to fix how far the corrosion runs, and the survey is not credited until they are in. One number on its own is not a pattern: where a single reading sits on the margin, the surveyor has to take more and weigh them together with what close visual examination shows. Two carve-outs sit beside the main rule. A ship built with reduced scantlings may already be inside the band as built; ABS then holds back the extra survey and gauging until the coating over that area falls to FAIR — 7-A1-4/17.1, PDF p. 585. And on Great Lakes vessels the follow-up gauging is split by member: longitudinal strength members within 0.6L amidships at every annual survey, the rest at special and intermediate surveys at intervals not over three years — 7-A1-4/17.3, PDF p. 585. 7-A1-4/17.1 · PDF p. 585, with 7-3-2/7 · PDF p. 154 onward |
| BVNR467 Part A — R26, July 2026 | The percentage form as the URs have it — the pattern showing wastage past 75 % of the allowable margins, the member still inside the limits BV accepts — and then a thickness form, which BV extends to eight named ship groups, well beyond CSR ships: a measured thickness between t_renewal and t_renewal + 0,5 mm, t_renewal being the thinnest, in mm, a member may become before renewal. The eight groups reach far beyond CSR ships: liquefied gas carriers contracted on or after 1 Jul 2022; oil tankers to NR523; bulk carriers to NR522; bulk carriers or oil tankers to NR606; cargo ships other than container ships over 65 m contracted on or after 1 Jul 2022; container ships over 65 m contracted on or after 1 Jul 2016; ships with alternate light or heavy cargo loading conditions of any length contracted on or after 1 Jul 2022; non-cargo ships over 90 m contracted on or after 1 Jul 2022. Pt A Ch 2 Sec 2 [2.2.7] · PDF p. 107 | 75 % of allowable margins — named groups: t_renewal + 0,5 mm Pt A Ch 2 Sec 2 [2.2.7] · PDF p. 107 | Areas judged to be carrying the finding are measured further. How far the extent is widened is left to the Surveyor's satisfaction, with Ch 3 Sec 3 Tab 4 as the guide, and Tab 1 repeats that line for the annual, intermediate and class renewal survey alike. The location also becomes a suspect area under [2.2.11]. Pt A Ch 2 App 3 §2 · PDF pp. 130–132, with Pt A Ch 3 Sec 3 Tab 4 · PDF p. 182 |
| CCSGuidelines for Thickness Measurement of Hull — Ver 6.6, March 2025 | Both forms, in a Guidance document rather than in the Rules. Percentage first: the pattern showing more than 75 % of the allowable margins gone, the member still acceptable. For CSR ships CCS words the second form as a wastage — not as a thickness, which is how everyone else writes it — running between t_ren and t_ren + 0.5 mm. The clause is drawn twice, as Sketch Map 1.1 for non-CSR ships and Sketch Map 1.2 for CSR ships, on the following page. Chapter One, definitions item 8 · PDF p. 7 | 75 % of allowable margins — CSR ships: t_ren + 0.5 mm Sketch Map 1.1 splits the allowable band into four quarters and marks the top quarter as the substantial corrosion area. Chapter One, definitions item 8, with Sketch Maps 1.1 and 1.2 · PDF pp. 7–8 | The duty falls on the measurement team, not on the surveyor. They check the readings again, work out how far the affected areas run, draw them on a sketch and tell the surveyor in charge of the job. And once a result nears or reaches the band, the owner is told at once so that the measurement can be extended and the average condition of the suspect area settled. Chapter Two §IV Thickness measurement on site · PDF pp. 21–22 |
| DNVDNV-CG-0182 Allowable thickness diminution for hull structure | older edition — Jul 2022not published The term "substantial corrosion" appears nowhere in this class guideline. Its Table 4 names four corrosion types — edge, general, groove and pit — carries "grooving" and "pitting" as second names for two of them, and sets separate criteria and repairs for each type. The band itself is stated without the label; see the trigger column. whole document searched · 46 pages | older edition — Jul 2022Gauged thickness within the range t_net + 0.5 mm to t_net + 1.0 mm Stated for hatch covers and coamings. Where the corrosion addition t_c = 1.0 mm, the guideline states the coating-or-annual-gauging band as t_net to t_net + 0.5 mm. Sec.8 · PDF p. 25 | older edition — Jul 2022Steel has to be renewed. DNV lets two things stand in its place: coating laid on to the coating manufacturer's requirements, or gauging repeated every year. Where the coating route is taken, it has to be kept in good condition to the standard DNV-RU-SHIP Pt.7 Ch.1 Sec.1 Table 1 sets. Sec.8 · PDF p. 25 |
| INSBPart I — Issue 04 Rev.03, December 2024 | Two chapters, worded differently. Chapter 3 gives the percentage alone — the pattern showing wastage past 75 % of the allowable margins, the member still acceptable. Chapter 5, which holds the periodical survey regulations for the ESP ship types, repeats that and then adds the CSR band: from t_ren + 0.5 mm down to t_ren, with t_ren set on the next line as the lowest thickness a member may keep before it has to be renewed. Two chapters, two definitions. The general classification chapter stops at the percentage; the ESP chapter carries both forms. Pt I Ch 3 Sec 1 §1.2.8 · PDF p. 47, with Pt I Ch 5 Sec 1 §1.2.1 · PDF p. 89 | 75 % of allowable margins — CSR ships: t_ren + 0.5 mm A CSR-built ship in INSB class takes the CSR band, printed in the Chapter 5 definitions list. Reading only the Chapter 3 definition would leave that band out. Pt I Ch 3 Sec 1 §1.2.8 · PDF p. 47, with Pt I Ch 5 Sec 1 §1.2.1 · PDF p. 89 | The extra readings follow Table E. Suspect areas carried forward from earlier surveys are examined, anything already in the band is gauged again, and the gauging widens until the affected areas have been marked out — all of it before the annual survey can be credited. Pt I Ch 3 §2.2.14 · PDF p. 50 and Sec 13 §13.1.5 · PDF p. 80, Table E · PDF p. 83 |
| KRRules for the Classification of Steel Ships, Part 1 — 2026 | Both forms, with KR naming its own CSR parts. Percentage first: the pattern showing wastage past 75 % of the allowable margins, the member still acceptable. For a ship built to the Common Structural Rules — KR Pt 11, Pt 12 or Pt 13 — the test becomes a measured thickness between t_ren and t_ren + 0.5 mm, and the same item fixes t_ren as the thinnest, in mm, a member may become before renewal. Pt 1 Ch 2, definitions item 14 · PDF p. 30 | 75 % of allowable margins — CSR ships: t_ren + 0.5 mm Guidance, not Rule, and written in the "should" form: the term is used again on the repair side, in the Annex 1-5 pages that print the Guidance footer. Where a member is renewed or reinforced, the diminution at the butt joint of each member it lands on should stay outside the substantial corrosion range, which the Guidance reads as 75 % of that member's allowable diminution; where a strap is fitted, the deck or bottom plating it reinforces should likewise be outside the range. Pt 1 Annex 1-5 (4)(B) · PDF p. 281 and (4)(C)(b) · PDF p. 282. The definitions item 14 at PDF p. 30 carries no repair rule. Pt 1 Ch 2, definitions item 14 · PDF p. 30 | Once a reading lands in the band, the gauging widens until the corroded areas have been mapped out, and Table 1.2.5 is offered as a guide for the extra readings. That work has to be finished before the Annual Survey can be credited. The location is also a suspect area under item 13. The item does not reach the cargo tanks of oil tankers, chemical tankers and double hull oil tankers: those are surveyed under Ch 3 Sec 3, Sec 4 and Sec 5, and a CSR-built bulk carrier is sent to the CSR clauses of Ch 3 instead. The carve-out is printed as a note under item (34): the requirements are not applicable to the cargo tanks of those three ship types. A surveyor reading item (34) on a tanker is reading the wrong clause. The CSR redirection in the same item sends single skin bulk carriers to Ch 3 202.5, 203.4 (c), 204.5 (2) and double skin bulk carriers to Ch 3 602.5, 603.4 (c), 604.5 (2). Pt 1 Ch 2, Annual Survey item (34) Suspect areas · PDF p. 44, with definitions item 13 · PDF p. 30 |
| LRThickness Measurement and Close-up Survey Guidance, Part 1 — Ver 7.3 | older edition — Nov 2016 v7.3LR takes the percentage off a different quantity and applies it item by item: a plate or a stiffener is in the band once more than 75 % of the diminution permitted for that item has been lost. "permissible diminution", not "allowable margins", and it is named per plate and per stiffener rather than per corrosion pattern. LR is the only source in this set that drops the "but within acceptable limits" tail from the non-CSR sentence. Pt 1 Ch 5 Sec 1 §1.5 · PDF p. 25 | older edition — Nov 2016 v7.375 % of the permissible diminution — CSR ships: t_renewal < t_gauged < t_renewal + t_reserve The CSR form is written with strict inequalities at both ends, where the Z10 series and CSR itself close the band at the lower end. Pt 1 Ch 5 §1.5 · PDF p. 25 and the CSR definitions · PDF pp. 38–39 | older edition — Nov 2016 v7.3The extra gauging the Rules and Regulations for the Classification of Ships call for has to be done, and until it is the survey stays open. Any structure carrying the finding is then taken to close-up survey and gauging at the annual survey and at every survey after it, with representative readings taken to show how far the corrosion reaches. On CSR ships LR accepts coating laid on to the coating manufacturer's requirements, or gauging every year, in place of steel renewal — the coating kept in good condition and examined annually by the surveyor. Pt 1 Ch 5 §1.5 · PDF p. 25, with §1.2.2.1 · PDF p. 6 |
| ClassNKGuidance for Undergoing Class Maintenance Surveys | older edition — 2014The percentage form, printed inside a Guidance document: wastage read off the pattern running past 75 % of the allowable margins while the member stays inside acceptable limits. Figure 1 at PDF p. 16 restates it and adds a second form — a gauged thickness between the renewal thickness and a value above it — for three named ship groups. Appendix 3.5 (3) · PDF p. 177 | older edition — 201475 % of allowable margins The allowable margins are the wastage allowances of Appendix 3.5 (1) on the same page: 20 % + 1 mm, 25 % or 30 % of original thickness by member. Appendix 3.5 (3) · PDF p. 177 | older edition — 2014Anything past 75 % of the allowable margin has to be written up as a suspect area. From then on the area is gauged, and inspected as far as is necessary, at every survey that follows — annual, intermediate and special alike. The clearest statement in this set that the finding travels forward to every survey, not only to the next special survey. Appendix 3.5 (3)(ii) · PDF p. 178 |
| ClassNKRules and Guidance, Part B — Class Surveys | older edition — 2022The percentage form first — the pattern showing wastage past 75 % of the allowable margins, the member still acceptable. For three groups the clause then overrides that with a thickness test, expressed as a 0.5 mm range measured up from the renewal thickness the relevant provisions set. The three: ships built to Part CSR-B, Part CSR-T or Part CSR-B&T; hatch covers and hatch coamings for cargo holds of ships the Society has stipulated otherwise for; and transverse watertight bulkheads in cargo holds built to Chapter 31A or Chapter 31B of Part C. The second form is written as a range measured upward from the renewal thickness rather than as a thickness between two named values. The same item fixes the renewal thickness as the lowest a member may reach before it is renewed. Pt B Ch 1 §1.3.1 (6) · PDF p. 10 | older edition — 202275 % of allowable margins — named groups: 0.5 mm above the renewal thickness Pt B Ch 1 §1.3.1 (6) · PDF p. 10 | older edition — 2022The gauging widens under Tables B5.16 through B5.20, taken by the structural member the corrosion was found in. On a bulk carrier built under Part CSR-B or Part CSR-B&T, the areas found go one of two ways: coated to the coating manufacturer's requirements and checked every year that the coating is holding, or gauged every year. Pt B Table B4.4 Thickness Measurements · PDF p. 76, with Tables B5.16–B5.20 cited at PDF p. 8 |
| PRSPart I — Classification Regulations, January 2026 | The percentage form only: the pattern showing wastage past 75 % of the allowable margins, the member still inside acceptable limits. One form only. PRS Part I gives no separate wording for CSR-built ships; the definition sits in the alphabetical general list, not in a survey chapter. Pt I Ch 1, Definitions · PDF p. 15 | 75 % of allowable margins Pt I Ch 1, Definitions · PDF p. 15 | Where gauging shows the band has been entered, or wastage beyond the allowable diminution, the surveyor picks the further locations needed to map the corroded areas out and to settle which members go for repair or renewal. The pattern is written into the Rules: over plating, the suspected area and the plates beside it, five points to the square metre; over a stiffener, three readings in line on the web and three on the flange. Pt I §5.1.3.7 · PDF p. 44, with §5.1.3 · PDF p. 43 |
| RINAPart A Ch 2 App 2 — 1 July 2024 interactive set | older edition — 1 Jul 2024not published Appendix 2 cites the definition rather than carrying it. Where it lists what a class renewal survey has to measure, the last item is the extra measurement on areas found to be affected, and for the meaning of the term it sends the reader to Sec 2, [2.2.9]. Pt A Ch 2 Sec 2 is not held, so the wording itself was not read. Pt A Ch 2 App 2 §2.2.1 and §2.3.1 · PDF p. 1 | older edition — 1 Jul 2024not published The number sits in Pt A Ch 2 Sec 2 [2.2.9], outside the file held. Pt A Ch 2 App 2 §2.2.1 · PDF p. 1 | older edition — 1 Jul 2024Measurements are added on the areas found to be affected, and the extent may be widened as far as the Surveyor is satisfied. Which table guides that widening is decided by service notation in Tab 1 of the appendix, not by whether the ship is under ESP: Ch 3 Sec 5 Tab 3 serves every notation Tab 1 does not list separately, while each ESP notation has its own set — Ch 4 Sec 2 Tab 7 to Tab 11 for bulk carrier and ore carrier ESP, Ch 4 Sec 9 Tab 5 to Tab 8 for double skin bulk carrier ESP, Ch 4 Sec 3 Tab 5 to Tab 8 for oil tanker and combination carrier ESP, Ch 4 Sec 4 Tab 4 to Tab 8 for double hull oil tanker ESP, Ch 4 Sec 5 Tab 5 to Tab 8 for chemical tanker ESP. Ch 4 Sec 8 Tab 3 belongs to the "general dry cargo ships" line of Tab 1, which is not an ESP notation. The liquefied gas carrier line names no substantial corrosion table at all. Pt A Ch 2 App 2 §2.2, §2.3 and Tab 1 · PDF pp. 1–3 |
Where the band opens — the arithmetic as the sources state it
The 75 % rule is a percentage of a percentage, and it is easy to compute the wrong way round. Only the sourced statements are here; nothing in this block is derived. 12 rows · every value carries its clause and PDF page
| Society · document | What the source statesno arithmetic added |
|---|---|
| The rule itselfThe percentage is taken of the allowable margin, not of the as-built thickness | Wastage running beyond 75 % of the allowable margins, the member still inside acceptable limits — the percentage bites on the margin, never on the plate §1.2.11 · PDF p. 7 |
| Worked example — plate still acceptable12 mm transverse bulkhead plate on a tanker, individual plate wastage allowance 25 % | Gauged 9.75 to 12 mm — diminution 0 % to 18.75 %, that is 75 % of 25 %. The plate had some corrosion. 7-A1-4/17.1 · PDF p. 585 |
| Worked example — substantial corrosion rangeSame plate | Gauged 9.0 to 9.75 mm — diminution 18.75 % to 25 %. The wastage of the plate was in the substantial corrosion range. The band opens at 9.75 mm exactly, with no measurement margin added. 7-A1-4/17.1 · PDF p. 585 |
| Worked example — renewalSame plate | Gauged less than 9.0 mm — diminution greater than 25 %. The wastage of the plate was in the renewal limit. 7-A1-4/17.1 · PDF p. 585 |
| One reading is not a patternThe definition turns on a corrosion pattern, and a pattern needs more than one number | One reading on its own does not make a corrosion pattern. Where a single reading sits on the margin, more have to be taken and judged alongside the surveyor's close visual examination, so that the reach of the corrosion can be settled. 7-A1-4/17.1 · PDF p. 585 |
| The band drawn as quartersCCS splits the allowable band into four and marks the last quarter | Sketch Map 1.1 divides the range between the renewal thickness and the original thickness into four quarters; the quarter nearest the renewal thickness is marked "substantial corrosion areas" and labelled "75 % of allowable limits". Everything below the renewal thickness is marked "excessive corrosion areas". Sketch Map 1.1 · PDF p. 8 |
| On a CSR ship the band does not scaleThe width is fixed by the reserve thickness, whatever the member and whatever the as-built thickness | t_res = 0.5 mm, a flat figure independent of member and of as-built thickness Pt 1 Ch 3 Sec 3, Symbols · PDF p. 101, with Pt 1 Ch 13 Sec 2 [2.1.3] · PDF p. 723 |
| The allowable margins the percentage is taken of — ABSThe 7-1-1/3.64 note names them | The wastage allowances themselves sit in 7-A1-4/35.3, Tables 1A through 4 7-1-1/3.64, Note · PDF p. 21 |
| The allowable margins the percentage is taken of — ClassNKUniform wear of principal structural hull members | older edition — 201420 % of original thickness + 1 mm for shell plates, strength deck plates, longitudinal beams on sheer strake and strength deck, tight bulkheads in deep tanks and inner bottom plates; 25 % for floors and girders in double bottom, primary members, hold frames and watertight bulkhead plates; 30 % for other frames, longitudinal beams, stiffeners and brackets, effective deck plates, hatch covers and hatch beams Appendix 3.5 (1) · PDF p. 177 |
| Local corrosion sits outside the band — ClassNKPitting and stress corrosion are handled separately | older edition — 2014Local corrosion — stress corrosion and pitting among it — is left to the Surveyor's discretion rather than to the table. For local corrosion other than stress corrosion the standard limit is 40 % of the original thickness Appendix 3.5 (1)(iv) · PDF p. 177 |
| What the percentage is taken of — LRLR names a different quantity | older edition — Nov 2016 v7.375 % of the permissible diminution, read plate by plate and stiffener by stiffener. LR adds that the maximum diminutions in its tables already carry an allowance for the average corrosion expected over another five years. Pt 1 Ch 5 Sec 1 §1.5 · PDF p. 25 |
| Thinner members reach the band firstA consequence of expressing the limit as a percentage | A limit set as a percentage is reached earlier by a thin member than by a thick one, at any given rate of loss per surface. A member losing metal off both faces at that rate runs to the limit twice as fast. That is why internals in tankers — thinner to start with, and attacked on both sides — usually get there well ahead of the plating beside them. 7-A1-4/17.1 · PDF p. 585 |
What the finding costs in measurement — the expansion each society calls for
Every society expands the measurement once the band is entered. The pattern is close to identical across the general tables; the ESP ship-type tables are member by member and are summarised here by their first sheet. 14 rows · every value carries its clause and PDF page
| Society · document | Platingextent and pattern | Stiffenersextent and pattern | Where the table sitsand whether it binds |
|---|---|---|---|
| IMO 2011 ESP CodeBulk carriers, Annex A part A — shell plating sheet | Bottom and side shell plating: the suspect plate together with four plates next to it. Five points on each panel between longitudinals. Annex 10 · PDF p. 83 | Bottom and side shell longitudinals: at least three longitudinals in way of the suspect areas. Three readings in line across the web and three on the flange. Annex 10 · PDF p. 83 | Annex 10 in each of the two bulk carrier parts; Annex 4 in each of the two oil tanker parts. The Code is mandatory under SOLAS chapter XI-1, so these are not guidance tables. The sheets are the parent of the UR Z10 tables and read the same, member for member. Annex A Pt A Annex 10 · PDF pp. 83–85, Annex A Pt B Annex 10 · PDF pp. 168–169, Annex B Pt A Annex 4 · PDF pp. 194–198, Annex B Pt B Annex 4 · PDF pp. 271–272 |
| IACS UR Z7General hull classification surveys | Suspect area and adjacent plates. 5 point pattern over 1 square meter. Table 2 · PDF p. 18 | Suspect area. 3 measurements each in line across web and flange. Table 2 · PDF p. 18 | Table 2, headed "guidance for additional thickness measurements in way of substantial corrosion" — it may be used as guidance §2.2.11 · PDF p. 10, Table 2 · PDF p. 18 |
| IACS UR Z10.1Oil tankers, ESP — bottom structure sheet | Bottom plating. Extent: three bays at least, spread across the tank, the aft bay among them, and every bell mouth gauged all round and underneath. Pattern: 5 points to each panel framed by longitudinals and webs. Table IV / Sheet 1 · PDF p. 28 | Bottom longitudinals. Extent: three at least in every bay whose bottom plating was gauged. Pattern: three readings in line across the flange, three more on the web. Table IV / Sheet 1 · PDF p. 28 | Table IV, four sheets — bottom, deck, side shell and longitudinal bulkheads, transverse and swash bulkheads. Provisions "are given", not offered as guidance. §2.4.2 · PDF p. 11, Table IV · PDF pp. 28–31 |
| IACS UR Z10.2Bulk carriers, ESP — shell structures sheet | Bottom and side shell plating. Extent: the suspect plate and the four plates around it. Pattern: five points to each panel between longitudinals. Gauging in way of tanks and cargo holds is set out on the other sheets. Table VIII / Sheet 1 · PDF p. 60 | Bottom and side shell longitudinals. Extent: three at least serving the suspect areas. Pattern: three readings in line on the web, three on the flange. Table VIII / Sheet 1 · PDF p. 60 | Table VIII, five sheets — shell, transverse bulkheads in cargo holds, deck structure with hatch covers and coamings, double bottom and hopper, cargo holds Table VIII · PDF pp. 60–64 |
| IACS UR Z10.3Chemical tankers, ESP — bottom, inner bottom and hopper sheet | Plating of the bottom, the inner bottom and the hopper structure. Extent: no fewer than three bays taken across the tank, the aft bay among them, and every suction bell mouth gauged around and beneath. Pattern: 5 points to each panel framed by longitudinals and floors. Table IV / Sheet 1 · PDF p. 33 | Longitudinals of those same three structures. Extent: no fewer than three in each bay whose bottom plating was gauged. Pattern: three readings in line across the flange, three on the vertical web. Table IV / Sheet 1 · PDF p. 33 | Table IV, four sheets — bottom / inner bottom / hopper, deck, side shell and longitudinal bulkheads, transverse watertight and swash bulkheads §2.4.2 · PDF p. 12, Table IV · PDF pp. 33–36 |
| IACS UR Z10.4Double hull oil tankers, ESP — deck structure sheet | Deck plating. Extent: two bands run transversely across the tank. Pattern: in each band, every plate takes three readings at least. Table IV / Sheet 2 · PDF p. 42 | Deck longitudinals. Extent: every third one in each of the two bands, and never fewer than one. Pattern: three readings in a vertical line on the web, two on the flange where a flange is fitted. Table IV / Sheet 2 · PDF p. 42 | Table IV, four sheets — bottom / inner bottom / hopper, deck, structure in wing ballast tanks, longitudinal bulkheads in cargo tanks §2.4.2 · PDF p. 13, Table IV · PDF pp. 41–44 |
| IACS UR Z10.5Double skin bulk carriers, ESP — bottom, inner bottom and hopper sheet | Plating of the bottom, the inner bottom and the hopper structure. Extent: three bays at least taken across the double bottom tank, the aft bay among them, plus readings all round and under every suction bell mouth. Pattern: five points to each panel framed by longitudinals and floors. Table IV / Sheet 1 · PDF p. 50 | Longitudinals of those same three structures. Extent: three at least in every bay whose bottom plating was gauged. Pattern: three readings in line across the flange, three on the vertical web. Table IV / Sheet 1 · PDF p. 50 | Table IV, four sheets — bottom / inner bottom / hopper, deck with hatch covers and coamings, structure in double side spaces, transverse bulkheads in cargo holds. The Rev.20 text prints the number as "Table III IV". Table IV · PDF pp. 50–53 |
| ABSPart 7 — July 2026 | A five point pattern of thickness measurements on each panel of the strake within the bay, once a strake is identified with wastage in the substantial corrosion range 7-A1-4/17.1 · PDF p. 586 | If substantial corrosion is found on plating, additional measurements extend not only to surrounding plating but also to internal members; where it is identified on internals, measurements extend to other similar and thinner internals 7-A1-4/17.1 · PDF p. 585 | 7-3-2/7, the clause headed "Extent of Thickness Measurement Requirements for those Areas of Substantial Corrosion" and applying inside the cargo area — split by ship type, and binding ABS varies the density of readings with the loss found: the full five point pattern only comes in once a first assessment turns up readings inside the band. 7-3-2/7 · PDF p. 154 onward, contents at PDF p. 44 |
| BVNR467 Part A — R26, July 2026 | Suspect area and adjacent plates. 5 point pattern over 1 square metre. Pt A Ch 3 Sec 3 Tab 4 · PDF p. 182 | Suspect area. 3 measurements each in line across web and flange. Pt A Ch 3 Sec 3 Tab 4 · PDF p. 182 | Tab 4, "guidance for additional thickness measurements in way of substantial corrosion areas" — the extent may be increased to the Surveyor's satisfaction using it as guidance Pt A Ch 2 App 3 §2 · PDF p. 131, Tab 4 · PDF p. 182 |
| INSBPart I — December 2024 | Suspect area and adjacent plates. Five-point pattern over 1 square metre. Pt I Ch 3 Sec 13 Table E · PDF p. 83 | Suspect area. Three measurements in line across each flange and web. Pt I Ch 3 Sec 13 Table E · PDF p. 83 | Table E, "guidance for additional thickness measurements in way of substantial corrosion" §2.2.14 at PDF p. 50 points to "Table E of Sec 12"; Table E is printed in Section 13. The cross-reference in the held edition does not match the table's own section number. Pt I Ch 3 Sec 13 §13.1.5 · PDF p. 80, Table E · PDF p. 83 |
| KRRules Part 1 — 2026 | Suspect area and adjacent plates. 5 point pattern over 1 m². Pt 1 Ch 2 Table 1.2.5 · PDF p. 69 | Suspect area. 3 measurements each in line across web and flange. Pt 1 Ch 2 Table 1.2.5 · PDF p. 69 | The guide offered is Table 1.2.5. On CSR bulk carriers the identified areas are instead handled under Ch 3, 202.5, 203.4(c) and 204.5(2) for single side skin, or 602.5, 603.4(c) and 604.5(2) for double side skin. Pt 1 Ch 2, item (34) · PDF p. 44, Table 1.2.5 · PDF p. 69 |
| PRSPart I — January 2026 | Plating: the suspected area plus the plates adjoining it, 5 points to the square metre. Pt I §5.1.3.7 · PDF p. 44 | Stiffener suspect area: 3 readings in line on the web and 3 on the flange. Pt I §5.1.3.7 · PDF p. 44 | No table — the pattern is written into the clause as a requirement, in the shall form PRS is the only source in this set that states the pattern as rule text rather than as a guidance table. Pt I §5.1.3.7 · PDF p. 44 |
| CCSTM Guidelines Ver 6.6 — March 2025 | Shell plating and longitudinal bulkheads: five-point pattern over about 2 m² areas ship-type measurement table, item c) · PDF p. 64 (printed p. 59) | General corrosion in the web plating pushes the pattern out to five points. Each section then takes 2 readings at least on the flange or faceplate, and the plating the stiffener is attached to takes one at least. ship-type measurement table · PDF p. 102 (printed p. 97) | The measurement extent sits in the ship-type sections of the Guidelines; the substantial corrosion duty is a reporting and notification duty on the measurement personnel Chapter Two §IV · PDF pp. 21–22, with the ship-type measurement tables · PDF pp. 64, 102 |
| RINAPart A Ch 2 App 2 — 1 July 2024 | older edition — 1 Jul 2024not published Tab 1 only names the tables — Ch 3 Sec 5 Tab 3 for the ordinary notations, and a different Ch 4 table for each ESP notation. None of those chapters is held, so no pattern was read. Pt A Ch 2 App 2 Tab 1 · PDF pp. 2–3 | older edition — 1 Jul 2024not published Same reason — the table sits outside the file held. Pt A Ch 2 App 2 Tab 1 · PDF pp. 2–3 | older edition — 1 Jul 2024Tab 1 routes each service notation to its own table. Ch 3 Sec 5 Tab 3 covers every notation Tab 1 does not list on a line of its own. "General dry cargo ships" have their own line and go to Ch 4 Sec 8 Tab 3 — that is a dry cargo table, not an ESP one. The ESP notations take Ch 4 Sec 2 Tab 7 to Tab 11 (bulk carrier and ore carrier), Ch 4 Sec 9 Tab 5 to Tab 8 (double skin bulk carrier), Ch 4 Sec 3 Tab 5 to Tab 8 (oil tanker, OBO and OOC), Ch 4 Sec 4 Tab 4 to Tab 8 (double hull oil tanker) and Ch 4 Sec 5 Tab 5 to Tab 8 (chemical tanker). All of them are named as guidance for widening the extent. Pt A Ch 2 App 2 Tab 1 · PDF pp. 2–3 |
Two forms, one name. Every source in this table carries the pre-CSR form — wastage past 75 % of the margin the society allows, the member still acceptable — and most of them add a second form for ships built to Common Structural Rules, expressed as a measured thickness band above the renewal thickness. The two are not interchangeable: the first is a percentage of a wastage allowance the society publishes elsewhere, the second is a fixed 0.5 mm band above a thickness computed from the as-built scantling. Which form applies is decided by the ship, not by the surveyor. What CSR itself says. The IACS Common Structural Rules never use the term "substantial corrosion". Pt 1 Ch 13 Sec 2 [2.1.3] gives the band t_ren ≤ t_m < t_ren + t_res and heads the consequence "Alternative solutions", sending the reader on to whichever of the individual Society's rules or documents enact UR Z10.2, Z10.4 and Z10.5. The label is applied to that band by the Z10 series, not by CSR. Every row below states what its document requires, in this handbook's words; the clause and page against each cell take the reader to the document's own wording. Symbols differ across IACS's own documents. UR Z10.2, Z10.4 and Z10.5 write the CSR band as t_ren + 0.5 mm to t_ren. IACS Rec 96 writes the same band as t_net + 0.5 mm to t_net, and so does the IMO ESP Code. DNV-CG-0182 also uses t_net. The two symbols are not defined identically — t_ren in CSR is t_as-built − t_c − t_vol_add, while t_net is the net thickness. Read the symbol the document itself defines. Editions. Five rows carry an older-edition badge on every cell; the badge names the edition held. A newer edition exists for each of them and sits behind a member login. Values from those rows should be re-checked against the society's current text before being quoted to a surveyor. Guidance versus requirement. Cells tagged guidance come from IACS Recommendations, which are not Unified Requirements and bind no society by themselves.