What counts as substantial corrosion in each class?

A band, not a limit

Say this first, because it is the part that gets lost. A plate in substantial corrosion is still acceptable. The wording every society uses closes with the words "but within acceptable limits" — the wastage has passed 75% of what is allowed, and it has not passed 100%. No renewal is owed on that reading alone. What changes is what the surveyor has to do next. Z10.2 §1.2.11 · Z7 §1.2.8

Three things follow from the finding, and they are the whole cost of it. The measurement is expanded on the spot, to a pattern the society prints in a table, and the survey is not credited until the expansion is done. The location becomes a suspect area — every society in this set defines a suspect area as a location showing substantial corrosion or considered by the surveyor to be prone to rapid wastage. And the area is carried forward, to be examined and re-gauged at the surveys that follow. Z10.2 §1.2.8 / §2.4.2

One more consequence catches people out because it is filed under a different definition. On bulk carriers and double skin bulk carriers, a space used for both cargo and salt water ballast is treated as a ballast tank once substantial corrosion has been found in it — which pulls the whole ballast-tank survey regime, coating condition and annual examination onto a space that was being surveyed as a hold. Z10.2 §1.2.2

Two forms under one name

The term carries two completely different definitions, and most societies print both of them in the same clause, one sentence after the other. They are not interchangeable and they do not produce the same answer on the same plate. Which one applies is decided by the ship — how and when it was built — not by the surveyor and not by the member being gauged.

The IACS wording for the second form is a single added sentence — for vessels built under the Common Structural Rules, substantial corrosion is a measured thickness between t_ren + 0.5 mm and t_ren, with t_ren defined one clause earlier as the minimum allowable thickness below which renewal is to be carried out. Only three of the five Z10 texts carry it — Z10.2, Z10.4 and Z10.5, the bulk carrier, double hull tanker and double skin bulk carrier requirements. Z10.1 for oil tankers, Z10.3 for chemical tankers, Z7 for general hull classification surveys and the two ship-type companions of Z7 — Z7.1 for general dry cargo ships and Z7.2 for liquefied gas carriers — carry the first sentence only. Neither of those two ship types falls under the Common Structural Rules, so there is nothing for a second sentence to bite on. Z10.2 / Z10.4 / Z10.5 §1.2 · Z7.1 §1.2.9 · Z7.2 §1.2.8

The band also sits in the one text on this page that is not a class document. The IMO 2011 ESP Code is mandatory through SOLAS chapter XI-1, so it reaches the flag State and the Administration and not only the society's contract with the owner — and it carries both forms. Three of its four parts add the CSR sentence, and all three write the band against t_net where Z10.2, Z10.4 and Z10.5 write t_ren. The fourth part, the one for oil tankers that are not double-hull, stops at the percentage. ESP Code §1.2.9 ×3 · §1.2.8

Two societies do not stop at CSR ships. BV attaches the band form to eight named ship groups, most of which are not CSR ships at all — liquefied gas carriers contracted on or after 1 July 2022; oil tankers built to NR523; bulk carriers built to NR522; bulk carriers or oil tankers built to NR606; cargo ships other than container ships over 65 m contracted on or after 1 July 2022; container ships over 65 m contracted on or after 1 July 2016; ships with alternate light or heavy cargo loading conditions of any length contracted on or after 1 July 2022; and non-cargo ships over 90 m contracted on or after 1 July 2022. On a BV container ship of 200 m the band form applies even though CSR does not reach container ships. BV Pt A Ch 2 Sec 2 [2.2.7]

ClassNK does the same thing with a three-item list — ships complying with Part CSR-B, CSR-T or CSR-B&T; hatch covers and hatch coamings for cargo holds where the Society so stipulates; and transverse watertight bulkheads in cargo holds under Chapter 31A or 31B of Part C. So on a ClassNK ship the two forms can be live in different parts of the same hull, the band applying to the corrugated bulkhead and the hatch coaming while the percentage applies to everything else. NK Pt B §1.3.1 (6) · 2022 ed.

The one thing you cannot do is choose. There is no surveyor's discretion in which form applies. Establish the build regime from the ship's papers and the class notation before you work out where the band sits, and if the ship is CSR-built, everything in the rest of this page about percentages is the wrong arithmetic for it.

The arithmetic, worked through

The pre-CSR form is a percentage of a percentage, and it gets computed the wrong way round constantly. The 75% is taken of the allowable margin — the wastage the society permits on that member — and not of the as-built thickness. Read it as a proportion of the plate and the band lands nowhere near the right place. Z10.2 §1.2.11

ABS works the example out in full, and it is the clearest statement of the arithmetic in any of the twenty-five documents. Take a transverse bulkhead plate on a tanker where the individual plate wastage allowance is 25% and the original thickness was 12 mm. ABS 7-A1-4/17.1

allowance      25%  of the original thickness
band opens at  75%  of that allowance   =  18.75% diminution
band closes at 100% of that allowance   =     25% diminution

gauged 12   to 9.75 mm   diminution 0% to 18.75%    the plate had some corrosion
gauged 9.75 to 9.0  mm   diminution 18.75% to 25%   substantial corrosion range
gauged        < 9.0  mm   diminution over 25%        renewal limit

Three quarters of the way through the allowance, not three quarters of the way through the plate. The band opens at 9.75 mm exactly and the source adds no measurement margin to it. ABS 7-A1-4/17.1

CCS draws the same thing rather than writing it. Sketch Map 1.1 divides the range between the renewal thickness and the original thickness into four quarters and marks the quarter nearest the renewal thickness "substantial corrosion areas", labelled 75% of allowable limits. Everything below the renewal thickness is marked "excessive corrosion areas". Sketch Map 1.2 on the same page draws the CSR case separately, because the geometry is different. CCS Ch 1, Sketch Map 1.1

IACS Recommendation 96 is the only source in this set to put a percentage on each end of the band. It treats a member as substantially corroded once its wastage has run past 75% of the diminution allowed for that particular member and has not yet reached 100% of it. Same band as the definition clause, stated from the other direction. Note what it is — a Recommendation, not a Unified Requirement, so it binds no society by itself. Rec 96 §2.1.5 · guidance

Then the question nobody asks until it matters — which allowable margin. Neither Z7 nor the Z10 series publishes one. They define the term against "allowable margins" and leave the figures to each Society, which is why the same plate on two ships under two flags of class can enter the band at two different readings. The ESP Code does the same and goes one step further on the paperwork: its recommended procedures require the maximum allowable diminution to be stated on the reporting forms, and allow it to be carried on an attached document instead of on the form itself. Nowhere does the Code print the figure. ESP Code Annex 8 · Annex 10 ABS names its own tables in a note to the definition itself — the wastage allowances are in 7-A1-4/35.3 Tables 1A through 4. ClassNK's allowances sit in Appendix 3.5 (1) of the Guidance, at 20% of original thickness plus 1 mm for shell, strength deck, sheer strake and strength deck longitudinals, deep tank tight bulkheads and inner bottom; 25% for double bottom floors and girders, primary members, hold frames and watertight bulkhead plating; and 30% for other frames, longitudinal beams, stiffeners and brackets, effective deck plating, hatch covers and hatch beams. ABS 7-1-1/3.64 · NK App 3.5 (1) · 2014 ed.

ClassNK also puts local corrosion outside the band altogether. The wastage allowance for local corrosion such as stress corrosion and pitting is left to the surveyor's discretion, and the standard diminution limit for local corrosion other than stress corrosion is 40% of the original thickness. A pit is not judged by the substantial corrosion arithmetic. NK App 3.5 (1)(iv) · 2014 ed.

Two consequences of writing the limit as a percentage, both spelled out by ABS and both worth having ready before you argue about a reading. First: at the same loss rate per surface, the thinner member hits its limit first. Second: a member wasting from both faces under the same conditions runs at twice that speed towards it. Put the two together and you have the tanker internal — thinner than the plating around it to begin with, and corroding from each side into cargo tank conditions, so it reaches its limiting wastage well ahead of that plating. ABS 7-A1-4/17.1

On a CSR ship none of that scaling happens, because the band is not a proportion of anything. The reserve thickness t_res is 0.5 mm, flat, independent of the member and of the as-built thickness. A 30 mm plate and a 9 mm stiffener web get the same half millimetre. CSR Ch 13 Sec 2, Symbols

Society by society

Twenty-five documents were read for this page. Most of them copy the IACS sentence word for word, which is the point of a Unified Requirement. Two never use the term, two cite a definition clause they do not themselves carry, one is an amendment that leaves the definitions alone, and the remaining departures change the wording rather than the substance. One of the twenty-five is not a class document at all: the IMO ESP Code, which is where the sentence the class texts copy comes from. Here is where each one puts it and whether it carries the CSR sentence.

Clause locations verified in the documents named in the Sources list. Older-edition rows carry the edition in the row label and in the chips below.

The one that is not a class rule

The IMO ESP Code is where the sentence starts. Every ESP text above is a society's implementation of it, and it is the only document on this page that binds through SOLAS rather than through a class contract — chapter XI-1 makes it mandatory, so on a bulk carrier or an oil tanker the definition reaches the Administration and the flag State as well as the society. It is built as four parts, each with a definitions list of its own: Annex A part A for bulk carriers of single-side skin construction, Annex A part B for those of double-side skin, Annex B part A for double-hull oil tankers, and Annex B part B for oil tankers that are not double-hull. The term appears on 66 of the Code's 326 pages. One drafting habit to know before quoting it: the 2011 text is written in "should" from end to end, including the clause that widens the measurement. That is the Code's house style and not an offer of choice — the paragraphs MSC.525(106) later replaced come back in "shall". ESP Code, four parts · SOLAS XI-1

What the Code does with the finding is what the Z10 series does with it. The measurement is widened over the area; the widened readings are taken before the survey can be credited; suspect areas left on the record by earlier surveys are examined; and areas already logged as substantially corroded are gauged again. Where the pattern for that widening is printed depends on which part you are in, and the numbering is a trap. The two bulk carrier parts carry it in their Annex 10. The two oil tanker parts carry it in their Annex 4 — and in those parts Annex 10 is a different thing altogether, the recommended procedure for recording measurements. Open the annex by its title, not by its number. ESP Code §2.6.3 · §2.5.2 · Annex 10 vs Annex 4

MSC.525(106) is in this set because the Code cannot be read without it, not because it changes this page. Adopted 10 November 2022 and in force 1 July 2024, it leaves the definition clauses exactly where they were and exactly as they read. What it moves sits next door to them: the coating trigger for annual examination of bulk carrier ballast tanks, which now bites at less than GOOD condition rather than at POOR; a new duty on double-side skin void spaces bounding cargo holds on bulk carriers over 20 years of age and of 150 m and upwards, taking those spaces to annual examination where the coating is POOR; the condition evaluation report; and the IACS Recommendations the technical assessment guidelines cite. Quote A.1049(27) for the definition. Check MSC.525(106) before quoting anything about coating. MSC.525(106) · definitions untouched

The two ship-type requirements outside the ESP regime

Z7.1 and Z7.2 sit alongside Z7 rather than under ESP, and they are the two people forget when they reach for a Z10 sheet on a ship that is neither a tanker nor a bulk carrier. Z7.1 reaches general dry cargo ships that are self-propelled, of 500 gt and upwards that carry solid cargo, behind a list of nine exclusions: any ship already under Z10.2 or Z10.5; carriers dedicated to containers, to wood chips or to cement; ro-ro and refrigerated cargo ships; livestock carriers; deck cargo ships; and general dry cargo ships whose double-side skin runs the length of the cargo area and the full height of the hold to the upper deck. On a hybrid ship, with some holds single-side skin and some double, Z7.1 reaches only the structure in way of the single-side skin holds. Z7.2 covers self-propelled ships carrying liquefied gases in bulk. Both take the Z7 sentence with nothing added, and neither has a second form to give, because neither ship type is built to the Common Structural Rules. Mind the clause numbers when you copy a reference between the three: the same sentence is §1.2.8 in Z7, §1.2.9 in Z7.1, and §1.2.8 again in Z7.2. Z7.1 §1.1.1 / §1.2.9 · Z7.2 §1.1.1 / §1.2.8

One consequence of that runs straight into the BV row above, and it is worth having before you tell an owner which arithmetic his gas carrier takes. The IACS requirement for liquefied gas carriers gives one form; BV gives that ship type two. Z7.2 stops at the percentage for every gas carrier. BV's own definition reaches liquefied gas carriers contracted on or after 1 July 2022 and puts them in the renewal-thickness band. Same hull, same reading, two different bands depending on whose book you are working to — so establish the society before the arithmetic, not after. Z7.2 §1.2.8 vs BV [2.2.7]

The ones that word it differently, and how

Lloyd's Register is the largest departure. Its non-CSR sentence works one plate or one stiffener at a time, and the figure it takes 75% of is what LR calls the permissible diminution, not the allowable margins the IACS texts name. The term is pinned to the member rather than to the corrosion pattern. LR is also the only source in this set to stop the sentence there, with no "but within acceptable limits" tail on the end of it. One more line in the same clause changes how you read every LR percentage: those numbers are not a picture of what is acceptable on the day of the survey, because LR has already worked into them the average corrosion it expects the structure to pick up over the five years that follow. LR Pt 1 Ch 5 §1.5 · Nov 2016 v7.3

On the CSR side LR states the same band three times and does not agree with itself. This is a finding about one document, not a comparison between two, and it is worth knowing before you argue a reading with anybody.

• The definition, at §1.5.7.4, puts the gauged thickness between the renewal thickness and the renewal thickness plus the reserve. It uses the word and no symbols, so read on its own it takes both ends in.
• The local strength criterion, one page later at §1.5.7.5, writes the same band out as t_renewal < t_gauged < t_renewal + t_reserve. Two less-than signs, so both ends are shut out. A reading sitting exactly on the renewal thickness is inside the band by the definition and outside it by the criterion.
• The figure printed on that same page labels the band against t_net instead — the substantial corrosion range running from t_net + 0.5 mm down to t_net, with the half millimetre spelled out as a number rather than as the reserve symbol.

Three statements of one band on two facing pages of one document, with two different symbols and two different treatments of the endpoints. The Z10 series and CSR itself close the lower end of the band, so the definition wording is the one that lines up with IACS and the formula is the one that does not. What LR is consistent about is the size of the reserve: 0.5 mm, described as the wastage held back for the two and a half years between an intermediate and a special survey. Where a reading lands exactly on the renewal thickness on an LR CSR ship, the document does not settle it — put the number in front of the surveyor rather than deciding it on the sheet. LR §1.5.7.4 vs §1.5.7.5 vs fig. p.39

DNV does not use the term. The words "substantial corrosion" appear nowhere in the 46 pages of DNV-CG-0182. Its definitions table sets out four corrosion types — edge, general, groove and pit — and carries two further rows, grooving and pitting, which do nothing but point back to groove corrosion and pit corrosion. Diminution criteria are set for each of the four. The band is there in substance for hatch covers and coamings, where a gauged thickness in the range t_net + 0.5 mm to t_net + 1.0 mm calls for steel renewal, or alternatively coating applied to the manufacturer's requirements or annual gauging, with the coating maintained in good condition. 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. Do not go looking in this document for a definition to quote — there is none. DNV-CG-0182 Sec.8 · Tbl 4 · Jul 2022

The symbol is not stable, and the split reaches the statutory text. UR Z10.2, Z10.4 and Z10.5 write the CSR band against t_ren. The IMO ESP Code — the SOLAS text those requirements sit under — writes the same band against t_net in every one of the three parts that carry it, and calls the reading a gauged or measured thickness where IACS says a measured thickness. IACS Recommendation 96 uses t_net as well, and DNV and CCS use it too in places. The two symbols are not defined the same way — in CSR, t_ren = t_as-built − t_c − t_{vol_add}, while t_net is the net thickness. Read the symbol the document in front of you defines, and do not carry a value across from one document to another because the letters look similar. ESP Code §1.2.9 · Rec 96 §3.2.1 (h) vs Z10.4 §1.2.10

IACS Recommendation 76 is the only IACS text in this set to write "material wastage" and to spell "per cent" out. Rev.2 dates from June 2004 and predates CSR, so it carries no second form. Like Rec 96, it is a Recommendation — it is not a Unified Requirement and binds no society by itself. Rec 76 §3.2.1 (h) · guidance

ClassNK Part B writes the second form as a range measured upward from the renewal thickness — a gauged thickness within the range of 0.5 mm to the renewal thickness stipulated in the relevant provisions — rather than as a thickness lying between two named values. Same band, different construction of sentence. CCS makes a smaller shift, saying the corrosion pattern indicates "a wastage between t_ren + 0.5 mm and t_ren" where IACS says a measured thickness between them. NK Pt B §1.3.1 (6) · CCS Ch 1 item 8

KR takes the term somewhere none of the others do — into the repair, in its Guidance rather than its Rules, and in the "should" form. Take a member that has been replaced. At the butt joints where the new piece meets the old, forward and aft, KR looks at how much each joining member has itself wasted — and none of those readings should already sit inside the substantial corrosion range. The Guidance puts the figure in brackets: 75% of the allowable diminution for the member in question. The same test is applied to straps — the deck or bottom plating being reinforced should not be inside the range either. That is a constraint on the repair drawing, not on the gauging, and it is worth knowing before the yard proposes one. KR Pt 1 Annex 1-5 (4)(B) · guidance

RINA defines the term somewhere we cannot reach, and two held appendices prove it rather than one. Appendix 2 tells the reader to gauge further over any area found to be suffering it, then routes the meaning of the term to Sec 2, [2.2.9]. Appendix 3 — the CSR appendix, covering ships subject to Ch 13 of the CSR for Bulk Carriers, Sec 12 of the CSR for Double Hull Oil Tankers or Pt 1 Ch 13 Sec 2 of the combined CSR — lists what a CSR ESP ship has to have gauged and puts extra measurement over substantially corroded areas fourth on that list, pointing at the same Sec 2, [2.2.9]. Pt A Ch 2 Sec 2 is in neither file. So the wording and the numeric trigger stay off this page. That is a gap in what is held, not a gap in RINA's rules. RINA App 2 §2.2.1 · App 3 §2.2.1 · 1 Jul 2024

What Appendix 3 does give you is the CSR routing, and it is worth having on the day. For acceptance, App 3 keeps nothing of its own for general corrosion on a CSR ship — it hands the criteria back to the CSR text that applies to the notation and the contract date, so the band is CSR's band and not a RINA number. RINA's own figures in that appendix are for pitting, edge and grooving corrosion on CSR ships. For extent, its reference table sends you by service notation: Ch 3 Sec 5 Tab 3 as guidance for every notation, Ch 4 Sec 2 Tab 7 to Tab 11 for bulk carrier ESP CSR, Ch 4 Sec 9 Tab 5 to Tab 8 for the double skin version, and Ch 4 Sec 4 Tab 4 to Tab 8 for double hull oil tanker ESP CSR, picked by where on the ship the corrosion is. None of those tables is in the files held. RINA App 3 Tab 1 · §3.2.1 · §5.2.1 · 1 Jul 2024

CSR never uses the term

This one is worth stating flatly, because it is quoted the wrong way round in a lot of procedures. The Common Structural Rules do not contain the words "substantial corrosion". A full-text search of the 812 pages of the 1 January 2024 version returns three occurrences of the word "substantial", and all three are ordinary English usage about buckling deflections, web frames and inner bottom thickness. None of them is the survey term. CSR 1 Jan 2024, whole text

What CSR does carry is the band, without a name for it. The clause is headed "Alternative solutions". Where a measured thickness lands inside the band, CSR lets something other than renewal be adopted. What that something can be is set by the society itself, in its own rules or in whichever of its documents implement IACS UR Z10.2, Z10.4 and Z10.5. CSR Ch 13 Sec 2 [2.1.3]

CSR Pt 1 Ch 13 Sec 2

[2.1.1]  renew if  t_m < t_ren        t_ren = t_as-built − t_c − t_vol_add
[2.1.3]  "Alternative solutions"      t_ren ≤ t_m < t_ren + t_res
Symbols  t_res = 0.5 mm

So the label is attached to that band by the Z10 series, not by CSR. CSR supplies the arithmetic and hands the survey consequence straight back to the society. That division is deliberate and it is stated elsewhere in the same chapter — the hull survey requirements for a CSR ship are those of the individual society's rules incorporating UR Z10.2, Z10.4 and Z10.5. On a CSR ship you are working to two documents at once, and the one that gives you the word "substantial" is not the one that gives you the number. CSR Ch 13 Sec 2 [2.1.3] · Z10.2 §1.2.11

Practical effect on a report. If a procedure or a client's format says "substantial corrosion per CSR Part 1 Chapter 13", it is citing a clause that does not use the term. The honest citation is the society's implementation of Z10.2, Z10.4 or Z10.5 for the label and the consequence, and CSR Pt 1 Ch 13 Sec 2 for the band itself.

What the finding costs in measurement

Every society expands the measurement once the band is entered. The short-form tables all carry the same shape: five points to the square metre on the suspect plate and on the plates beside it, and three readings in a line across the web plus three across the flange of the suspect stiffener. Z7 prints it, and so do Z7.1 and Z7.2 for their own ship types, in the same two lines. Do not carry that shape onto every ship, because two sets of sources depart from it materially. The ESP sheets — the Code's own annexes and the Z10 tables that follow them — drop the short form and put a schedule in its place, setting the extent and the pattern separately for plating, longitudinals, girders, floors, webs, brackets and bulkheads. And CCS sizes the plating area at roughly 2 m² rather than one. What differs after that is where the pattern is printed and how hard it binds. Z7 Tbl 2 · Z7.1 Tbl III · Z7.2 Tbl V · BV Tab 4 · INSB Tbl E · KR Tbl 1.2.5 · PRS §5.1.3.7

Read the "binds" column before you argue about scope on board. The ESP Code says the provisions for the widened measurement are given in its annex, and the Z10 ESP tables repeat that; Z7, Z7.1, Z7.2, BV, INSB, KR and RINA all use the softer form, the table being guidance and the extent left to the surveyor to increase. Note what is soft and what is not in that second group: the widening itself is required once the band is read, and it is the printed pattern that is offered as guidance. PRS writes the pattern as rule text with no table at all. That is a real difference in how much room there is to argue when the extent of a job is being decided at the gangway. ESP Code §2.6.3 · Z10.1 §2.4.2 vs Z7 §2.2.11 · Z7.1 §2.4.2

One waiver in the chemical tanker requirement is filed where people do not look for it, and it is not part of the expansion. The relief for stainless steel hull structure and piping belongs to the minimum thickness measurement requirement for the special survey — the Table II duty at §2.4.1 — and it can be waived there, with clad steel plating excepted. The Table IV expansion for substantial corrosion is the next clause down and carries no waiver of its own. So the waiver decides what you gauge as a matter of course on a chemical tanker; it does not decide what you gauge once a band reading appears. Z10.3 §2.4.1 vs §2.4.2

Two documents put a duty on the measurement crew rather than on the surveyor. CCS requires the thickness measurement personnel themselves to verify the accuracy of the data, determine the extent of the substantial corrosion areas, record them in sketch drawings and inform the surveyor undertaking the work; and where a result approaches or reaches the band, to notify the owner promptly so the measurement can be extended. ABS puts the same idea the other way round — an individual reading does not constitute a corrosion pattern, and a single marginal reading requires additional readings taken and evaluated together with close visual examination by the surveyor. CCS Ch 2 §IV · ABS 7-A1-4/17.1

One cross-reference fault in the edition held, worth knowing before someone sends you to the wrong section. INSB §2.2.14 points to "Table E of Sec 12", while Table E is printed in Section 13. Go to Section 13. INSB §2.2.14 vs Sec 13 Tbl E

The row-by-row grid — every one of the twenty-four documents against the wording, the numeric trigger, the duty and the expansion, with each cell carrying its clause and PDF page — is in handbook table T02, What counts as substantial corrosion, class by class. This page gives you the reading; the table gives you every cell.

What it does to the survey regime

The expansion is the immediate cost. The lasting cost is that the area stops being a one-off reading and joins the ship's survey record. Three separate mechanisms do that, and they run in parallel.

The survey is not credited until the expansion is done. Every Z10 text and Z7 say it in the same words; so do Z7.1 and Z7.2, so does the ESP Code, and so do LR and INSB in their own. The rule bites at the annual too. If gauging on an annual turns up the band, the extent has to be widened to the ship-type table, and none of that widened work can be pushed to a later date — it has to be in hand before the annual can be credited. Two duties ride along with it at each survey: the surveyor examines the suspect areas the earlier surveys left on the record, and every area already logged as substantially corroded gets gauged again. Z10.2 §2.4.2 / §3.2.4.1 c) · Z7.1 §3.2.4

The area is examined and re-gauged at the surveys that follow. ClassNK puts it plainest. Once wastage runs past 75% of the allowable margin, ClassNK has the location named as a suspect area, and it has that area gauged again and inspected as needed at every survey after it — annual, intermediate and special. LR imposes the same duty but hedges the front of it, and the hedge is part of the requirement: in general and where considered necessary, structure carrying the finding gets close-up survey plus gauging at the annual and at the surveys after it, with enough readings taken to show how far the corrosion runs. Rec 76 writes the same idea as guidance and pairs two things to be recorded for attention during the next survey cycle unless they are repaired at the survey itself — protective coating that has deteriorated in salt water ballast spaces, and structural areas showing substantial corrosion and/or judged by the surveyor prone to rapid wastage. Note the "and/or": one item, either condition. NK App 3.5 (3)(ii) · LR §1.2.2.1 · Rec 76 §2.4.4

On a tanker the tank itself changes regime. Find it inside a ballast tank on an oil tanker, a chemical tanker or a double hull oil tanker, and that tank goes onto annual examination from then on. It is the third of four triggers in one clause — the other three being no hard coating applied from the time of construction, a soft or semi-hard coating applied, and a hard coating found in less than GOOD condition and left unrepaired. Read the scope of this one before you carry it across a fleet. It lives in the three tanker requirements only, Z10.1, Z10.3 and Z10.4, each at §2.2.3.1, and the same four-item list comes back in their intermediate survey clauses at §4.2.2.2. The split is not IACS's own invention. The ESP Code draws the same line: both oil tanker parts carry the four-item list, with substantial corrosion found inside the tank as the third item, and both bulk carrier parts leave that item out and keep the coating conditions alone. Z10.2, Z10.5, Z7, Z7.1 and Z7.2 print the shorter version too: the three coating items, and no corrosion item at all. So on a bulk carrier, a double skin bulk carrier, a general dry cargo ship or a liquefied gas carrier, this finding on its own does not put a ballast tank onto an annual cycle. Rec 96 restates the tanker version, as guidance. What the two bulk carrier requirements do instead is move the space — a hold used for both cargo and salt water ballast is treated as a ballast tank once substantial corrosion is found in it, as said at the top of this page, and the tank regime follows from there. Z10.1 / Z10.3 / Z10.4 §2.2.3.1 c) · ESP Code Annex B §2.3

On a CSR ship the annual regime splits — and the two texts do not agree

This is the sharpest divergence in the whole Z10 series and it is easy to miss because the clause numbers match. Take a CSR bulk carrier or double skin bulk carrier, and an area already identified as substantially corroded. Z10.2 and Z10.5 leave two ways of carrying that area forward. Coat it, to whatever the coating maker specifies, and then look at that coating once a year to confirm it is still sound over the area; or leave it bare and gauge it once a year. Both texts write the alternative with may, and neither of them says who picks — the clause opens the door and names no hand on it, so do not tell an owner the decision is his. What is clear is the effect: coat it properly, keep the coating good, and the annual gauging comes off. Z10.2 §2.4.2 · Z10.5 §2.4.2

On CSR-built double hull oil tankers there is no such option, and Z10.4 conditions its clause on the build regime exactly as Z10.2 does. Every area found in the band has to be looked at again, and gauged again, at each annual and each intermediate survey. The clause is written as a requirement, not a choice, and no coating alternative is offered against it. The same ship, the same reading, the same half-millimetre band — and on the tanker the crew comes back every year. Z10.4 §2.4.2 / §4.2.1.3

Both societies that publish the CSR case at member level follow their side of the split. DNV offers steel renewal or, alternatively, coating to the manufacturer's requirements or annual gauging, with the coating maintained in good condition. ClassNK Part B offers the same either-or for bulk carriers built under Part CSR-B. KR routes CSR bulk carriers out of its guidance table entirely and into the ship-type chapters. DNV Sec.8 · NK Pt B Tbl B4.4 · KR item (34)

And what it does to the survey planning document

The survey programme is worked out by the owner with the society before any part of a special survey, or of an intermediate survey on a bulk carrier over 10 years of age, and the survey does not commence until it is agreed. Two things in it bear directly on this page. Extended measurements for areas with substantial corrosion are those of the ship-type table and anything the survey programme adds — so the programme can widen the expansion beyond the table. And the ship is told its own numbers: Z10.2 puts the society under a duty to advise the owner of the maximum structural corrosion diminution it will accept on that hull. That is where the allowable margin behind the 75% actually comes from for that ship. Z10.2 §5.1.1 / §5.1.4 / §2.4.2

The survey planning meeting held before the special or intermediate survey has an indicative agenda of nine items, and two of them are this subject by name — item .5, which puts on the table how much close-up survey and how much gauging the job needs once the state of the coating, the suspect areas and the areas of substantial corrosion have all been weighed; and item .8, the mapping of areas of substantial corrosion. The measurement firm's representative is a named attendee at that meeting where involved. If you are the operator, item .8 is your instruction to produce a drawing, not a column of figures. Z10.2 §5.7.3 · Z10.4 §5.7.3

What this means when you fill the sheet

A reading in the band is not a failure and it is not clean. Buried in a column of numbers it looks like every other entry, and the whole survey consequence — the expansion, the suspect area, the annual re-gauging — hangs on somebody noticing it. So flag it where it happens, not in the office two days later.

The as-built column decides it, on both sides of the CSR line. On a pre-CSR ship the band is a percentage of an allowance which is itself a percentage of the original thickness, so an as-built value out by a millimetre moves both ends of the band. On a CSR ship the renewal thickness is derived straight from the as-built, and the band sits half a millimetre above it. Either way, the reading you took is not the number that decides anything on its own — it only means something against the original. If the as-built on the plan and the as-built on the sheet disagree, that gets resolved before the reading is judged, not after.

And one reading is not a pattern. The definition every society uses turns on the assessment of a corrosion pattern, and ABS says outright that an individual reading does not constitute one — a single marginal reading calls for additional readings evaluated together with close visual examination by the surveyor. Do not enter "substantial corrosion" against one number. Take the extra readings first, then say it. ABS 7-A1-4/17.1

Six things the office cannot judge a reading without, in the order they get forgotten:

• Whether the ship is CSR-built. It changes which definition applies and which arithmetic goes with it, and it is decided by the contract date and the notation, not by the size of the ship or the year of delivery.
• The as-built thickness of the member, per member and not per area. Where the plan carries a renewal column as well, record both.
• The allowable margin for that member, from the society's own table or from the diminution levels the society advised the owner for this ship. IACS publishes none — the number comes from the class, and it is member-specific.
• The exact location, in the society's own terms — strake, frame number, bay, tank or hold, and which side. The allowance and, on a CSR ship, the corrosion addition both change with it.
• The compartment on both sides of every plate gauged on a CSR ship. The corrosion addition is built from the two faces, so ballast one side and sea the other is a different number from ballast one side and cargo hold the other.
• The previous report. Areas of substantial corrosion identified at earlier surveys have to be re-gauged, and suspect areas have to be examined. If nobody brings the last set of drawings on board, that requirement cannot be met and the survey cannot be credited.

Two habits pay for themselves. Sketch it, do not just list it — CCS puts the duty on the measurement personnel to determine the extent of the substantial corrosion areas and record them in sketch drawings, and the IACS survey planning meeting agenda has "mapping of areas of substantial corrosion" as a named item. A marked-up shell expansion or bulkhead sketch is what the surveyor needs to set the extent of the expansion. Say when you are in the band, in words, on the sheet and in the summary — not only as a number that happens to fall between two thresholds the reader has to compute.

Last thing, for the office, and mind the condition on it. On a bulk carrier built under the Common Structural Rules the finding can be closed out at annuals by coating instead of gauging; on a double hull oil tanker it cannot, CSR-built or not. Every clause that offers the coating route opens by naming vessels built under the IACS Common Structural Rules, so a pre-CSR bulk carrier gets no such option either — check the build regime before you write the relief into a recommendation. That is a scheduling difference worth a sentence, because it decides whether the owner is booking a crew every year for the next five. Z10.2 §2.4.2 / §3.2.4.1 c) · Z10.4 §2.4.2

Sources

1. ESP Code Annex A Pt A §1.2.9IMO Resolution A.1049(27) — International Code on the Enhanced Programme of Inspections during Surveys of Bulk Carriers and Oil Tankers, 2011 (2011 ESP Code); mandatory under SOLAS chapter XI-1official text ↗
2. MSC.525(106)IMO Resolution MSC.525(106) — Amendments to the 2011 ESP Code, adopted 10 November 2022, in force 1 July 2024official text ↗
3. Z10.1 §1.2.9IACS UR Z10.1 Rev.25 (Feb 2023) — Hull Surveys of Oil Tankersofficial text ↗
4. Z10.2 §1.2.11IACS UR Z10.2 Rev.37 (Feb 2023) — Hull Surveys of Bulk Carriersofficial text ↗
5. Z10.3 §1.2.9IACS UR Z10.3 Rev.21 (Aug 2023) — Hull Surveys of Chemical Tankersofficial text ↗
6. Z10.4 §1.2.10IACS UR Z10.4 Rev.18 (Feb 2023) — Hull Surveys of Double Hull Oil Tankersofficial text ↗
7. Z10.5 §1.2.11IACS UR Z10.5 Rev.20 (Feb 2023) — Hull Surveys of Double Skin Bulk Carriersofficial text ↗
8. Z7 §1.2.8IACS UR Z7 Rev.29 Corr.1 (May 2024) — Hull Classification Surveysofficial text ↗
9. Z7.1 §1.2.9IACS UR Z7.1 Rev.15 Corr.1 (May 2024) — Hull Surveys of General Dry Cargo Shipsofficial text ↗
10. Z7.2 §1.2.8IACS UR Z7.2 Rev.8 (May 2019) — Hull Surveys of Liquefied Gas Carriersofficial text ↗
11. Rec 76 §3.2.1 (h)IACS Recommendation 76 Rev.2 Corr.1 — Bulk Carriers, Guidelines for Surveys, Assessment and Repair of Hull Structure (Rev.2 June 2004, Corr.1 September 2007)official text ↗
12. Rec 96 §3.2.1 (h)IACS Recommendation 96 Rev.2 (2023) — Double Hull Oil Tankers, Guidelines for Surveys, Assessment and Repair of Hull Structuresofficial text ↗
13. CSR Pt 1 Ch 13 Sec 2IACS Common Structural Rules for Bulk Carriers and Oil Tankers, 1 January 2024 version (in force 1 July 2024) — Acceptance Criteriaofficial text ↗
14. ABS 7-1-1/3.64ABS Rules for Survey After Construction, Part 7 — July 2026official text ↗
15. BV Pt A Ch 2 Sec 2 [2.2.7]BV NR467 Rules for the Classification of Steel Ships, Part A — R26, July 2026official text ↗
16. CCS Ch 1, def. 8CCS Guidelines for Thickness Measurement of Hull — Ver 6.6, March 2025official text ↗
17. DNV-CG-0182 Sec.8DNV-CG-0182 Allowable thickness diminution for hull structure — Edition July 2022, the newest edition heldofficial text ↗
18. INSB Pt I Ch 3 Sec 1 §1.2.8INSB Rules and Regulations for the Classification and Construction of Steel Ships, Part I — Issue 04 Rev.03, December 2024official text ↗
19. KR Pt 1 Ch 2, def. 14KR Rules for the Classification of Steel Ships, Part 1 — 2026 editionofficial text ↗
20. LR Pt 1 Ch 5 §1.5Lloyd's Register Thickness Measurement and Close-up Survey Guidance, Part 1 — Ver 7.3, November 2016, the newest edition heldofficial text ↗
21. NK Appendix 3.5ClassNK Guidance for Undergoing Class Maintenance Surveys — 2014 edition, the newest edition heldofficial text ↗
22. NK Pt B Ch 1 §1.3.1 (6)ClassNK Rules and Guidance for the Survey and Construction of Steel Ships, Part B — Class Surveys, 2022 edition, the newest edition heldofficial text ↗
23. PRS Pt I Ch 1PRS Rules for the Classification and Construction of Sea-going Ships, Part I — Classification Regulations, January 2026official text ↗
24. RINA Pt A Ch 2 App 2RINA Rules for the Classification of Ships, Part A Ch 2 App 2 — 1 July 2024 interactive set, the newest edition heldofficial text ↗
25. RINA Pt A Ch 2 App 3RINA Rules for the Classification of Ships, Part A Ch 2 App 3 — Thickness Measurements: Number, Locations and Acceptance Criteria for Ships Built under the Common Structural Rules, 1 July 2024 interactive set, the newest edition heldofficial 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 class thickness limit?K5.1
• 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 cargo ships, class by class?K5.5
• What are the pitting acceptance criteria in each class?K5.6
• At what thickness must steel be renewed on a CSR ship?K5.7
• What wastage allowances apply to ships under 90 m?K5.8
• What changes for thickness measurement on CSR ships?K3.9