In this Annual Report on Form 10-K (this "Report"), unless the context otherwise requires, references in these notes to consolidated financial statements to "Kaiser Aluminum Corporation," "we," "us," "our," "the Company" and "our Company" refer collectively to Kaiser Aluminum Corporation and its subsidiaries.

Organization and Nature of Operations. Kaiser Aluminum Corporation specializes in the production of semi-fabricated specialty aluminum mill products, such as aluminum plate and sheet and extruded and drawn products, for the following end market applications: aerospace and high strength ("Aero/HS products"), automotive ("Automotive Extrusions"), general engineering ("GE products") and other industrial ("Other products"). Our business is organized into one operating segment. See Note 16 for additional information regarding our business, product and geographical area information and concentration of risk.

Principles of Consolidation and Basis of Presentation. The consolidated financial statements include the accounts of our wholly owned subsidiaries and are prepared in accordance with United States generally accepted accounting principles ("GAAP") and the rules and regulations of the Securities and Exchange Commission ("SEC"). Intercompany balances and transactions are eliminated. We have reclassified certain amounts in prior-period financial statements to conform to the current period's presentation. On the Statements of Consolidated Income, prior period presentation of Unrealized (gain) loss on derivative instruments is now contained within Cost of products sold (see Note 8 for additional details).

Use of Estimates in the Preparation of Financial Statements. The preparation of financial statements in accordance with GAAP requires the use of estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities known to exist as of the date the financial statements are published and the reported amounts of revenues and expenses during the reporting period. Uncertainties with respect to such estimates and assumptions are inherent in the preparation of our consolidated financial statements; accordingly, it is possible that the actual results could differ from these estimates and assumptions, which could have a material effect on the reported amounts of our consolidated financial position and results of operations.

Fair Value Measurements. We apply the fair value hierarchy established by GAAP for the recognition and measurement of certain financial assets and liabilities. An asset or liability's fair value classification within the hierarchy is determined based on the lowest level input that is significant to the fair value measurement. In determining fair value, we utilize valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible and consider counterparty risk in our assessment of fair value.

Financial assets and liabilities that we measure at fair value each period include our derivative instruments, equity investments related to our deferred compensation plan and debt investment securities classified as available for sale securities (see Note 5 and Note 8). Prior to our adoption of Accounting Standards Update ("ASU") No. 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities ("ASU 2016-01"), equity investments related to our deferred compensation plan were accounted for as available for sale securities. Subsequent to our adoption of ASU 2016-01 on January 1, 2018, changes in the fair value of these equity investments are recorded within Other expense, net (see Note 5 and Note 12). Additionally, we measure at fair value once each year at December 31 our Canadian defined benefit pension plan and the plan assets of the Salaried VEBA (defined in Note 5). In determining the fair value of the plan assets at an annual period end, we utilize primarily the results of valuations supplied by the investment advisors responsible for managing the assets of each plan, which we independently review for reasonableness. We record our remaining financial assets and liabilities at carrying value.

Goodwill is tested for impairment during the fourth quarter on an annual basis, as well as on an interim basis, as warranted, at the time of relevant events and changes in circumstances. Intangible assets with definite lives are initially recognized at fair value and subsequently amortized over the estimated useful lives to reflect the pattern in which the economic benefits of the intangible assets are consumed. In the event the pattern cannot be reliably determined, we use a straight-line amortization method. Whenever events or changes in circumstances indicate that the carrying amount of the intangible assets may not be recoverable, the intangible assets are reviewed for impairment. See Note 4 for a discussion of our business acquisition in 2018 and the goodwill impairment charges we recorded during 2019 and 2017.

For a majority of our remaining non-financial assets and liabilities, which include inventories, debt issuance costs and property, plant and equipment, we are not required to measure their fair value on a recurring basis. However, if certain triggering events occur, an evaluation of the affected non-financial asset or liability will be required, which could result in a reduction to the carrying amount of such asset or liability. See "Property, Plant and Equipment, Net" below for a discussion of impairment charges on idled assets recorded during 2019, 2018 and 2017.

Cash and Cash Equivalents. We consider only those short-term, highly liquid investments which, when purchased, have maturities of 90 days or less to be cash equivalents. Our cash equivalents consist primarily of funds in commercial paper, money market funds and other highly liquid investments, which are classified within Level 1 of the fair value hierarchy with the exception of commercial paper, which is classified within Level 2 of the fair value hierarchy (see Note 8).

Restricted Cash. We are required to keep on deposit certain amounts that are pledged or held as collateral relating to workers' compensation and other agreements. We account for such deposits as restricted cash (see Note 15). From time to time, such restricted funds could be returned to us or we could be required to pledge additional cash.

Available for Sale Securities. We account for investments in certain marketable debt securities as available for sale securities. Such securities are recorded at fair value (see Note 8), with net unrealized gains and losses, net of income taxes, reflected in Accumulated other comprehensive loss as a component of Stockholders' equity. Realized gains and losses from the sale of marketable debt securities, if any, are determined on a specific identification basis. Debt investment securities with an original maturity of 90 days or less are classified as Cash and cash equivalents (see Note 2). Debt investment securities with an original maturity of greater than 90 days are presented as Short-term investments on the Consolidated Balance Sheets. In addition to debt investment securities, we also hold assets in various investment funds managed by a third-party trust in connection with our deferred compensation program (see Note 5).

Trade Receivables and Allowance for Doubtful Accounts. Trade receivables primarily consist of amounts billed to customers for products sold. Accounts receivable are generally due within 30 to 90 days. For the majority of our receivables, we establish an allowance for doubtful accounts based upon collection experience and other factors. On certain other receivables where we are aware of a specific customer's inability or reluctance to pay, an allowance for doubtful accounts is established against amounts due, to reduce the net receivable balance to the amount we reasonably expect to collect. However, if circumstances change, our estimate of the recoverability of accounts receivable could be different. Circumstances that could affect our estimates include, but are not limited to, customer credit issues and general economic conditions. Accounts are written off once deemed to be uncollectible. Any subsequent cash collections relating to accounts that have been previously written off are typically recorded as a reduction to total bad debt expense in the period of payment. Write-offs for 2019, 2018 and 2017 were immaterial to the consolidated financial statements.

Inventories. Inventories are stated at the lower of cost or market value. Finished products, work-in-process and raw material inventories are stated on the last-in, first-out ("LIFO") basis. During the year ended December 31, 2019, we decremented a prior year, higher cost LIFO layer which resulted in a charge of $3.6 million. At December 31, 2019 and December 31, 2018, the current cost of our inventory exceeded its stated LIFO value by $11.9 million and $31.7 million, respectively. The stated LIFO value of our inventory represented its net realizable value (less a normal profit margin). Other inventories are stated on the first-in, first-out basis and consist of operating supplies, which are materials and supplies to be consumed during the production process. Inventory costs consist of material, labor and manufacturing overhead, including depreciation. Abnormal costs, such as idle facility expenses, freight, handling costs and spoilage, are accounted for as current period charges (see Note 2 for the components of inventories).

Replacement Parts. Replacement parts consist of preventative maintenance and capital spare parts, which are stated on the first-in, first-out basis. Replacement parts are recorded within Prepaid expenses and other current assets or Other assets depending on whether or not the expected utilization of the replacement parts is to occur within the next 12 months.

Property, Plant and Equipment, Net. Property, plant and equipment, net, is recorded at cost and includes construction in progress (see Note 2). Interest related to the construction of qualifying assets is capitalized as part of the construction costs. The amount of interest expense capitalized as construction in progress was $1.6 million, $1.7 million and $2.2 million during 2019, 2018 and 2017, respectively.

Depreciation is computed using the straight-line method at rates based on the estimated useful lives of the various classes of assets. Finance lease assets and leasehold improvements are depreciated on a straight-line basis over the shorter of the estimated useful lives of the assets or the lease term. The estimated useful lives are as follows:

Property, plant and equipment are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the asset or group of assets may not be recoverable. We regularly assess whether events and circumstances with the potential to trigger impairment have occurred and rely on a number of factors, including operating results, business plans, economic projections and anticipated future cash flow, to make such assessments. We use an estimate of the future undiscounted cash flows of the related asset or asset group over the estimated remaining life of such asset(s) in measuring whether the asset(s) are recoverable.

We recorded impairment charges of $0.9 million, $1.4 million and $0.8 million in 2019, 2018 and 2017, respectively, to reflect the scrap value of idled assets we determined not to deploy for future use. Asset impairment charges are included in Other operating charges, net, in the Statements of Consolidated Income.

Derivative Financial Instruments. Consistent with guidelines established by management and approved by our Board of Directors, we use derivative financial instruments to mitigate our exposure to changes in the market price of aluminum, alloying metals, energy and, to a lesser extent, foreign currency exchange rates. We do not use derivative financial instruments for trading or other speculative purposes. Hedging transactions are executed centrally on behalf of all of our operations to minimize transaction costs, monitor consolidated net exposures and allow for increased responsiveness to changes in market factors.

We reflect the fair value of all of our derivative instruments on our Consolidated Balance Sheets (see Note 8). The fair value of hedges settling within one year is included in Prepaid expenses and other current assets or Other accrued liabilities. The fair value of hedges settling beyond one year is included in Other assets or Long-term liabilities. Cash flows related to all of our derivative instruments are reported in the Statements of Consolidated Cash Flows within the same category as the items being hedged.

Prior to our adoption of ASU No. 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities ("ASU 2017-12"), we did not meet the requirements for hedge (deferral) accounting related to our aluminum and energy derivatives. Accordingly, we recorded unrealized gain or loss associated with these hedges in the Statements of Consolidated Income. Our adoption of ASU 2017-12 on January 1, 2018 has allowed our aluminum and energy derivatives to qualify for hedge (deferral) accounting and, as such, we designated such hedges as cash flow hedges. Forward swap contracts for zinc and copper ("Alloying Metals") used in our fabrication operations are also designated as cash flow hedges. Unrealized gains and losses associated with our cash flow hedges are deferred in Other comprehensive loss, net of tax and reclassified to Cost of products sold when such hedges settle (see Note 8).

From time to time, we enter into foreign currency forward contracts to protect the value of anticipated foreign currency expenses associated with cash commitments for equipment purchases. For the years ended December 31, 2019, December 31, 2018 and December 31, 2017, we did not meet the requirements for hedge (deferral) accounting related to foreign currency derivatives and, as such, gains and losses (both unrealized and realized) related to our foreign currency forward contracts have been reflected as an increase or reduction in Other expense, net.

Self Insurance of Workers' Compensation and Employee Healthcare Liabilities. We self-insure the majority of the costs of workers' compensation benefits and employee healthcare benefits and rely on insurance coverage to protect us from large losses on individual claims. Workers' compensation liabilities are based on a combination of estimates for: (i) incurred-but-not-

reported claims and (ii) the ultimate expense of incurred claims. Such estimates are based on judgment, using our historical claims data and information and analysis provided by actuarial and claims advisors, our insurance carriers and other professionals. Our undiscounted workers' compensation liabilities were estimated at $29.2 million and $27.6 million at December 31, 2019 and December 31, 2018, respectively. However, we account for our workers' compensation accrued liability on a discounted basis (see Note 2), using a discount rate of 1.75% at December 31, 2019 and 3.00% at December 31, 2018. Accrued liabilities for employee healthcare benefits, which are estimates of unpaid incurred medical and prescription drug costs as provided by our healthcare administrators, were $3.5 million and $3.6 million at December 31, 2019 and December 31, 2018, respectively.

Conditional Asset Retirement Obligations ("CAROs"). We have CAROs at several of our manufacturing facilities. Our CAROs can be separated into two primary categories: (i) legal obligations related to the removal and disposal of asbestos and (ii) asset retirement obligations related to future lease terminations. The majority of our CAROs relate to the first category and consist of incremental costs that would be associated with the removal and disposal of asbestos (all of which is believed to be fully contained and encapsulated within walls, floors, roof, piping or equipment insulation) of certain of our older facilities if such facilities were to undergo major renovation or be demolished. We estimate incremental costs for special handling, removal and disposal costs of materials that may or will give rise to CAROs and then discount the expected costs back to the current year using a credit-adjusted, risk-free rate. When it is unclear when or if CAROs will be triggered, we use probability weighting for possible timing scenarios to determine the probability-weighted liability amounts that should be recognized in our consolidated financial statements (see Note 10).

Environmental Contingencies. With respect to environmental loss contingencies, we record a loss contingency whenever a contingency is probable and reasonably estimable (see Note 10). Accruals for estimated losses from environmental remediation obligations are generally recognized no later than the completion of the remedial feasibility study. Such accruals are adjusted as information develops or circumstances change. Costs of future expenditures for environmental remediation obligations are not discounted to their present value. Accruals for expected environmental costs are included in Other accrued liabilities or Long-term liabilities, as appropriate (see Note 2). Environmental expense relating to continuing operations is included in Cost of products sold in the Statements of Consolidated Income. Environmental expense relating to non-operating locations is included in Selling, general, administrative, research and development ("SG&A and R&D") in the Statements of Consolidated Income.

Revenue Recognition. We adopted Financial Accounting Standards Board ("FASB") Accounting Standards Codification ("ASC") Topic 606 ("ASC 606") on January 1, 2018 using the modified retrospective method. The reported results for the years ended December 31, 2019 and December 31, 2018 reflect the application of ASC 606 guidance while the reported results for the periods prior to January 1, 2018 were prepared under the guidance of FASB ASC 605, Revenue Recognition ("ASC 605"). Subsequent to our adoption, we recognize revenue as we fulfill our performance obligations and transfer control of products to our customers. For products that have an alternative use and/or for which we do not have an enforceable right to payment (including a reasonable profit) during the production process, we recognize revenue at a point in time. For products that have no alternative use and for which we have an enforceable right to payment (including a reasonable profit) throughout the production process, we recognize revenue over time. In general, a majority of our Aero/HS products and our Automotive Extrusions are recognized over time, with the remainder of our products recognized at a point in time.

For the majority of our business, contracts with customers begin when we acknowledge a purchase order for a specific customer order of product to be delivered in the near term. These purchase orders are short term in nature, although they may reference a longer term "blanket purchase order" or a "terms and conditions" agreement, both of which may span multiple years. For revenue recognized at a point in time, transfer of control usually occurs upon shipment or upon customer receipt of the product, depending on shipping terms. For contracts recognized over time, control transfer occurs incrementally during our production process as progress is made on fulfilling the performance obligation. We use the input method of determining our progress, capturing direct costs beginning at the point that billet or cast ingot is introduced into production at either the extrusion phase or the rolling phase, respectively. We believe the input method more accurately reflects the transfer of control as it represents the best information available of work completed to date for which we have an enforceable right to payment. For products in production, we recognize revenue using estimates of the cost incurred to date plus a reasonable margin. As the duration of our contracts for accounting purposes is typically less than one year, we do not present quantitative information about the aggregate transaction price allocated to unsatisfied performance obligations at the end of the reporting period.

We adjust the amount of revenue recognized on all products, regardless of timing of revenue recognition, for variable price consideration, which could include metal market price adjustments, volume rebates and sales discounts. We estimate rebate and discount values based on forecasted order data and historical payment trends for specific customers, adjusted as necessary at each reporting period. Accounts receivable is recorded when our right to consideration becomes unconditional. Payment terms for a majority of our domestic customers are 30 days after invoice date. To accommodate shipping time to overseas locations, payment terms for foreign customers are typically several weeks longer than domestic terms. As such, we do not adjust the promised amount of consideration for the effects of a significant financing component as we do not expect the period between the transfer of control of products to our customers and receipt of payment will be greater than one year.

Contract assets primarily relate to our enforceable right to consideration for work completed but not billed at the reporting date on contracts for products recognized over time. Contract assets also include amounts related to our contractual right to consideration for finished goods recognized over time that were in transit as of period end.

Stock-Based Compensation. Stock-based compensation in the form of service-based awards is provided to executive officers, certain employees and non-employee directors and is accounted for at fair value. We measure the cost of services received in exchange for an award of equity instruments based on the grant-date fair value of the award and the number of awards expected to ultimately vest. The grant-date fair value is determined based on the stock price on the date of grant, adjusted for expected dividends or dividend equivalents to be paid during the vesting period.

We also grant performance-based awards to executive officers and other key employees. The methodology used to value these performance-based awards is based on the nature of the performance conditions within those awards. Awards that are subject to performance conditions pertaining to total shareholder return are valued on the date of grant using a Monte Carlo valuation model. The key assumptions in applying this model are an expected volatility and a risk-free interest rate. Awards with performance conditions pertaining to our cost performance and awards with performance conditions pertaining to our economic value added performance are valued based on our stock price at the date of grant. Holders of performance-based awards receive a one-time payment at the time of issuance of vested shares based on the total dividends they would have received if the vested shares had been held of record from the date of grant through the date of issuance. For more information on our stock-based compensation, see Note 7.

The cost of service-based awards, including time-vested restricted stock and performance shares, is recognized as an expense over the requisite service period of the award on a straight-line basis. Adjustments to expense related to forfeitures are recorded in the period in which they occur. For performance shares with performance conditions pertaining to our cost performance and economic value added performance, the related expense is updated quarterly by adjusting the estimated number of shares expected to vest based on the most probable outcome of the performance condition (see Note 7).

ASU No. 2016-02, Leases (Topic 842): Amendments to the Financial Accounting Standards Board Accounting Standards Codification ("ASU 2016-02"), was issued in February 2016 (with amendments issued in 2018) and requires lessees to recognize a right-of-use asset and a lease liability for virtually all of their leases (other than leases that meet the definition of a short-term lease). For income statement purposes, a dual model was retained, requiring leases to be classified as either operating or finance. Operating leases result in straight-line expense (similar to operating leases under the previous guidance) while finance leases result in a front-loaded expense pattern (similar to capital leases under the previous guidance). We adopted ASU 2016-02 and its subsequent amendments (together "ASC 842") during the quarter ended March 31, 2019 using the

transition approach provided for under ASU No. 2018-11, Leases (Topic 842): Targeted Improvements, which allowed us to apply the new lease standard as of January 1, 2019, rather than the beginning of the earliest period presented. We elected the package of practical expedients, which permitted us to: (i) not reassess whether any of our contracts contained leases; (ii) carry forward the historical lease classification of our existing leases; and (iii) not reassess initial direct costs for our existing leases. We did not elect to apply the hindsight practical expedient when determining lease term and assessing impairment of right-of-use assets. Due to our adoption of ASC 842, we recorded an operating lease right-of-use asset of $29.0 million, a current operating lease liability of $4.1 million and a long-term operating lease liability of $27.4 million on our Consolidated Balance Sheets as of January 1, 2019. There was no cumulative-effect adjustment to Retained earnings required. The standard did not materially impact our consolidated net earnings and had no impact on cash flows. Comparative information in this Report has not been adjusted and continues to be reported under the previous lease accounting rules. See Note 3 for details of the significant changes and quantitative impacts of the changes, as well as other required disclosures related to our adoption of ASC 842.

There were no material impacts on our consolidated financial statements resulting from our adoption during the year ended December 31, 2019 of: (i) ASU No. 2018-07, Compensation-Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting; (ii) ASU No. 2018-08, Not-for-Profit Entities (Topic 958): Clarifying the Scope and the Accounting Guidance for Contributions Received and Contributions Made; (iii) ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement; (iv) ASU No. 2018-14, Compensation - Retirement Benefits - Defined Benefit Plans - General (Subtopic 715-20): Disclosure Framework - Changes to the Disclosure Requirements for Defined Benefit Plans; and (v) ASU No. 2019-07, Codification Updates to SEC Sections: Amendments to SEC Paragraphs Pursuant to SEC Final Rule Releases No. 33-10532, Disclosure Update and Simplification, and Nos. 33-10231 and 33-10442, Investment Company Reporting Modernization, and Miscellaneous Updates.

ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments ("ASU 2016-13"), was issued in June 2016. Under ASU 2016-13, existing guidance on reporting credit losses for trade and other receivables and available for sale debt securities will be replaced with a new forward-looking "expected loss" model that generally will result in the earlier recognition of allowances for losses. We will adopt ASU 2016-13 in 2020 using the modified retrospective transition approach and do not expect it to have a material impact on our consolidated financial statements.

ASU No. 2018-15, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40): Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract ("ASU 2018-15"), was issued in August 2018. Under ASU 2018-15, requirements for capitalizing implementation costs incurred in a hosting arrangement (cloud computing) that is a service contract are to be aligned with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. We will adopt ASU 2018-15 in 2020 using the prospective transition approach and do not expect it to have a material impact on our consolidated financial statements.

ASU No. 2019-12, Simplifying the Accounting for Income Taxes ("ASU 2019-12"), was issued in December 2019. Under ASU 2019-12, the accounting for income taxes is simplified by eliminating certain exceptions and implementing additional requirements which result in a more consistent application of ASC 740. We are currently in the process of evaluating the impact of adopting ASU 2019-12 in 2021, but do not expect it to have a material impact on our consolidated financial statements.